Terence R. Anthoney
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Since there are already a plethora of resources on neuroanatomy, many of which you will have ready access to, why burden you with another? The attempt to put together a brief treatment of human neuroanatomy which tries to put forth in simple terms the major structural and functional features of the system, with emphasis on clinical relevance and stressing methods of organizing material to aid recall and prevent common pitfalls of misunderstanding, is probably similar to the aim behind, for example, Goldberg's Clinical Neuroanatomy Made Ridiculously Simple or Barrow's Guide to Neurological Assessment. (The latter, however, also includes and focuses on the procedures used to look for findings.) Since these texts (and others) are done well, a felt need to produce another is probably to some extent, parochial--looking for a resource that seems to "just fit" our curricular needs--written at just the right level of detail, stressing what we consider important, reflecting the particular "mix" of neuroanatomy, neurophysiology, neuroradiology, and clinical relevance being aimed for in our curriculum. Beyond that, the major differences from other resources I am familiar with will be to have you participate actively throughout--much as when using programmed self-learning texts, such as Sidman and Sidman's Neuroanatomy: A Programmed Text and to have you review what you've already learned over and over--each time a new functional system is added--by working relevant problems.
In a sense, I am at cross-purposes in writing this workbook. On the one hand, it is meant to be an overview of neuroanatomy only, and so should have a strong structural focus. Yet functional neuroanatomy is probably the main component of the Neuro block, which is constructed on the premise that neuroanatomy in isolation, like neuro-physiology, neuropharmacology, neurochemistry, etc., in isolation, is not very worthwhile clinically. Since it is the integration that is not only worthwhile but also most difficult, I am continually tempted to weave in material that is not strictly neuroanatomy and to leave out complicating structural details that do not seem useful. For now, I will simply suggest that as soon as possible, you should be tying your anatomy into the other disciplines necessary to handle clinical problems. This will allow you to begin judging for yourself what level of neuroanatomical detail you will need to know. Indeed, assuming that you will use neuroanatomy as a physician, the natural starting point will be a finding, not a structure. Thus, a natural progression in your studying might be to begin with findings, immediately ascertain and learn how to do the procedures necessary to demonstrate the findings, then study the mechanisms underlying the findings--including anatomy, physiology, biochemistry, etc. If working in reverse, trying to "make sense" of neuroanatomy, for example, then similarly become involved in the relevant physiology, testing procedures, and findings (normal and abnormal) as soon as possible. In this regard, you will probably want to keep your set of Neuro Clinical Skills modules (and perhaps a copy of Barrows, Guide to Neurological Assessment) handy, along with your black bag and a willing (?) subject (girl and boy friends, spouses, and children are historical favorites).
The faculty here has been impressed with how much better students seem to learn when they assume an active role--continuously asking questions about material they encounter in lectures, books, or patients and forcing themselves to make drawings, examine patients, etc.--rather than simply listening or reading, hoping that when they find the "magic resource" or have gone over the material enough times or in enough different ways, the material will suddenly fall together, all make sense, and be at their command. How often one hears a student say--if only someone would put all the brainstem structures in one clear diagram or 3-D model (despite availability of such resources!) or if only someone would tell us what's important to know (despite many people trying to do so!), etc. The bottom line is--the material is vast and complexly interwoven enough that you must use it to learn it. And use it means working with it until you feel comfortable that you know what's important and why, until you could make your own clear diagrams or 3-D models of the brainstem and defend why you chose the structures you did and put them where you did! At that point, one might say that you've "digested" the material, and it's the digestion that's critical. A student sometimes seems to think he can digest an integrative lecture by simply reading the lecture notes over till memorized, or that she can digest a diagram or model by passively memorizing what it looks like. No, memorization is not digestion. Memorization is duplication. One might just as well take a photograph of the notes or diagram and put the photo in his pocket. No matter how big a bite is taken, as long as the student relates it to nothing else, it is isolated. To assimilate material into your way of thinking about problems that require the information, you must reorganize it-- personalize it by forcing it to encounter and to make sense with the rest of your relevant knowledge. Only then will the material have been digested and become something that is uniquely yours. Good luck!
NEUROANATOMICAL TERMINOLOGY: SOME COMMENTS
Even if functional neuroanatomy had been "discovered" by a close-knit team of educationalists, very concerned about making what they found out easy to learn, the number of structures and functions alone would make for a hefty list of new terms to be learned. When you add to the picture the changes in structures that occur during development, embryologic and later, the procedures needed to examine the structures and functions for normality, and the various abnormalities of structure and function that are found, you are well on your way to learning a whole new language. Now, top this all off with the fact that many structures, functions, procedures, findings, etc., have names that are not descriptive, and may, in fact, have more than one name, sometimes not quite synonymous: and there you have it! The wonderful world of neurologic jargon!
Don't underestimate this hurdle. The sooner you become comfortable using these terms--speaking them and writing them in meaningful context, as well as understanding them when you hear them or see them--the sooner you can start meaningfully thinking about observations and problems that involve them. How can we make this necessary process easier for you? Hard to say, but here are some suggestions!
STRUCTURE OF THE NERVOUS SYSTEM: AN OVERVIEW
On to the structures themselves! This workbook will make very heavy use of diagrams and make no use of actual photographs. Consequently, make sure that you use your cadavers and fixed brains, as well as various models of the nervous system and anatomy texts, often enough that you are in fact "picturing" structures accurately as you think about them, their locations, and their functions.
Figure 2 is a diagrammatic frontal section of the nervous system, showing many of the major divisions and structures. Remember it is only accurate enough to allow meaningful labelling of these major divisions and structures. There is no attempt to draw it to scale. Most of the terms used to label Figure 2 (and Figure 3and Figure 4 as well) are "biggies" and therefore should be learned soon. You'll be meeting them over and over again.
Note that in terms of gross structures the nervous system is bilaterally symmetrical around the midsagittal plane. This means that there are "duplicates" of many structures--one on each side of the midline. This can be confusing for the beginning student, for we often talk of the paired structures as if there is only one--e.g., the optic nerve (cranial nerve II), the frontal-lobe (of the cerebral cortex), the caudate nucleus (one of the basal ganglia), the corticospinal tract (carrying information from cerebral cortex to spinal cord). This is just a convention, where it is assumed that the reader or listener knows that there are really two such structures. Some names, however, refer to only one structure on both sides of the body. Cerebrum is an example; other examples are brain stem, cerebellum, and spinal cord. As you learn the name of each structure, be sure to learn whether there are two of them or only one. To make it more confusing, there are a few cases where clinicians and neuroanatomists use a name differently. For example, clinicians tend to use "thalamus" to refer to thalamic structures in both cerebral hemispheres--i.e., there is only one thalamus. Neuroanatomists, however, often refer to a thalamus in each cerebral hemisphere- -i.e., there are two thalami (plural of thalamus). We will speak of only one thalamus, since you are clinicians-in-training. But now, if you run into the term "thalami" in a neuroanatomy text, it shouldn't surprise or bother you.
The "cerebrum" is what in layman's terms might be called the "brain." It is divided into right and left cerebral hemispheres. Its outer surface (think of it as a peel or rind) is called the cerebral cortex. Since it is loaded with cell bodies and dendrites, which are unmyelinated, the cerebral cortex looks gray--it is an example of gray matter. Other major areas of gray matter in the cerebral hemispheres are the basal ganglia and thalamus. Since the rest of the cerebral hemispheres (excepting the ventricles--cavities filled with cerebrospinal fluid--to be mentioned later) consists of axons, most of them myelinated, these areas look white--they are examples of white matter. The axons in these white areas act as wires, carrying information (neural impulses) from cell bodies in some portion of gray matter to cell bodies (and/or their dendrites) in some other portion of gray matter. Students frequently confuse the terms "cerebral" and "cortical." "Cerebral" refers to the entire cerebrum, so "decerebrate" refers to a condition in which the entire cerebrum is thought to be functionally disconnected from the rest of the nervous system. "Cortical," on the other hand, refers to only the cerebral cortex, so "decorticate" refers to a condition in which the cerebral cortex (but not the entire cerebrum) is thought to be functionally disconnected from the rest of the nervous system. (An illustration of the postures typical of decerebrate and decorticate rigidity are given on p. 674 of Curtis, Jacobson, and Marcus, An Introduction to the Neurosciences.) As a side note, "cortical" could theoretically refer to cerebellar cortex as well (see below); but as an isolated term, it always refers to cerebral cortex only.
Caudal to the cerebrum is the brain stem. The brain stem connects the cerebrum to the spinal cord, which begins by definition where the brain stem ends--at the foremen magnum. The brain stem is usually broken down into three major subdivisions. The one adjacent to the cerebrum (thalamus) is the midbrain, also known as the mesencephalon. Caudal to the midbrain is the pons. And caudal to the pons, joining it with the spinal cord, is the medulla--also called medulla oblongata. Although clinicians tend to consider only the midbrain, pons, and medulla as divisions of the brainstem, neuroanatomists often include the thalamus (thalami) as part of the brainstem. Keep this in mind to prevent confusion as you read neuroanatomy texts. The brain stem, like the cerebrum, is made up of areas of white matter and areas of gray matter. However, unlike the cerebrum, there is no outer covering of gray matter. The gray matter, instead, is found internally, grouped into a lot of distinct areas called nuclei (plural of nucleus).
Lateral and caudal to the brain stem is the last major portion of the central nervous system within the skull--the cerebellum. Cerebellum means "little cerebrum," and like the cerebrum, the cerebellum is mostly white matter inside (except for some deep nuclei--to be discussed later), surrounded by gray-looking cortex at the surface. Notice the similarity between the terms cerebrum and cerebellum, also between the terms cerebral cortex and cerebellar cortex. As you might imagine, it's easy to use the wrong term or hear the wrong term without even realizing it. If you're sensitive to this potential problem, you will probably have no difficulty with it.
The only portion of the central nervous system below the head is the spinal cord. Rostrally the spinal cord is defined to end at the foremen magnum, where the brain stem begins. Caudally, it ends in the adult at about the level of the twelfth thoracic or first lumbar vertebra. (See NEURO EMBR 1 for the embryologic development leading to this placement and NEURO ANAT 3 for more information on the relevance of this placement for doing spinal taps, also known as lumbar punctures.)
As a quick review, then, we've now covered the major divisions of the central nervous system (CNS). They are: the cerebral hemispheres (including the basal ganglia and thalamus), the brain stem (including midbrain, pons, and medulla), the cerebellum, and spinal cord. There now, that wasn't so bad, was it?
The peripheral nervous system (PNS) is much simpler to describe than the CNS, for it consists of two sets of structures only: 12 pairs of cranial nerves (and their roots, branches, and ganglia) and 31 pairs of spinal nerves (and their roots, branches, and ganglia). By definition, spinal nerves attach to the spinal cord, and cranial nerves attach to a part of the CNS inside the cranium.
Having gone over the CNS and PNS, we have covered the entire nervous system. There are no other neural structures left. But wait a minute, you may say. What about the autonomic nervous system, the limbic system, the sensory system, the extrapyramidal system, etc.? Good point. The answer is: any additional subdivisions of the nervous system that you hear about are simply different sets of structures within the CNS and/or PNS. Usually, a particular set of structures is singled out and given a particular name because someone has thought that they form a functional unit. For example, the autonomic nervous system (also called vegetative nervous system) was thought to regulate vital internal functions, such as pulse and respiratory rate, blood pressure, and digestion. The limbic system was thought to regulate emotional and complex innate behaviors, such as those associated with aggression, eating and drinking, and mating. And so on. As we progress further, we will describe some of these functional sets of structures. But for now, the message should be--if you've named the major components of the CNS and PNS (as we already have), you have named the major components of the entire nervous system.
So much for Figure 2. Now let's look at Figure 3, which is a schematic diagram of a cross-section (transverse section) through the spinal cord. The spinal cord, like the brain stem, has its white matter (myelinated neural processes) on the outside and its gray matter (cell bodies and unmyelinated neural processes) inside. Unlike the brain stem, however, the internal gray matter is continuous (not broken up into a lot of discrete nuclei) and has a characteristic shape, somewhat like an "H". The upper (dorsal) limbs of the H are called the dorsal or posterior horns and the lower (ventral) limbs of the H are called the ventral or anterior horns. In the thoracic, lumbar, and sacral cord (not the cervical), there is a lump of gray matter protruding laterally off the middle of the H; it's called the intermediolateral cell column and will be mentioned later, when we talk about the autonomic nervous system. Just as the gray matter is divided mainly into the dorsal and ventral horns, the white matter is divided basically into three sets of funiculi (plural of funiculus = a small cord or bundle). On the two sides of the midline between the dorsal horns are the two (right and left) dorsal funiculi. On each side, between the ipsilateral dorsal and ventral horns is one of the two (right or left) lateral funiculi. And on the two sides of the midline between the ventral horns are the two (right and left) ventral funiculi.
Figure 4 is a cross-section through one lateral half of a spinal cord, with a spinal nerve attached. Each spinal nerve is formed from the union of a dorsal nerve root and a ventral nerve root. The dorsal nerve root itself comes off the dorsal spinal cord as a series of small rootless. Similarly, the ventral nerve root comes off the ventral spinal cord as a series of small rootless. The nerve fibers running in the dorsal root are all sensory; the nerve fibers running in the ventral root are all motor. Thus, if a patient had a lesion near the dorsal spinal cord that destroyed several adjacent dorsal roots on one side, we would expect him to have sensory loss on that side of the body, but no motor problems. Similarly, if a lesion wiped out only several adjacent ventral roots on one side, we would expect to find motor loss on that side of the body, but no sensory deficits. Once the dorsal and ventral roots of a spinal nerve join, however, motor and sensory fibers tend to run together, so that typically a patient with functional impairment of one or more spinal nerves (or both dorsal and ventral nerve roots) has both motor and sensory problems, with similar distributions in the body.
To avoid confusion, we should probably discuss right away various classifications of sensory systems that you will be encountering. One of the simplest and most clear-cut classifications distinguishes among exteroception, proprioception, and interoception. Exteroceptors convey information about the external environment; examples are receptors in the eyes, ears, nose, and skin. Proprioceptors convey information about location and movement of different parts of the body; examples are receptors in muscles, tendons, and joints. Interoceptors convey information about internal organs, such as the heart, lungs, blood vessels, stomach and intestines, etc.; often, these sensations are poorly differentiated as to quality and location, but deep pressure and deep pain are common types of interoceptive experience. A second classification distinguishes between general sensory modalities and the special senses. This classification completely cries-crosses that of extero-, proprio-, and interoception. Within exteroception,, touch, superficial pain and pressure, and temperature--all conveyed from receptors in the skin--are general senses; whereas vision, audition, and olfaction are special senses. Within proprioception, information conveyed from receptors in joints, ligaments, muscles and fascia is general; whereas information on orientation and movement of the head conveyed from the vestibular system is special. Within interoception, deep pain and deep pressure are general senses; whereas taste is a special sense. As a rule, if a type of sensation is conveyed from various points of the body, it's called general; whereas sensations which arise only from a specific organ or tissue, which is usually specialized in some way, are called special. Clinically, another common category of sensation is somatosensation (also called the somesthetic system, some = body, esthesia = capacity for sensation). Somatosensation is a subclass of the general sensations, which includes only the general exteroceptive senses and general conscious proprioception (i.e., sense of position and sense of movement conveyed from receptors in joints, ligaments, and fascia).
The primary (first-order) general sensory neurons begin at their distal end in the form of sensory endings or adjacent to specialized receptor cells. Once an action potential is begun in this region, it must travel into the CNS before the information it represents is processed in any way. Two specializations help to hasten the speed with which sensory impulses reach the CNS. First, the neural process from the sensory endings (or receptor cell) to the cell body, which is embryologically a dendrite, has been structurally made into an axon: it can carry action potentials and many of the processes are even myelinated to allow much faster velocities (see NEURO PHYS 2 to learn how myelination speeds up velocities of action potentials). Indeed, these dendrites are not even called dendrites. The fibers running in nerves are all called axons. Second, the sensory first-order neurons have become unipolar: that is, the cell body has migrated to one side, allowing the embryological dendrite to join directly to the embryological axon (see Figure 5). This means that the action potentials do not have to be transformed into local potentials on the cell body, and then summate adequately before a new action potential is generated in the embryological axon (see NEURO PHAR 2 to learn about local potentials and summation). Instead, each single action potential can speed on its way, having bypassed the cell body. The cell bodies of all first-order sensory neurons traveling in a given spinal nerve are found in a special bulge on the dorsal root of that nerve: the bulge is called the dorsal root ganglion (ganglion = swelling). Note that in these sensory ganglia, there are no synapses. [NOTE: Unlike spinal nerves, cranial nerves do not usually have clearly separate sensory and motor roots--the trigeminal (cranial nerve V) is an exception. However, except for the olfactory and optic nerves (don't worry about them now), cranial nerves that have general sensory fibers do have sensory ganglia. In most cases, these ganglia are also outside (though close to) the CNS and contain cell bodies of the unipolar first-order neurons--i.e., no synapses. The two exceptions are as follows. (1) The first-order sensory neurons of the stato-acoustic nerve (cranial nerve VIII) are bipolar, though the cell bodies are still segregated into sensory (spiral and vestibular) ganglia. For these neurons, another specialization has substituted for the lack of unipolarity: the cell bodies have become myelinated, so that action potentials in the embryologic dendrites can simply "jump" across the cell bodies to the embryologic axons (see NEURO PHYS 2 to learn about saltatory conduction). (2) The cell bodies of the unipolar first-order sensory neurons associated with proprioception in muscles innervated by the trigeminal nerve (cranial nerve V) make up a small nucleus in the midbrain--called the mesencephalic nucleus of V. This is the only true ganglion in the CNS; all the rest are in the PNS. The "basal ganglia" don't count, for they are really nuclei, despite their name.
Many motor neurons that send their axons out of the CNS through the ventral root of a spinal nerve innervate striated muscle fibers. They have their cell bodies in the ipsilateral ventral (anterior) horn at the same level of the spinal cord where the ventral root containing their axons comes off. Because of the location of their cell bodies, these neurons are sometimes called anterior horn cells. They are the only motor neurons that innervate striated muscle fibers. A more common name for these neurons is lower motor neurons (or lower motoneurons--LMNs). Since they are called lower, you might expect that there are upper motor neurons (UMNs) as well. There are. We won't discuss them till a little later, but for now, just realize that a lot of CNS neurons with their cell bodies above the spinal cord send axons down into the spinal cord to influence the firing of the lower motor neurons. [Don't get confused into thinking that all motor neurons above the spinal cord are upper motor neurons. Not true! Motor neurons whose axons travel in cranial nerves to innervate striated muscles are also lower motor neurons, even though their cell bodies reside in the brain stem. Remember, then, that you can have LMN lesions in the brain stem as well as in the spinal cord and peripheral nerves; likewise, you can have upper motor neuron lesions in the spinal cord (the axons of UMNs must travel in the spinal cord to reach the appropriate LMNs) as well as in the brain stem and cerebrum. These points will become important a bit later, when we describe the differential findings in LMN and UMN lesions. Although there are a variety of neurons from various parts of the nervous system that do influence the activity of striated muscles, the only way they can do so is to act upon the lower motor neurons. For this reason, the lower motor neurons have been referred to as the "final common pathway." In other words, an LMN may be receiving impulses from some neurons "telling it" to fire faster and impulses from other neurons "telling it" to fire more slowly, but it is the net effect of all these impulses on the LMN that constitutes its "decision" as to how rapidly it will fire. Another way to assess the significance of LMNs would be to sever them. If all LMNs innervating a muscle were severed or destroyed, the CNS would have no way to make the muscle contract: the muscle would be denervated. [As a side-note, sensory fibers from the muscle might still be intact (see NEURO PHYS 3 and 4 to learn about sensory innervation of striated muscles), but the muscle would still undergo denervation atrophy and display denervation hypersensitivity (see NEURO PHYS 2 and NEURO PHAR 9 to learn about these).
There are other motor neurons that send their axons out of the CNS through the ventral root of spinal nerves, to innervate smooth muscles and glands. Although their cell bodies are also in the ipsilateral spinal cord at the same level where the ventral root containing their axons comes off, the cell bodies are in the intermediolateral cell column (see Figure 3), not the anterior horn. These neurons are part of a very important functional neural system, called the autonomic (vegetative) nervous system. Though autonomic fibers (axons) are found all over the body, the autonomic system is particularly important in regulating a variety of vital functions--including those served by the heart (e.g., pulse rate), lungs (e.g., respiratory rate), stomach (e.g., rate of acid production), intestines (e.g., force and rate of peristaltic movements), blood vessels (e.g., blood pressure), urinary bladder (e.g., ability to urinate), and reproductive organs (e.g., erection and ejaculation in the male), as well as many others.
In the broadest sense, the autonomic nervous system can be (and is) considered to have both afferent (sensory) and efferent (motor) components, the afferents being the sensory neurons innervating the variety of smooth muscle-containing and gland-containing structures (mostly, internal organs) served by autonomic efferents. These autonomic (vegetative) afferents, in fact, make up the interoceptors, carrying sensory information about internal organs, as opposed to the proprioceptors and exteroceptors mentioned earlier. In the narrower sense, however, the autonomic nervous system is often spoken of as strictly an efferent system. For example, when you read or hear that the autonomic system is basically a two-neuron system, consisting of preganglionic neurons and postganglionic neurons (as in NEURO ANAT 4), be aware that both the preganglionics and postganglionics are efferent, neurons. Similarly, when you hear or read about autonomic neurotransmitters (as in NEURO PHAR 3), be aware that it's the neurotransmitters of the preganglionics and postganglionics only, i.e., efferents, that are meant. In the brief overview to be given here, also, it is only the efferents that will be described.
The autonomic nervous system is divided into two major components: the sympathetics and the parasympathetics. These two components, though having some characteristics in common, show major differences in both anatomy and function. Both the sympathetic and parasympathetic systems consist of both preganglionic and postganglionic neurons. The cell bodies of all preganglionic neurons are in the CNS. At the spinal cord level, the preganglionic cell bodies are in the intermediolateral cell columns; in the brain stem, they are in distinct nuclei. Preganglionic cell bodies of sympathetic and parasympathetic systems are not found together, however. Preganglionic cell bodies of the sympathetic system are found only in the thoracic and lumbar segments of the spinal cord; while those of the parasympathetic system are found only in the brain stem and in the sacral spinal cord. Note that there are no preganglionic cell bodies in the cervical spinal cord; thus there are no intermediolateral cell columns in the cervical cord. Although some organs of the body may be innervated by sympathetics only or by parasympathetics only, most autonomically innervated structures receive both sympathetic and parasympathetic fibers. This is an important point, as in any given structure, the effects of sympathetic firing tend to be opposite to the effects of parasympathetic firing. As just one example of this, increased sympathetic firing speeds the heart rate, while increased parasympathetic firing slows it down. The sympathetic system has been called the flight or fight system, which prepares an animal under stress to take appropriate action. The parasympathetic system, on the other hand, seems to promote functions associated with non- stressful daily existence, such as digestion. (See NEURO PHAR 3 to learn more about autonomic functions.)
But if the parasympathetics originate only in the brain stem and sacral cord, how do they come to innervate all the structures in between? And if the sympathetics originate only in the thoracolumbar cord, how do they reach the head (and neck) and lower body? Let's take the parasympathetics first. The tenth cranial nerve (the vagus nerve) carries a large number of autonomic fibers (all are parasympathetic, of course). Although it is a cranial nerve, it travels down through the neck into the thorax and abdomen, innervating a variety of structures down to (but not including) the sacral level. The sympathetics, for their part, move toward the head and sacral regions in two bundles of sympathetic fibers, one on each side of the vertebral column, called the paravertebral sympathetic chains. At various points along these chains, not only at the ends, bunches of sympathetic fibers leave to accompany blood vessels, which carry them to their organs of destination. Since it is the sympathetics that tend to be distributed along blood vessels, it may not be surprising that the main autonomic innervation of the limbs, which terminates primarily in the blood vessels themselves (especially the arterioles) and in the skin (especially sweat glands--sudomotor fibers--and smooth muscle that erects the body hair-- pilomotor fibers) is sympathetic.
Next we will consider the autonomic ganglia and postganglionic neurons. All autonomic ganglia are found in the peripheral nervous system. In them, the axonal endings of preganglionic neurons synapse with the dendrites/cell bodies of postganglionic neurons. Note, therefore, that autonomic ganglia, unlike sensory ganglia, contain synapses. The parasympathetic ganglia tend to be found near or in the organ to be innervated by the postganglionic neurons. Thus, the parasympathetic preganglionics (as in the vagus nerves) tend to be long; whereas the parasympathetic postganglionics tend to be very short. Since the parasympathetic ganglia tend to occur toward the peripheral end of the system, after the preganglionics have already broken up into rather small bundles going to the various organs, the ganglia are relatively small, often microscopic. Discussion of the sympathetic ganglia and sympathetic postganglionic neurons takes us back to Figure 4.
Although Figure 4 was meant to represent any level of the spinal cord, what you've already read about the autonomic system lets you know that it is really accurate only for the thoracolumbar spinal cord. Since the cervical cord has no cell bodies of preganglionic autonomic neurons, it would not have the intermediolateral cell column shown in Figure 4. And although the sacral cord does have cell bodies of preganglionic autonomic neurons, they are parasympathetics, so would not have axons going into the chain of sympathetic ganglia lying along the vertebral column (paravertebral ganglia), as does the axon of the representative preganglionic neuron in Figure 4. Notice in Figure 4 that each thoracic and lumbar spinal nerve is attached to the paravertebral ganglia by two small bundles of fibers, called rami (plural of ramus, meaning "branch"). One of the rami is called a white ramus, the other is called a gray ramus. Why? Well, the preganglionic sympathetic fibers course from the spinal cord through the white ramus to the paravertebral chain. Since all preganglionic fibers are myelinated (this is true of parasympathetics, too!), the ramus in which they run appears whitish. The postganglionic sympathetic fibers, however, whose cell bodies are found in the paravertebral ganglia, leave the paravertebral chain through the gray ramus, to enter the spinal nerve and go peripherally. Since all postganglionic fibers are nonmyelinated (this is true of parasympathetics, also), the ramus in which they run appears grayish. Let me hasten to point out, however, that not all sympathetic fibers synapse in the paravertebral ganglion nearest the spinal nerve in which they exit. Some go up or down the paravertebral chain to other paravertebral ganglia before synapsing; others go to one of several large sympathetic ganglia anterior to the vertebral column (prevertebral ganglia), and a few are even found in small terminal ganglia near the organ to be innervated (e.g., in the urinary bladder). This concludes our brief overview of the autonomic nervous system (see NEURO ANAT 4 and NEURO PHAR 3 to learn more about it).
SOME LONG-TRACT SYSTEMS: THE FOUNDATION
Now that we've finished our first quick journey through the nervous system, let's go through it again. But this time, we'll concentrate on function of structures, particularly functions that are often tested clinically to find out what, if anything, is wrong with the nervous system, and where the problem is located. The process of finding out where problems exist in the body is called anatomical diagnosis. As you will soon discover, the process of anatomical diagnosis is central to the clinical neurosciences. Although there are some more or less "silent" areas in the nervous system, where damage may not give rise to abnormal findings during the taking of the history (symptoms) or during the physical examination (signs), damage in most areas does produce some abnormal symptoms and/or signs. Because of the way structures subserving various functions are organized in the nervous system, the findings associated with damage to a particular area of the nervous system are often unique to that area. Often, by specifying abnormal findings carefully and knowing the functional anatomy of the nervous system, a physician can locate neural damage to within a centimeter or less. Although the degree of sophistication attained by many clinical neuroscientists is awe- inspiring, one can achieve considerable skill at neuroanatomical diagnosis rather quickly by focusing on major organizing facts and principles. We will try to maintain that focus on this, our second, journey through the nervous system.
We will start our journey by consideration of three sets of structures, each of which is centrally important clinically, as damage to it will produce a specific constellation of abnormal findings. As a short-hand convention, we will refer to each of these three sets of structures as a "long-tract" system, to signify that each set stretches through a considerable portion of the nervous system. What these three sets have in common, then, is that damage at a variety of locations in the nervous system may disrupt their functions! Two of the long-tract systems are general afferent (sensory)--the "lateral spinothalamic" system and the "dorsal columns" system. The third system is efferent (motor)--the "corticospinal (corticobulbar)" system.
Let's consider the lateral spinothalamic system first (Figure 6 and Figure 7). This system carries information on pain and temperature. Clinically, these modalities are tested on the skin, light pin-pricks being used to measure ability to sense superficial pain and warm/cool objects being used to measure ability to sense temperature (see NEURO HxPx 5 to learn more about the clinical testing). In all examinations of general sensory systems, the patient's eyes should be closed.
Just from information you've learned so far, the initial course of the lateral spinothalamic system should not surprise you. As all general sensory systems do, the lateral spinothalamic begins with first-order neurons, whose unipolar cell bodies reside in a sensory ganglion. The impulses conveyed by these neurons begin at the distal ends, which may be specialized themselves or be adjacent to specialized receptor cells, and pass proximally right through the sensory ganglion and into the CNS.
Having entered the ipsilateral dorsal horn of the spinal cord, the axons of the first-order neurons end (without ascending or descending) by synapsing on dendrites and/or cell bodies of second-order neurons in the dorsal horn. The axons of these second-order neurons begin to ascend and at the same time to move towards the anterior white commissure (Figure 6), where they cross to attain a position in the ventral half of the contralateral lateral funiculus. It takes about 2-3 spinal segments of ascension before all the axons of second-order neurons associated with first-order lateral spinothalamic neurons in a given spinal nerve have crossed. For example, for first-order neurons in the left tenth thoracic nerve, some axons of associated second-order neurons will be crossing the midline at the T10 level, but others will be crossing at T9 or T8. This is clinically important to remember. For example, if you tested a patient and found her unable to sense superficial pain and temperature on the left side from the foot on up to the level innervated by the tenth thoracic nerve, would damage to the right half of the cord at the T10 spinal level be consistent with the findings? No! To wipe out pain and temperature on the left up to the level innervated by the tenth thoracic nerve would require that the right cord be damaged at about T7-T8 (or above, if it didn't wipe out the entire pathway). If this doesn't make sense to you, go over the above material on the lateral spinothalamic system again.
Since you will meet neural fibers crossing the midsagittal plane (midline) again and again in various functional systems, let's lay out two general definitions now. Several anatomically distinct regions in the CNS which join together structures on the two sides (left and right) are called commissures (commissura = a joint or a joining together). Often, a commissure is the site of many neural fibers crossing the midline, as in the case of the anterior white commissure of the spinal cord. If a bundle of fibers (actually one bundle on either side) crosses the midline while simultaneously moving anteriorly or posteriorly, the two bundles together tend to form an "X" shape as viewed in cross-section. Since "decussate" means to cut or cross in the form of an X, the crossing fibers are said to "decussate" and the place of crossing is called a "decussation." The decussation of the second-order axons in the lateral spinothalamic system, which takes place in the anterior white commissure, is an example (Figure 6). You will soon meet several other important decussations. Although you may at times hear the word decussate used loosely to mean simply "cross the midline," purists will try to maintain the original distinction.
The actual "lateral spinothalamic tract" itself is situated in the ventral half of the lateral funiculus of the spinal cord. It originates in the lower sacral cord and runs all the way up the cord to the brain stem. Since it is gaining fibers from more and more of the body as it ascends, the lateral spinothalamic tract becomes increasingly larger as it approaches the medulla.
Since this is the first tract we have encountered, let's digress briefly to note some general features of tracts. We'll start with the names. Fortunately, most names of tracts have some descriptive aspects, making it easier to recall them correctly (assuming you know something about the tracts, that is). Many names of tracts tell you both their origins and destinations--in that order. In that regard, some common combining forms are cortico- (= cerebral cortex), spino- (= spinal cord), bulbo- (= brain stem), thalamo- (= thalamus), and cerebello- (= cerebellum). The name "spinothalamic," for example, means that the tract originates in the spinal cord and travels to the thalamus. The "lateral" in front of "spino-thalamic" indicates that this tract runs in the lateral funiculus of the spinal cord. Another spinothalamic tract, called the "anterior" spinothalamic, runs in the anterior funiculus of the spinal cord. Besides giving information about origin, location, and destination, which is clinically quite useful, some names give information about shape of the tract as it appears on cross-section--not particularly helpful. Examples are the fasciculus cuneatus (cuneatus = wedge-shaped) and internal arcuate fibers (arcuate = shaped like a bow). Least helpful are terms that are basically synonyms for "tract," such as bundle, fasciculus, and lemniscus.
Since most tracts were identified as anatomical entities long before much was known of their function, the correlation between tracts and functional pathways is often far from 1:1. For example, you can see from Figure 6 and Figure 7 that the lateral spinothalamic tract makes up only a portion of the lateral spinothalamic system. Although a given tract usually contains only uninterrupted axons (i.e., no synapses), it often does not even include the entire axons. In the case of the lateral spinothalamic tract, for example, the axons of the second- order neurons travel from one dorsal horn, through the anterior white commissure to the contralateral lateral funiculus, before they officially enter the tract. Also, in some cases, there are nuclei essentially within a tract, so that the tract doesn't end at one location, but rather ends partially in stages as some of its fibers end in synapses at each nucleus. (The descending (spinal) tract of the trigeminal is an example, to be discussed later.)
While fibers within a given tract (whether called a tract, bundle, fasciculus, lemniscus or whatever!) tend to run together and are often visible on cross- sections as separate structures, the fibers may at times be separated from each other by intervening structures, sometimes widely so. As you might imagine, this can have important clinical consequences. Where fibers of a tract are running close together, a relatively small lesion may completely destroy the tract and its functions; whereas in areas where the fibers are diffusely spread out, a lesion of similar size may result in almost no apparent functional deficit. The corticospinal system, an effector system to be described soon, provides an example here, for a cut through the dorsal part of one lateral funiculus in the cervical cord (1-2 millimeters) can completely destroy ability to move the ipsilateral arm, leg, or body below the neck, whereas a lesion near the cerebral cortex would have to extend over much of the precentral gyrus and paracentral lobule (8-10 centimeters!) to cause the same deficit.
A last point to be mentioned is that fibers in several important tracts are organized somatotopically--i.e., fibers within the tract are grouped physically according to the part of the body they influence (some = body, topos = a place). For example, a common organization for tracts while in the cerebrum and rostral brain stem is to have fibers associated with the feet and legs at one end, fibers associated with the arms and trunk in the middle, and fibers associated with the head and neck at the opposite end. Often, where the fibers associated with a particular part of the body will be located within a tract can be reasoned out, so that you don't have to memorize the information.
For example, think about how the lateral spino-thalamic tract is formed as it ascends (Figure 8). Since at any level of the cord, the axons just joining the tract come to its medial side first, having just decussated, one could say that it "makes sense" to have the fibers already in the tract "pushed" further laterally and dorsally. Thus, fibers associated with the perineum, which had already joined the tract in the sacral cord, would be found most laterally, while fibers associated with the legs, trunk, arms, and neck would, respectively, be located more and more medially. Fortunately, this organization not only makes sense, but is true. The lateral spinothalamic tract will also be used to illustrate the possible clinical significance of somatotopic organization. If you remember, we determined a few pages back where a lesion might produce loss of ability to sense superficial pain and temperature on the left side from the toes on up to the level of the trunk innervated by the tenth thoracic nerve. At that point, we decided on a lesion in the ventral half of the right lateral funiculus at T7-T8, wiping out the entire right lateral spinothalamic tract at that level. Now that you know about the somatotopic organization of the tract, however, you might see another possibility. Right! A lesion higher up in the spinal cord (e.g., at the level in Figure 8) may have mimicked T-7 damage by destroying only the dorso-lateral part of the right lateral spinothalamic tract.
When the lateral spinothalamic tract goes from the upper cervical spinal cord into the medulla, the only major change that occurs is in its name (Figure 7). When entering the medulla from below, fibers of the anterior and lateral spinothalamic tracts merge, so they are now jointly called by the same name--the spinothalamic tract. The spinothalamic tract courses on up through the medulla, pons, and midbrain to the thalamus, where its axons end by synapsing in the ipsilateral ventral posterolateral nucleus (the VPL).
As you learn more about the nervous system, you will discover that several different sensory systems that send information to the cerebral cortex do so via the thalamus, where the cell bodies of the neurons whose axons will actually reach the cortex are segregated into distinct nuclei. These nuclei are collectively known as thalamic relay nuclei. Each sensory system has its own, and the VPL is the relay nucleus for the somatosensory system, which includes the lateral spinothalamic. Since the axons in the spinothalamic tract belong to second-order neurons, the cell bodies in the VPL belong to third-order neurons. Because axons from cells in thalamic relay nuclei tend to fan out as they approach the cortex, they are called thalamic radiations. Because the axons coming from the VPL are associated with somatosensation, they are called the somatosensory radiations. During the first part of their trip, as they pass around the basal ganglia, they are within a structure called the internal capsule. The internal capsule contains essentially all neural fibers connecting the cerebral cortex with other parts of the nervous system. It is very important clinically and you'll be learning more about it soon. After passing the basal ganglia, the somatosensory radiations become much more diffuse as they fan out and intermingle with a variety of other fiber systems. In this location, they form a small part of the myriad fibers radiating out in all directions to the cortex. These myriad fibers are jointly called the corona radiate (corona = crown, radiate = radiating). The part of the corona radiate formed by the somatosensory radiations ends up synapsing largely in the ipsilateral post-central gyrus (and posterior paracentral lobule) of the parietal lobe of the cerebral cortex.
Don't worry about all the special names now. The most important facts to remember about the anatomy of the lateral spinothalamic system are that fibers in the system cross the midline within 1-3 spinal segments above their entry point into the cord, are somatotopically organized, and remain crossed (i.e., contralateral to their receptors) all the way up through the spinal cord, brain stem, and thalamus, to the cerebral cortex. As for function of the lateral spinothalamic system, remember two things. First, the system conveys information on pain and temperature, which are tested clinically with a pin and warm/cool objects, respectively. Second, lesions in the system up to and including the VPL in the thalamus can destroy all ability to sense temperature and pain in the relevant parts of the body, but lesions above the thalamus (somatosensory radiations or cortex destroy primarily the ability to tell where stimuli presented to the relevant parts of the body are located, even though the stimuli themselves may be correctly perceived as painful, hot, or cold. In other words, if you examined a patient and found that when you touched him on the right side of the body with a pin, he could feel it was sharp but couldn't locate very well where he'd been touched, you'd think of a lesion ... Where? Exactly! In the left somatosensory radiations or left somatosensory cortex.
Now that you've completed going over your first functional system, try posing some relevant clinical problems to yourself and working out the answers. To get you started, I'll give you a couple; but you should be able to come up with others yourself.
A quick comparison of Figure 9 with Figure 6 will show you that up to the point of entering the spinal cord through the dorsal roots, the dorsal columns system is organized just like the lateral spino-thalamic system. To repeat: As in all general sensory systems, the dorsal columns system begins with first-order neurons, whose unipolar cell bodies reside in a sensory ganglion. The impulses conveyed by these neurons begin at the distal ends, which may be specialized themselves or be adjacent to specialized receptor cells, and pass proximally right through the sensory ganglion and into the CNS. Unlike the lateral spinothalamic system, however, first-order neurons do not synapse soon after entering the cord. Instead, their axons go into the ipsilateral dorsal funiculus and ascend in it all the way up to the junction of the spinal cord with the medulla. Unfortunately, anatomists sometimes call the dorsal (posterior) horns, which consist of gray matter, the dorsal (posterior) columns. Be aware that whenever you encounter the term dorsal (posterior) columns clinically, it is the ascending long tracts making up most of each dorsal funiculus, i.e., white matter, that is meant.
The dorsal columns themselves are somatotopically organized. Given that as the dorsal columns ascend, axons newly joining them approach from the lateral side, where do you think that fibers from various parts of the body are found within the dorsal columns? If you guessed that the lower sacral fibers from the perineum, which enter first, become most medial, while fibers from the leg, trunk, arm, and neck lie, in that order, more and more lateral, then you guessed right! Although the dorsal columns on either side form a functional-whole, fibers that come from the perineum and leg (L1 on down) on one side are separated from the fibers originating in the ipsilateral trunk, arm, and neck by a deep narrow crevice. In neuroanatomy, a minor crevice, such as this one, is called a sulcus (= furrow, plural = sulci), a major crevice is a fissure. The name of the sulcus we've just described is not important. What is important is the unfortunate consequence of the sulcus' presence: the fibers on one side have a different name than the fibers on the other side. The fibers medial to the sulcus, emanating from the perineum and leg, are jointly called the fasciculus gracilis. Those fibers lateral to the sulcus, emanating from the trunk, arm, and neck, are jointly called the fasciculus cuneatus. When pairs of structures are arbitrarily assigned different names, it is usually very difficult to remember which name goes with which structure. In such situations, it's nice to have 2 mnemonic (= a device to aid your memory). Though I'm not sure it's worthwhile expending much effort to keep the fasciculus gracilis and fasciculus cuneatus straight, my own mnemonic is as follows. Gracilis and ground both start with "gr," cuneatus and ceiling with "c." The legs touch the ground, while the ceiling is higher, so the fasciculus gracilis is the medial column with fibers from leg and perineum, the fasciculus cuneatus is the lateral column with fibers from trunk, arm, and neck. A student suggested another mnemonic--namely, the gracilis muscle is in the leg. If you come up with a mnemonic you like better, let me know.
At the junction of the spinal cord with the medulla, the first-order axons in the dorsal columns end by synapsing on cell bodies/dendrites of ipsilateral second-order neurons. These synapses occur in a pair of nuclei: fibers in the fasciculus gracilis synapse in the nucleus gracilis, fibers in the fasciculus cuneatus synapse in the nucleus cuneatus. That seems almost too easy, doesn't it? Well, now the fun begins!
As you can see from Figure 10, the axons of the second-order neurons in the nucleus gracilis and nucleus cuneatus immediately cross the midline (decussate, remember?) before starting to ascend up through the medulla, pons, and midbrain. Unfortunately, name changes accompany these changes in position. As the axons are leaving the nuclei and crossing the midline, they are called internal arcuate fibers (because they are located far internal to the surface of the CNS and are bow-shaped). Once they begin to ascend, and until they reach their thalamic relay nucleus, the fibers are jointly called the medial lemniscus (because it is medially located during much of its ascent). Notice that axons leaving the nucleus gracilis do not have different names than axons leaving the nucleus cuneatus. From here on up, fibers associated with perineum, leg, trunk, arm, and neck all travel in structures with the same names. By comparing Figure 7 and Figure 10, you will see that once axons in the medial lemniscus end by synapsing on cell bodies of third- order neurons in the ipsilateral ventral posterolateral (VPL) nucleus of the thalamus, the rest of the structures are the same as in the lateral spinothalamic system. The axons of the third-order neurons form part of the somatosensory radiations, traveling first through the internal capsule and then through the corona radiate, finally synapsing largely in the ipsilateral post- central gyrus (and posterior paracentral lobule) of the parietal lobe of the cerebral cortex.
As in the case of the lateral spinothalamic system, don't worry about all of the special names of dorsal columns structures now. The most important facts to remember about the anatomy of the dorsal columns system are that fibers in the system are somato-topically organized, all of them decussate at the spino-medullary junction, and they remain crossed (i.e., contralateral to their receptors) all the way up through the brain stem and thalamus, to the cerebral cortex. As for function of the dorsal columns system, remember two things. First, the system conveys information on conscious proprioception, vibration, and discriminative touch, which are tested clinically by moving fingers and toes, using a tuning fork, and testing ability to localize tactile stimuli precisely. Second, lesions in the system up to and including the VPL in the thalamus can destroy all of the above--conscious proprioception, vibratory sense, and discriminative touch--in the relevant parts of the body (though crude touch will be spared in CNS lesions below the VPL); but lesions above the thalamus (somatosensory radiations or cortex will leave ability to perceive vibrations intact, though the patient may not be able to tell where the vibrating stimulus is located on the body.
This is a good time to pose some clinical problems involving only the dorsal columns system. Again, be sure to work through each problem, defending your answers. I'll only give you one this time.
Let's start the process now. By comparing the anatomy and the function of the lateral spinothalamic and dorsal columns systems, you will already be able to refine your anatomical diagnoses considerably. Remember that until now, loss of sensations referable to just the lateral spinothalamic system or just the dorsal columns system, in part of the right leg, for example, could be caused by problems in the nerves to the leg themselves, in the spinal cord from the sacral level all the way up to the cervical, in the brain stem, or in the thalamus. (You already know how to tell if the problem is in the somatosensory radiations or cerebral cortex, right? If not, review briefly to refresh your memory.) Comparing Figure 7 and Figure 10 should help make the following functional-anatomical pattern clear. When you test sensations on the body (exclude the head for now), if the areas of loss for the two systems are about the same, then without consideration of other findings, you might expect that the problem is either peripheral (since the fibers for the two systems run together in the nerves) or in the cerebrum (since the fibers for the two systems are somatotopically integrated from the VPL nucleus on up to the cortex). Conversely, if you find an area of loss for one system only, it reflects damage in the spinal cord (where the lateral spinothalamic system is mostly contralateral, the dorsal columns system ipsilateral) or brain stem (where the two systems, though both now contralateral, don't run together). (I might point out, by the way, that even though the two systems cross at very different levels of the CNS, there is one striking similarity. Each of them crosses just beyond the cell bodies of the second-order neurons.)
Time to pose some more problems for yourself. Be sure to make some clinical (i.e., a patient presents with ... , so where's the problem?) and some anatomical (i.e., if there is damage to ..., what findings would the patient have?). I'll give you a couple.
Although the corticospinal-corticobulbar neurons and lower motor neurons all subserve function in striated muscles, damage to UMNs produces a different set of findings than damage to LMNs. (A few important exceptions will be given shortly.) To aid comparison of the two sets, some of the most commonly noted similarities and differences between them are listed in Table 1.
|Characteristic LMN Findings|
|Characteristic UMN Findings|
|1.||paresis (i.e., partial paralysis) or paralysis (also called palsy)||paresis or paralysis (esp. of distal musculature--e.g., of fingers or toes)|
|2.||decreased superficial reflexes||decreased superficial reflexes|
|3.||decreased deep tendon reflexes (DTRs)||increased DTRs (also called hyperreflexia), possible clonus|
|4.||no pathologic reflexes||pathologic reflexes (e.g., Babinski and Hoffman)|
|5.||hypotonia (decreased tone)||hypertonia (increased tone)|
|6.||marked muscular atrophy (denervation atrophy)||mild muscular atrophy (disuse atrophy)|
|7.||fasciculations (and fibrillations)||no fasciculations (or fibrillations)|
Your first reaction to this table might be--Oh, my God! Not only is it long, but it's full of unfamiliar terms, whose dictionary meanings are not very useful until you know how the relevant observations and maneuvers are actually done. It's not really as bad as it looks. Let's start by considering an LMN lesion, involving the muscles in, say, the right foot and leg. If you look down the list of characteristic LMN findings in Table 1, you'll notice that all of them (with the exception of fasciculations, but see below) are "absence" or "less than normal" findings. (Hardly surprising, since they reflect partial to complete absence of innervation to the muscles!)
|1.||Paralysis is inability to contract muscles voluntarily. (If a patient can contract the affected muscles, but it's unclear whether the contractions are normal or weakened (paresis), you can do standardized tests of muscle strength--see NEURO HxPx 6 to learn more about them and the other clinical observations and maneuvers mentioned below.)|
|2-4.||Superficial reflexes are ones in which you stimulate skin receptors (e.g., with a light scratch) to make the muscles contract; deep tendon reflexes (also called myotasis or muscle stretch reflexes) are ones in which you stimulate muscle receptors by stretching the muscle slightly (e.g., by tapping the tendon with a rubber hammer) to make the muscles contract. Obviously, if direct innervation to the muscles is damaged, both kinds of reflexes will be decreased. Similarly, pathologic reflexes (i.e., presence of reflexes normally absent) cannot occur.|
|5.||Muscle tone is defined as the resistance to passive movement (e.g., to the examiner moving the foot, lower leg, etc., back and forth with the patient relaxed). Since a denervated muscle cannot resist movements that stretch it, the muscle is hypotonic. Active movement, by the way, is movement for which the patient provides the force. In tests of muscle strength, as in 1. above, it is strength of active movements (or of active isometric contractions) that is being measured.|
|6.||Atrophy (a = lack of, trophic = nutrition) means wasting, and muscle wasting can be observed grossly by comparing to the normal side of the body (if there is one). More precise measurement can be done with a tape measure. When a muscle is denervated, it diminishes greatly in size, presumably because the nerves to it somehow provide needed nutrients or necessary minimal activity.|
|7.||A fasciculation (from fasciculus = small bundle) is the simultaneous contraction (usually a very brief jerk) of a bundle of muscle fibers, all innervated by the same LMN. As you know, one LMN with all the muscle fibers it innervates is called a motor unit; and the muscle fibers in one motor unit are usually grouped together. When an LMN is damaged, changes in the membrane permeability to ions may cause action potentials, which occur independent of firing in neighboring LMNs, thus producing an isolated simultaneous twitch of the motor unit. Such fasciculations are often grossly visible to the eye. Once an LMN is dead, however, no further fasciculations occur; so fasciculations are a sign of damaged, yet viable LMNs. Fibrillations are similar autonomous firings, but in single denervated muscle fibers. Fibrillations are not grossly visible (except perhaps in the tongue), so demonstration is usually accomplished with electromyography--recording electrical activity in the muscle. Once a denervated fiber completely degenerates, it no longer fibrillates. Fibrillations, then, are a sign of denervated, yet viable muscle cells.|
Now let's look at the characteristic UMN findings in Table 1. As you run down the list, notice that they can be divided into three groups--"less than normal" findings (paresis, decreased superficial reflexes, atrophy), a "normal" finding (no fasciculations or fibrillations), and "more than normal" or "release" findings (hypertonia, hyperreflexia and clonus, pathologic reflexes). As you can imagine, the "less than normal" findings, being similar to their LMN counterparts, aren't particularly helpful differentiators. Lack of fasciculations may also not be very helpful, as we noted above their absence in LMN lesions after the neurons are dead. Thus, it is the increased resistance to passive movement, the increased deep tendon reflexes, and the presence of usually absent reflexes that are the most useful indicators of UMN lesions.
You may be puzzled as to why a destructive lesion leads to an increase in activity, especially when it also produces paresis. This brings up two very important concepts, which you'll use repeatedly. First, if a set of neurons are inhibitory for a function, then their firing decreases the function and their electrical silence or destruction increases the function. If the function is normally inhibited by the neurons, then destruction of the neurons is said to "disinhibit" or "release" the function. In UMN lesions, for example, deep tendon reflexes (responses to stretch) are disinhibited, leading to hyperreflexia and hypertonia; and some pathologic reflexes (which are components of "normal" reflexes present in early infancy), such as the Babinski response, are released (see NEURO PHYS 4 and NEURO ANAT 17 to learn more about these reflexes).
But, you might say, what about the paresis and decreased superficial reflexes? If the UMNs are an inhibitory system, why don't we see more movement than normal and increased superficial reflexes? This brings up the second important concept. A set of neurons may influence more than one function; and if it does, some functions may be facilitated at the same time that others are inhibited. UMNs, for example, apparently facilitate voluntary movements and superficial reflexes, while inhibiting deep tendon and normal infantile reflexes. If our patient with weakness in the right foot and leg had an upper MN lesion, instead of a lower MN lesion, what would we find in addition to the paresis, the absent cremasteric reflex, some possible atrophy ("disuse" atrophy, which is less marked than "denervation" atrophy), and no fasciculations? There would be hypertonia on passively flexing and extending the foot and lower leg, hyperreflexia (e.g., of the patellar reflex and ankle jerk), and an extensor response of the large toe to planter stimulation (a Babinski response).
As in the case of the LMN lesion, however, the exact findings will depend on the time-course of the injury. Most importantly, you must realize that when a lot of damage to UMNs occurs suddenly, as, e.g., in a cerebrovascular accident (CVA = stroke) or trauma to the head or spine, the CNS goes into "shock," which masks the "release" phenomena. For example, if our patient with weakness in the right foot and leg had been fine until he suffered a stroke the previous day, we would not find hypertonia, hyperreflexia, and a Babinski response in the right lower extremity. Instead, we'd find hypotonia, hyporeflexia, and a normal planter response. Only over the course of days to weeks would his flaccid paralysis gradually be replaced by the signs of spastic paralysis.
After that long clinical-functional preamble, let's get back to the anatomy of the corticospinal-corticobulbar neurons. Just from the names, you can tell that they all have their cell bodies in the cerebral cortex, that the corticospinals end in the spinal cord, and that the corticobulbars end in the brain stem. Many of the cell bodies are found in the pre-central gyrus (and anterior paracentral lobule) of the frontal lobe (Figure 11). Let's deal with the corticospinal fibers first--those associated with function in muscles below the head. The axons coming from cell bodies in one cerebral hemisphere remain ipsilateral all the way to the medullospinal junction--traveling through the corona radiate somewhat diffusely, then becoming compact as they course through the internal capsule, and next passing through the midbrain pons, and medulla. Within the medulla, this corticospinal tract is somewhat pyramidal-shaped in cross-section, so has the alternate name of pyramidal tract. At the medullospinal junction, the vast majority of the corticospinal fibers decussate, in the so-called "pyramidal decussation," and form the lateral corticospinal tract, from which the various fibers are given off gradually and continuously down the length of the spinal cord (Figure 12). As you might imagine, if there is a lateral corticospinal tract, it suggests there is another corticospinal tract--and there is. It's called the anterior corticospinal tract and contains both crossed and uncrossed fibers (relative to side of origin). Though this relatively small tract may be important in aiding recovery of UMN function after injury to axons in the lateral corticospinal tract, it is much less helpful in clinical management and can be largely ignored. It is mentioned here only for completeness.
After a fiber is given off from the lateral corticospinal tract, it goes directly to the ipsilateral anterior horn and ends at that level, with synapses near the LMNs it will influence. Obviously, corticospinal fibers associated with neck musculature are given off first, followed in order by those associated with musculature of the upper extremity, trunk, lower extremity, and perineum. As in the lateral spinothalamic and dorsal columns systems, the corticospinal- corticobulbar fibers are somatotopically arranged. If fibers from the lateral corticospinal tract pass ventro-medially to get to the anterior (ventral) horn, where would you guess that fibers associated with the various parts of the body are located within the tract? As you can see from Figure 12, if you guessed that the fibers associated with neck are most ventro-medial, while those associated with arm, trunk, leg, and perineum are, respectively, more and more dorsolateral, you are correct! Hurray, another piece of information that doesn't need to be memorized!
In summary, the most important facts to remember about the anatomy of the corticospinal neurons are that their axons are somatotopically organized, all of them decussate at the spinomedullary junction, and they remain crossed (i.e., contralateral to their origins) until they end at various levels of the spinal cord. [Clinically, however, it is probably more practical to think of the fibers above the decussation as being contralateral, namely, to the muscles influenced, for it is the location of the muscles (effectors) on which the findings center, just as the findings in sensory systems center on the location of the receptors. Stated another way, you will reason from location of abnormal findings, largely associated with receptors and effectors (because these are where the systems are being tested during the clinical examination), back to the location of the lesion(s) responsible. If these last statements aren't clear to you, ponder them a while. Be sure to ask questions if the fog still hasn't cleared! As for function of the UMNs and LMNs, remember three things. First, both sets of neurons influence voluntary strength, muscle tone, and reflexes (among other things), as tested clinically by measuring ability to resist external force, by measuring resistance to movement when relaxed, and by measuring responses to stimulating nearby skin or stretching the muscles themselves with a reflex hammer, respectively. Second, whereas LMN findings are all basically "less than normal," some UMN findings are "greater than normal" and apparently represent disinhibition. Third, in both LMN and UMN lesions, the time-course of injuries must be taken into account to avoid being mislead by the findings.
Now, before integrating your knowledge of all three long-tract systems, pose and solve some problems related only to corticospinal and LMN fibers. A couple of starters follow:
Notice further patterns regarding the long tracts. For example, (1) if you have abnormalities in all three systems and they are all on the same side of the body (below the head), though not with identical distributions in the two sensory systems, where do you think the lesion would most likely be? (Since all three systems are on the same side of the CNS from the spinomedullary junction on up, but the fibers of the two sensory systems don't integrate somatotopically till near the thalamus, the lesion would most likely be in the brainstem, on the side contralateral to the findings.) (2) If you have abnormalities in the motor system and only one sensory system, is the lesion peripheral or central? Central, of course. Why? (3) If the abnormalities (motor and in one sensory system) are on one and the same side of the body (below the head) and involve the dorsal columns system, where might the lesion be? What difference would it make if the sensory system involved were the lateral spinothalamic? Why? (4) On the other hand, if the motor abnormalities were on one side of the body and the lateral spinothalamic abnormalities were on the other side, where would you suspect the lesion to be? Why?
If you can answer the above questions handily, you should be ready to tackle specific patient problems, such as the following. I would point out that even with no further knowledge of neural structures (and their functions), integrated application of the above long tract systems will allow you to diagnose both the side of lesions and their general level--i.e., peripheral (including which spinal nerves), spinal cord (including specific level), brain stem, thalamus, or cortical (and subcortical)--in a surprisingly large proportion of patients with neurologic lesions. Even if you don't know what additional findings mean or imply anatomically, you can ignore them for now and reason based on the findings with which you are familiar.
There was no Babinski response on either side. Where is this patient's lesion probably located? Why?
Where might his lesion be located? Why?
We will next cover the cranial nerve systems. Don't forget to apply what you've learned from the anatomy of the long-tract systems, for many of the cranial nerve systems carry the same type of information, only localized to the head. You'll find that the anatomical patterns, too, in many ways resemble those of the analogous long-tract systems.
Even though the cranial nerves as a group do not form a functional unit, there are a few general patterns regarding their spatial organization that are helpful to know. Two will be given now. The first is perhaps so obvious that it doesn't need to be mentioned: each cranial nerve is ipsilateral to the structures it innervates. This means that if you find any abnormality, sensory or motor, on one side of the head or neck, and it's due to peripheral damage to a cranial nerve, the nerve involved is on the same side as the abnormality. This was, of course, also true of the spinal nerves discussed earlier. The second pattern reflects their rostral-caudal order of joining the CNS (Figure 13): I and II are most rostra!, joining the CNS above the brainstem; III and IV join the midbrain; V joins the mid-pons; VI, VII, and VIII join at the pontomedullary junction; and IX, X, XI, and XII join the medulla. Since they are thus approximately arranged in order. it is rather simple to recall their general location from memorizing where a few are, e.g., III-IV and VI-VII-VIII. A mnemonic (remember?) that just came to mind uses the number of syllables in a few descriptive words:
|Words:||above||midbrain||pons||in between||and medulla|
|No. of nerves:||2||2||1||3||4|
(Actually, cranial nerve XI is a bit of an oddball, for in addition to the part coming off the medulla, there's a component that comes off the C1-C5 levels of the ipsilateral spinal cord and enters the skull (through the foremen magnum) to join the fibers from the medulla. We'll discuss its anatomy a bit more later, as we consider its function.)
Since the sensory information in a given nerve (with one important exception, to be described soon) is processed at that same level or higher (i.e., more rostrally) and the motor information is also influenced primarily by structures that lie at the same level or more rostrally, abnormalities in function of that nerve can generally be assumed to reflect damage either peripherally or else centrally at or above its level of union with the CNS. Does this sound new? It shouldn't. Think about the levels of possible damage when sensory or motor abnormalities below the head were noted earlier. If the findings were referable to the distribution of certain spinal nerves, how often would the lesion be in the CNS below the corresponding segments of the spinal cord?
Let's start by considering the motor functions of the cranial nerves. It will serve as a review of motor function below the head and will also allow us to complete the "corticobulbar" portion of the corticospinal-corticobulbar (UMN) system described earlier. As you will recall, we said that corticobulbar neurons were the UMNs associated with cranial nerves, just as corticospinal neurons were the UMNs associated with spinal nerves. Also, the cell bodies of the LMNs traveling in a cranial nerve lie ipsilaterally at the level of the nerve's exit from the CNS. [One exception is the cell bodies of the LMNs in cranial nerve IV, which lie contralateral to their nerve. Fortunately, cranial nerve IV is relatively unimportant clinically . Another exception involves the cell bodies of the LMNs supplying the levator palpebrae superioris, some of which may be contralateral to their nerve (III), and those of the LMNs supplying the superior rectus, all of which may be contralateral to their nerve (III). Although these muscles are clinically important, it will turn out that other anatomical considerations make it unnecessary to remember this exception. In conclusion, then, the exceptions are rather unimportant clinically and can be disregarded. Sound reminiscent of anterior horn cells? Functionally, too, LMN lesions associated with cranial nerve function yield the characteristic findings of flaccid paralysis (Table 1) in the muscles involved. It's the UMNs--the corticobulbar fibers--that throw us a few new curves! But first, let's list the muscles we're talking about and describe briefly how to test their function. (See NEURO I-5 to learn more about clinical testing.)
As you can see from Table 2, the muscles used to test cranial nerve systems tend to group functionally according to the nerve of supply. Thus, nerves III, IV, and VI are associated with turning the eyeball, V--with moving the jaw, VII-- with making facial expressions, IX and X (and cranial portion of XI)--with swallowing and phonation (i.e., making vocal vibrations: X only), and XII--with moving the tongue. The main difference between the cortico-bulbars and corticospinals is in their level and extent of crossing the midline. Whereas most muscles supplied by spinal nerves are influenced primarily by UMNs (corticospinal fibers) originating in the contralateral cerebral cortex and decussating at the medullospinal junction; most muscles supplied by cranial nerves are influenced by UMNs (cortico-bulbars) originating in either cerebral hemisphere (roughly half of the fibers coming from each side), the fibers from the contralateral hemisphere crossing the midline at approximately the level of the LMNs they will influence. Thus, for example, corticobulbar fibers influencing the muscles of mastication come from both hemispheres, and the contralateral fibers decussate in the pons, about half-way between the midbrain and medulla. (Just so there's no confusion, remember that when we talk of corticobulbar or corticospinal fibers influencing muscles, we don't mean directly! Only LMNs innervate a muscle, so the UMNs exert their influence on the LMNs.) Thus, when most or all of the corticobulbar fibers from one cerebral hemisphere are wiped out (as happens frequently because they run in a compact bundle through the internal capsule and upper midbrain), most muscles innervated by cranial nerves may show little grossly measurable weakness, and what weakness they do show may clear within hours to a few days, presumably because of the extensive influence by corticobulbars from the other cerebral hemisphere. (For review and integration, how and why does this differ from the situation where all corticospinal fibers from one hemisphere are interrupted by a lesion above the spinal cord?)
|Table 2.||Major Striated Muscle Groups Innervated by Cranial Nerves and Some of Their Functions Often Tested Clinically.|
|MUSCLES (OR GROUP)||FUNCTION(S) TESTED||CRANIAL NERVE|
|Levator palpebrae superioris||Lift upper eyelid||III|
|Extraocular muscles:||Turn eyeball:|
|Muscles of mastication:||Move jaw:|
|Lateral pterygoid||Downward (open)|
|Medial & lateral pterygoids||Laterally|
(to opposite side)
|Muscles of facial expression: e.g.,|
|Orbicularis oculi||Shut eyelid||VII|
|Levator labii superioris||Show upper teeth|
|Orbicularis oris||Purse lips|
|Muscles of pharynx: e.g.,|
|Stylo-pharyngeus||Aid swallowing||IX, X, XI|
|Muscles of larynx: e.g.,|
|Vocalis||Aid phonation||X, XI|
|Sternocleidomastoid||Turn head to opposite side||XI|
|Trapezius (upper part)||Lift shoulder|
|Intrinsic tongue muscles: e.g.,||Move tongue:|
There are two important exceptions, where one muscle or set of muscles supplied by a cranial nerve is influenced almost entirely by corticobulbar fibers from the opposite cortex, i.e., in the pattern characteristic of muscles innervated by spinal nerves. One exception is the genioglossus, which protrudes the tongue forward and slightly to the opposite side. The other is the lower muscles of facial expression, such as the risorius and zygomaticus, which help bring the mouth corners up into a full smile, and the orbicularis oris, used to close or purse the lips.
The pattern of weakness in just the lower muscles of facial expression on one side is so important clinically that it is given a special name--central (or supranuclear) facial palsy. It is contrasted with weakness of all muscles of facial expression on one side, which is called peripheral (or infranuclear) facial palsy. The term "peripheral" facial palsy may be somewhat misleading for you, as any lesion of the facial LMNs, whether peripheral or in the brain stem, will tend to produce a "peripheral" palsy. If you remember that the central palsy is essentially an UMN problem and the peripheral palsy is a LMN problem, you should be okay.
Now let's consider a few combinations of clinical findings to illustrate the importance of the usual corticobulbar distribution and the two exceptions. In the process, we will discover two more useful rules for localizing lesions!
Assuming that there is only one lesion and that it's all on one side of the nervous system, where might it be located? Can we still entertain the possibility of a peripheral lesion, in left XII? No. How about in the left medulla? Why not? Because the corticospinal fibers in the left medulla would decussate at the medullospinal junction, influencing muscles on the right side of the body. Is the right medulla a possibility? Yes, it is. Remember, above the level where the UMNs associated with the genioglossus cross, those corticobulbar fibers are on the same side as the corticospinal fibers that will decussate to influence muscles on the left below the head. Remember, also, that the corticospinal-corticobulbar fibers run together in somatotopic order. So a rather small lesion could wipe out all of the UMN fibers in the right medulla and produce the findings shown by our patient. How about higher levels? Yes, the lesion could be in the right pons, midbrain, or cerebral hemisphere, for the UMN fibers influencing the left genioglossus and left body below the neck remain on the right all the way to their origins in the cortex.
Notice what a striking difference it makes, whether the hemiplegia is on the same side as the weak genioglossus or on the opposite side. It turns out that this reflects one very useful pattern. Whenever a motor problem referable to a cranial nerve (you will find out later that the same rule holds for sensory problems) is combined with hemiparesis on the opposite side (so- called crossed paresis), the lesion is at the level of the involved cranial nerve and on the same side as the cranial problem. This is because the LMNs traveling in the nerve are found only ipsilaterally and at that level, while the corticospinal fibers on that same side have not yet decussated. Since all LMNs traveling in cranial nerves arise only in the brainstem (with the exception of the spinal component of XI), crossed paresis is a hallmark of a brainstem lesion. To make sure you understand this important point, work out the location of the lesion in a patient with:
The converse of the above principle is also important. That is, whenever a motor problem referable to a cranial nerve is combined with hemiparesis on the same side, the lesion is above the level of the involved cranial nerve and on the side opposite the motor findings. Why? Because to be on the same side as the hemiplegia, which is caused by damage to corticospinal fibers on the opposite side, the cranial nerve problem must also be due to UMN damage, occurring before the UMNs associated with the cranial nerve have crossed the midline. This is where it is important to remember the bilateral UMN supply to most cranial LMNs. For example, if there were a lesion in the left upper medulla that wiped out the left cortico-spinal-corticobulbar fibers, would you see motor problems referable to cranial nerves IX and X (e.g., absent gag reflex), XI (e.g., inability to turn the head forcibly to the side), and XII (e.g., inability to protrude the tongue in the midline), along with the right hemiparesis? For nerves IX, X, and XI, the answer is "probably not." LMNs coursing in these nerves on the right receive considerable input from the right corticobulbar fibers (uncrossed), and LMNs coursing in these nerves on the left also receive considerable input from the right corticobulbar fibers (crossed). Only the LMNs supplying the genioglossus on the right (in nerve XII) have very little UMN input, because that input is almost entirely crossed--i.e., from the left corticobulbars. Similarly, if there were a lesion in the left upper pons that wiped out the left corticospinal-corticobulbar fibers, would you see motor problems referable to cranial nerves V (e.g., can't bite down forcibly), VI (e.g., can't gaze laterally), and VII (e.g., can't shut eyelids forcibly, give a big smile, or show his upper teeth well), as well as to cranial nerves IX-XII? Again, the answer for nerves V, VI, as well as IX-XI, is "probably not," for the same reason given above. In addition to the right hemiplegia and deviation of the protruded tongue to the right, only the muscles of facial expression in the lower right face would be weak, again because UMN input to their LMNs (in nerve VII) is almost entirely crossed. [It is probably not too soon to alert you that the neural regulation of the extraocular muscles is somewhat more complex, once you get above the LMN level. A set of neurons in the brainstem link together the actions of the various extraocular muscles, so that they work as a group and the movements of the two eyes are coordinated. As a result, any UMN lesion that does produce a deviation of normal gaze (and there are some, as you'll find out later), tends to produce a "conjugate" deviation. In other words, the eyes still move together, so as to maintain binocular fixation on objects. For example, if you find that a patient can't turn his left eyeball laterally, although his right eyeball turns medially (i.e., normally) as he attempts to look to his left, his gaze is disconjugate, suggesting an ipsilateral LMN problem--in this case, of the LMNs coursing in the left nerve VI. If, on the other hand, when the patient tries to look to the left, neither eyeball turns (i.e., medially for the right, laterally for the left), he has a conjugate gaze paralysis--connoting UMN damage. We'll discuss movements of the eyes more later. For now, simply realize that conjugate gaze paralyses reflect possible damage at various levels, from the pons on up to the cerebrum, and concentrate instead on using crossed paralyses and unilateral combinations of hemiplegia, genioglossal weakness, and central facial palsy (i.e., of lower face only) to reason out the locations of lesions.]
If we go even higher, wiping out all of the corticospinal-corticobulbar fibers in the left midbrain or left cerebral hemisphere, would we find any additional motor problems referable to cranial nerves? No, because the LMNs in cranial nerves III and IV on either side also receive considerable bilateral UMN input (though not directly: to be discussed later). Consequently, if a patient has hemiplegia, genioglossal weakness, and paralysis of the lower facial muscles-- all on the same side--, we know the lesion is at least as high as the pons and is on the side opposite the findings, but we can't locate it more precisely-- e.g., in the pons, midbrain, internal capsule, corona radiate, or cerebral cortex--without additional information.
This pattern is essentially an extension of the one discussed above in 3. Let's reason it out for our patient with right hemiparesis and tongue deviating to the left upon protrusion. If the paresis involved only the right arm, the tongue problem would still indicate a lesion above the spinal cord. At that level, the arm weakness would have to reflect an UMN problem, so points to the left side of the CNS--partial destruction of the somatotopically arranged fibers in the left corticospinal tract. Since the tongue problem should also reflect damage on the left, it is probably an LMN problem--thus, located at the level of the involved cranial nerve (XII) in the medulla. If we now take away the patient's hemiparesis and give him right-sided sensory abnormalities below the head, we find that our reasoning remains basically the same. Since the tongue problem still indicates a lesion above the spinal cord and both the lateral spinothalamic and dorsal columns systems have decussated by this time, thus lying on the same side as the left corticospinal tract we discussed above, the sensory problems also reflect damage to the left side of the CNS, consistent with a LMN weakness of the left genioglossus.
To generalize and state the pattern another way, since the ipsilateral dorsal columns system, lateral spinothalamic system, and corticospinal fibers above the spinal cord all influence the same (contralateral) side of the body below the head, findings involving them on one side of the body below the head become almost equivalent to each other when reasoning out locations of CNS lesions known to be above the spinal cord. Again, the converse is also true. Remember that hemiparesis and cranial motor findings on the same side suggest a contralateral lesion above the level of the involved cranial nerve. If we now substitute an ipsilateral monoplegia or ipsilateral sensory abnormalities below the head for the hemiparesis, the cranial motor findings still suggest a contralateral lesion above the level of the involved cranial nerve. Be sure that all of the preceding makes sense to you before going on. Explaining it to a colleague is a good way to test your ability to use the information.
First, let's consider the similarities between the trigeminal sensory system and the two sensory long-tract systems, because they will serve as a partial review and point up information that does not have to be learned separately for the trigeminal sensory system. The cell bodies of the first-order neurons are unipolar and reside in a sensory ganglion (called the trigeminal, semi-lunar, or Gasserian ganglion) located near the CNS. (The exception to this, namely, cell bodies of first-order proprioceptive fibers from muscles innervated by V, were mentioned earlier. They are rather unimportant clinically, but do reside inside the CNS, in case you want to know.) The first-order fibers pass through the sensory ganglion and enter the CNS (mid-pons level, remember?) to synapse on second-order neurons ipsilaterally. The axons of the second-order neurons soon decussate as they ascend toward their relay nucleus in the thalamus--the ventral posteromedial nucleus (VPM). The VPM is just medial to the VPL--the thalamic relay nucleus in the lateral spinothalamic and dorsal columns systems. In fact, the paired VPM and VPL nuclei on either side are functionally a single somatotopically-organized nucleus. In the VPM lie the cell bodies of third-order neurons, whose axons ascend in the somatosensory radiations (first through internal capsule, then corona radiate) to the ipsilateral post-central gyrus (but contralateral to the first-order neurons!) of the parietal lobe of the cerebral cortex, maintaining somatotopic relations with somatosensory fibers from other parts of the contralateral body all the way. There's even one further anatomic similarity to the sensory long-tract systems: trigeminal first-order axons entering the CNS split up to begin two anatomically distinct subsystems, roughly analogous to the lateral spinothalamic and dorsal columns systems, respectively, which go their separate ways and do not reintegrate somatotopically until they near their thalamic relay nucleus.
Before we trace the two distinct subsystems from entry into CNS up to contralateral VPM nucleus, let's give their names and discuss their functions briefly, for here we will find one important difference from the analogous sensory long-tract systems. One subsystem is called the principal (or chief) sensory subsystem, it carries the sensory modalities handled by the dorsal columns system--namely, vibratory sense, position sense, and discriminative touch. The other is called the descending (or spinal) subsystem; it carries the sensory modalities handled by the lateral spinothalamic system--namely, pain and temperature--and anterior spinothalamic system--namely, crude touch. The important functional difference from the analogous long-tract systems is as follows. Whereas a lesion of the dorsal columns system by itself usually does not produce a deficit in simple touch, as might be tested with a wisp of cotton, a lesion of the principal trigeminal subsystem does produce such a deficit. Apparently, the information on simple touch carried in the descending trigeminal subsystem can not substitute completely for similar information carried in the principal subsystem. For this reason, presumably, the routine clinical test for the principal trigeminal subsystem simply involves touching various areas of the face systematically with a light stimulus (e.g., a wisp of cotton) and comparing the intensities of sensation reported from the different areas. No tests for vibratory sense, position sense, or discriminative touch are routinely done.
The essential anatomical difference between the two subsystems can be expressed in terms of the levels at which anatomical cell bodies of the second-order neurons lie. Those for the principal subsystem (in the principal sensory nucleus) lie at the level where the trigeminal nerve joins the CNS--i.e., mid- pons, whereas those for the descending subsystem (in the descending nucleus of the trigeminal) lie strung out from the lower pons all the way down to the first two cervical segments of the spinal cord. As a consequence, the first-order axons in the descending subsystem do just that, descend to various levels in lower pons, medulla, or cervical cord, to reach the second-order neurons. As mentioned earlier, the axons of the second-order neurons in both subsystems tend to decussate just shortly above (rostra! to) the cell bodies, as is also true in the long-tract systems. In other words, the principal subsystem decussates in the upper (rostra!) pons and the descending subsystem decussates at various levels from lower medulla to midpons.
Now let's consider some hypothetical findings in order to reason through the clinical implications of these discrete functional-anatomical subsystems.
Remember that the autonomic nervous system in the narrow sense is a motor system--innervating smooth muscle and glands. The two main divisions are the sympathetics and parasympathetics, which often have opposing actions within the same organ. Both sympathetics and parasympathetics are two-neuron subsystems-- consisting of preganglionic neurons originating in the CNS and going to a peripheral ganglion, in which they synapse with the dendrites/cell bodies of post-ganglionic neurons with axons going to synapse on muscle or gland (i.e., effector cells). Within the CNS, sympathetic preganglionics originate at the thoracic and lumbar levels of the spinal cord, hence the alternative name-- thoracolumbar division; and preganglionic parasympathetics originate in the brain stem and sacral portion of the spinal cord, hence the alternative name-- craniosacral division.
The parasympathetic functions in the head are specialized. Fibers traveling in cranial nerve III innervate smooth muscle responsible for (1) changing the shape of the lens in the eye to allow focusing as we shift our gaze to objects at different distances and (2) constricting the pupil as a means of regulating the amount of light entering the eye. Fibers traveling in cranial nerve VII innervate the lacrimal gland and some salivary glands (submaxillary and sublingual); while fibers traveling in cranial nerve IX innervate the parotid salivary gland. Although cranial nerve X has a particularly large number of preganglionic parasympathetic fibers, their destination is, as noted earlier, below the head--primarily to vital organs of the thorax and abdomen. For various reasons, including difficulties in quantitative testing and bilateral innervation by the involved cranial nerves, problems with salivation (e.g., dry mouth) and functions of the vital organs--heart rate (e.g., tachycardia = rapid pulse: tachys = quick, kardia = heart), intestinal peristalsis (e.g., constipation), etc.--are hard to correlate with discrete neurologic lesions. Instead, such findings tend to reflect general (i.e., systemic) effects on the autonomies, such as produced by drugs that have anti-parasympathetic properties. Lacrimation can be tested by looking at the conjunctive of the eye. If it looks moist and shiny, lacrimation is probably normal. A dry, reddened conjunctive, however, may reflect decreased lacrimation. Though changes in shape of the lens cannot be observed directly, you can assess this function quickly by asking a patient if he's experienced any blurring of his vision. Constriction of the pupils can be observed directly, either by shining light in the eye (the pupillary light reflex) or having the patient change her focus suddenly from a far to a near object (the pupillary accommodation response). (See NEURO HxPx 8 to learn more about testing autonomic function.)
The parasympathetic preganglionic cell bodies lie at the level of the brainstem joined by and ipsilateral to the involved cranial nerve; thus, those with axons in nerve III lie in the upper midbrain, those with axons in nerve VII lie near the pontomedullary junction, and those with axons in nerves IX and X lie in the upper (rostra!) medulla. This should not be surprising, as we noted the same rule earlier for the preganglionic cell bodies in the spinal cord. As noted above, problems with salivation and functions of the vital organs are difficult to correlate with discrete neurologic lesions, so will not be considered further. Decreased lacrimation may reflect either a peripheral problem, along the course of nerve VII, or a CNS problem. The only area of the CNS in which a lesion produces decreased lacrimation is near the medullopontine junction. Thus, for purposes of localization of lesions, lacrimation acts like a LMN function with bilateral UMN innervation. Right? If you're not sure, review as needed.
If you find a pupil that doesn't constrict properly in response to light and accommodation, where might the lesion be? Well, one set of locations can be reasoned out pretty much as we just did for decreased lacrimation. It may reflect a peripheral problem, along the course of ipsilateral nerve III, or it may be an ipsilateral CNS problem, at the level joined by the involved nerve-- i.e., upper midbrain. What makes the situation here more confusing is that these are reflex responses. In any reflex, the entire arc, including receptors, afferent fibers, internuncial neurons (if any), efferent fibers, and effectors, must be intact or else the reflex won't occur. For the accommodation and light reflexes, the stimuli involve visual information, which comes in through cranial nerve II (to be discussed under special senses). Consequently, a lesion in the retina or one of the optic nerves can also cause a decrease in pupillary reflex responses. (Though it's not a neural problem, what effect do you think a large cataract in one eye might have on pupillary reflexes?)
Because visual information in either eye reaches the preganglionic neurons traveling in both nerve IIIs, shining a light in one pupil should elicit constriction of both pupils. The reflex in the eye ipsilateral to the stimulus is called the direct response, while that in the contralateral eye is called the indirect (or consensual) response. With this information, can you figure out a way to decide whether a decreased pupillary light reflex is due to a motor lesion (i.e., ipsilateral nerve III or upper midbrain) or a sensory lesion (i.e., either retina or nerve II)? Well, if you get a direct and consensual response upon shining light in the left eye, but neither response upon shining light in the right eye, where is the neural lesion? Obviously, it must be in the right retina or right nerve II. Next, assume that you get only a direct response upon shining light in the left eye and only a consensual response upon shining light in the right eye. Now where is the neural lesion? Obviously in the right nerve III or the right upper midbrain. Since the visual pathways influencing pupillary constriction pass directly into the upper midbrain, without entering the thalamus or other cerebral structures, deficits in the pupillary reflexes still point primarily to a cranial nerve- brainstem problem, as did the other parasympathetic deficits mentioned earlier.
Although sympathetic innervation of cranial structures does not arise directly in the brain stem and pass out of the CNS in cranial nerves, sympathetic problems in the head turn out to involve both brain stem and cranial nerves, so their anatomy and function will be described here. The cell bodies of the sympathetic preganglionics destined to influence the head arise in the ipsilateral spinal cord at the upper thoracic level (about T1-T3). Remember the intermediolateral cell column? Of course not. The preganglionic axons exit through the ventral roots of ipsilateral spinal nerves T1, T2, and T3, leaving the roots (via the white rami) to enter the chain of paravertebral sympathetic ganglia lying just to the side of the vertebral column. In the sympathetic chain, they ascend all the way to the top, where they synapse with dendrites/cell bodies of postganglionics lying in the superior cervical ganglion. The postganglionic fibers form a network around the nearby common carotid artery and accompany it and its branches as they ascend up into the head region. (Much of the foregoing is really review, for the main aspects of sympathetic anatomy and distribution were covered earlier; only the specific spinal levels, ganglion, and artery are unique to the cranial sympathetics.)
At the bifurcation of the common carotid into external and internal carotid arteries, the population of sympathetic postganglionic fibers also splits--along functional lines. Accompanying the external carotid on its journey to supply structures outside the cranium are fibers subserving the general sympathetic functions of sweating and vasoconstriction. As a result, damage to these fibers on one side of the body is reflected in ipsilateral lack of sweating (anhidrosis, an = lack of, hidrosis = sweating) and inability to constrict arterioles normally (e.g., in cold weather) over the face and anterior scalp. On the other hand, those postganglionics accompanying the internal carotid artery on up into the cranial vault subserve special functions associated with the eye. They follow the ophthalmic artery as it branches from the internal carotid, and then join fibers of the ophthalmic division (I) of nerve V (trigeminal). Some of these sympathetics innervate the dilator pupillae, so that damage to them results in unequal pupil size, the ipsilateral pupil being smaller (miotic). Others innervate Mčller's muscle, a short smooth muscle in series with the levator palpebrae superioris, so that damage to them results in a slight drooping (ptosis) of the upper eyelid. In summary, then, damage to these cranial sympathetics, anywhere from the upper thoracic cord on up to the bifurcation of the common carotid can produce ipsilateral miosis, ptosis, facial anhidrosis, and inability to vasoconstrict over the face. This important constellation of findings is given a special name--Horner's syndrome. (Often, enophthalmos-- retraction of the eyeball--is also listed as a finding.)
Obviously, if sympathetic fibers are damaged along only the external carotid or internal carotid artery, the ipsilateral Horner's syndrome will be incomplete. In this regard, if you examined a patient with a left ptosis (i.e., the left upper eyelid drooped), how might you decide if the problem was referable to nerve III pathways (levator palpebrae superioris, nerve III, upper brainstem) or to sympathetic pathways (Mčller's muscle, nerve V, ophthalmic artery, etc.)? One way would be to examine the pupils. If the left pupil were smaller than the right pupil, the lesion is probably in the sympathetic subsystem. If the pupils were equal in size, the lesion is probably in the nerve III pathways. What if the left pupil were larger than the right? How would you explain that? Well, the upper brainstem and nerve III also contain parasympathetic neurons associated with the constrictor pupillae, right? So a single lesion might very well knock out both those neurons and LMNs to the ipsilateral levator palpebrae superioris.
One very important consideration remains. It turns out that there are so-called autonomic regulatory fibers that course down through the brainstem (from the hypothalamus, which is a part of the thalamus) on either side to influence the cranial sympathetic preganglionics in the ipsilateral upper thoracic spinal cord. Damage to these fibers on one side also produces an ipsilateral Horner's syndrome. The main diagnostic significance of this fact is to not be misled! If other findings suggest a lesion in the brain stem or cervical spinal cord, presence of Horner's syndrome does not mean that you have to rethink the problem or decide that two discrete lesions exist.
This would be a good time to pose and solve some problems combining findings from the long-tract systems, cranial motor and somatosensory pathways, and cranial autonomic pathways. I'll give you two to start with.
Where might her lesion be located? Why?
Where might his lesion be located? Why?
Having finished the cranial autonomic functions, we will now discuss the "special senses"--all of which are associated with cranial nerves.
|Smell||--||cranial nerve I|
|Vision||--||cranial nerve II|
|Taste||--||cranial nerves VII|
| (anterior two-thirds of
(posterior one-third of tongue)
|Hearing||--||cranial nerve VIII||(auditory portion)|
|Equilibrium||--||cranial nerve VIII||(vestibular portion)|
Smell is tested by offering a non-stringent aromatic substance to each nostril, while the opposite one is blocked, and asking the patient to identify the substance. Two aspects of vision are routinely tested--acuity (i.e., resolving power) and visual fields (i.e., spatial distribution of sight). Acuity is tested by having the patient read some small print (e.g., from a newspaper) or lines of letters on a Snellen chart (see NEURO HxPx 3 to learn more about testing vision). (Be sure patients are wearing any corrective lenses, as you don't wish to measure refractive errors.) Visual fields are tested by having the patient look straight ahead, while you bring objects (e.g., your fingers) toward the visual field of each eye separately from various directions. From each direction, you continue to bring the object closer and closer to his eye, until he can see it. Taste is tested by placing the stimuli--e.g., sugar solution for "sweet," salt solution for "salty," vinegar for "sour," and quinine solution for "bitter"--on either side of the tongue, one at a time, then asking the patient to identify each stimulus. (Be sure tongue remains out of mouth, so patient can't utilize smell instead.) Hearing is tested by presenting a soft auditory stimulus (e.g., a watch, two fingers rubbing together) to either ear, starting at a distance where it can't be heard and bringing it closer until it is heard. Equilibrium, the proprioceptive system utilized to regulate muscle tone and motor coordination unconsciously, as in habitual postures and movements, relies heavily on vestibular input, for the vestibular end-organs sense both the position of the head and the speed and direction of head movements. Equilibrium is routinely tested by having the patient walk; he should be able to walk straight ahead and turn around without placing his feet far apart or veering to the side. (See NEURO HxPx 7 to learn more about testing vestibular functions.) Another routine test for equilibrium is carried out while testing the extraocular muscles. As the eyes follow your finger in the various directions, you look for rather rapidly repeated back and forth movements--either horizontal, vertical, or rotatory. These involuntary rhythmic movements are called nystagmus. (See NEURO HxPx 7 to learn more about nystagmus.) With the exception of a very fine nystagmus on extreme lateral gaze in either direction, which may be normal (so-called "physiologic nystagmus"), any nystagmus elicited by finger-following is abnormal. Lesions of the equilibrium system are also often accompanied by nausea, vomiting, and sensations that he (the patient) or his environment is spinning (vertigo). [Do you understand how these disorders might occur if information about position or movement of the head is inaccurate? Imagine how you might walk if you felt your head suddenly moving to one side. You might quickly put your ipsilateral foot out to the side to catch yourself. If this happened repeatedly, you would thus display a wide- based (i.e., feet far apart), veering gait. Now imagine you are looking at something and your eyes get the message that your head is moving. What would your eyes do reflexly to maintain your focus? Obviously, they would have to move in the opposite direction. But if your head isn't really moving, the reflex movements of your eyes would actually pull your gaze away from the point of focus, so that you would have to move your eyes back again. If the erroneous reflex movements alternate with the corrective movements, a rhythmic back-and- forth motion of the eyes--i.e., nystagmus--is produced. Imagine next you are getting the message that your head is turning continually in one direction. Could this not result in a feeling of spinning--i.e., vertigo? And could not a feeling of continual spinning lead to nausea or, if severe enough, to vomiting? You may come up with better explanations, but I hope you now see at least one way to reason from vestibular functions to the findings indicative of vestibular abnormalities.]
Now let's examine the localizing value of findings in these systems. We will start with taste and equilibrium, since discussion of hearing, vision, and smell will form a natural transition into the functional anatomy of cerebral structures. Taste can be treated very briefly. First-order neurons, with cell bodies in sensory ganglia near the CNS, enter the CNS from the pontomedullary junction to mid-medulla, and synapse ipsilaterally on dendrites/ cell bodies of second-order neurons at the same levels. Apparently because the axons of the second-order neurons ascend bilaterally to the thalamus (representation in cerebral cortex is unclear and not very useful clinically), the only CNS lesions that tend to produce taste deficits are at the level of the associated nerves themselves. In this regard, taste can be treated in neuroanatomical diagnosis much like a muscle served by cranial LMNs receiving bilateral UMN input. Can you convince yourself that this is true? If not, review as needed.
The vestibular first-order neurons travel from receptors in the inner ear, over their own bipolar cell bodies in a sensory ganglion near the CNS, into the CNS and, from the lower pons to mid-medulla, synapse ipsilaterally on the dendrites/cell bodies of second-order neurons. Some first-order axons, however, continue back into the cerebellum, as do some second-order axons, presumably to coordinate head position and movement with postures and movements of the rest of the body, since the cerebellum is a general motor coordination center. Other second-order axons ascend to connect with the LMNs innervating the extraocular muscles, so as to coordinate movements of the head and eyes. Still other second-order axons descend into the spinal cord to influence LMNs at all levels, presumably yet another pathway for coordinating positions and movements of the head with those in the rest of the body. As we noted above, symptoms often produced by equilibrium disorders are vertigo and nausea, while the accompanying signs often include nystagmus, a tendency to have feet far apart and to veer laterally while walking (and standing still, if severe enough), and vomiting. Unfortunately, nausea and vomiting are so general, being found in a large variety of non-neural and neural disorders, as to be of little help in neuroanatomical diagnosis. If they are due to a neural problem, however, the location is probably in a cranial nerve VIII or within the cranium. Within the cranium, any diffuse damage, whether due to meningitis, metabolic disorder, massive head injury, etc., may produce nausea and vomiting, especially in the presence of increased pressure (intracranial pressure--ICP). If the findings are due to a discrete neural lesion, however, it is probably in a cranial nerve VIII, the brain stem, or cerebellum--i.e., between the cerebrum above and the spinal cord below.
Although we will put off detailed discussion of the cerebellum until later, the tendency for vestibular-like problems to occur in some cerebellar lesions should be noted here. Spontaneous nystagmus, vertigo, and a veering, wide-based gait, though not as general as nausea and vomiting, can also accompany lesions in the cerebellum--in addition to lesions in nerve VIII or the brainstem. The similarity of vestibular findings to some cerebellar findings should not be surprising, since we just noted that the vestibular system sends first-order axons to the cerebellum. The presence of vestibular findings by themselves are also not too useful for localizing lesions within the brainstem, for they can occur from damage to first-order neurons in the lower pons to mid- medulla, as well as from damage to the second-order axons that ascend as far as the midbrain to influence LMNs innervating the extraocular muscles. The descending fibers, however, which enter the spinal cord, do not produce these particular findings and, by themselves, are not very helpful in neuroanatomical diagnosis. Where the vestibular system is represented in the thalamus and above is also not clear, and vestibular findings are not very useful in locating cerebral problems.
In summary, then, the vestibular system is found within all major parts of the CNS below the cerebrum, the apparent sparcity of its input to the cerebrum possibly reflecting the unconscious nature of its major functions. Although the vestibular findings of nausea and vomiting are too general to be of much use in neuroanatomical diagnosis by themselves, nystagmus, vertigo, and a broad-based veering gait are at least associated primarily with lesions of nerve VIII, the brainstem, or cerebellum. [You should be aware that details of vestibular findings, particularly of nystagmus, may at times be extremely helpful in neuroanatomical diagnosis. We will discuss one striking example later, but detailed treatment of the subject is outside the scope of this work. To learn more about the types of nystagmus and their clinical relevance, see pp. 184-186 in Adams and Victor, Principles of Neurology (2nd ed.). Other helpful material on vestibular abnormalities can be found on pp. 81-82 (abnormal gait) and 200-207 (vertigo) of Adams and Victor, also on pp. 5-7 (gait) and 114-117 (nystagmus, vertigo, and others) of Barrows' Guide to Neurological Assessment.]
First-order neurons for hearing carry information from receptors in the inner ear, past their own bipolar cell bodies in a peripheral ganglion (also in the inner ear), and into the CNS, synapsing on dendrites/cell bodies of second-order neurons lying at the level of the pontomedullary junction. From there, the second- and then higher-order neurons (third-, fourth-?) ascend bilaterally in various complex pathways to reach the auditory relay nucleus in the thalamus-- the medial geniculate body (MGB). Cell bodies in the MGB give rise to axons which form the auditory radiations, passing through internal capsule and corona radiate, to reach the primary auditory cortex--the transverse gyri of Heschl in the temporal lobe of the cerebral cortex. Note the similarities to the somatosensory systems we've already covered: in particular, a relay nucleus in the thalamus and thalamic radiations passing through the internal capsule and corona radiate to a specific area of cerebral cortex. One difference is that cerebral distribution is bilateral for auditory receptors, contralateral for somatosensory receptors. This turns out to be an important difference. The auditory system crosses and recrosses from the time it begins its ascent from the pontomedullary junction, so the only places at which neural lesions produce marked hearing deficit in one ear are in the ipsilateral nerve VIII, ipsilateral pontomedullary junction, and ipsilateral lower pons. Hence, as in the case of taste, hearing can be treated in neuroanatomic diagnosis much as a cranial muscle whose LMNs receive UMN input from both sides. (You should be aware that the above statements pertain primarily for the rather crude auditory testing done routinely in the office or at the bedside. By using sophisticated audiometric equipment or studying brainstem auditory evoked responses, one can often localize auditory damage above the mid-pons. Such procedures, however, are outside the scope of this work.) Before we go on to the senses of smell and vision, notice again that our preceding discussion of cranial autonomic functions and of taste, equilibrium, and hearing has primarily added breadth to the number of findings we can look for and evaluate. It has introduced no new patterns for use in neuroanatomic diagnosis, nor has it presented us with findings of particular value in localizing cerebral lesions.
Since we will be using terminology from cerebral anatomy in our discussion of smell and vision, and the succeeding topic will be cerebral functions anyway, let's describe the cerebral lobes and major fissures now. As you can see from Figure 16, there are a total of five lobes in each hemisphere--frontal, parietal, temporal, occipital, and insular. The names and relative locations of the first four lobes should be easy to remember, as they correspond roughly to cranial bones of the same names overlying them. The insula is an oddball in this regard, for it lies buried deep inside the lateral (Sylvian) fissure--a T-shaped crevice separating the frontal and anterior parietal lobes from the temporal lobe laterally (see Figure 16A, 16D). Other major fissures of functional importance are the central (Rolandic), which separates the frontal and parietal lobes laterally, and the calcarine, which lies within the medial surface of the occipital lobe. You need to remember that cortex covers parts of the medial and inferior surfaces of each hemisphere, as well as the entire lateral surface; and you need to remember that a major portion of the cortex lies buried in numerous sulci, as well as in fissures. An important word in this context is operculum, meaning lid or cover. The portions of frontal, temporal, and parietal lobe overlying the insula, like a lid, are called the operculum (see Figure 16A, 16D).
Cranial nerve I, which carries olfactory information, is only a few millimeters in length. The first-order neurons in it have their bipolar bodies in the nasal epithelium and end in the olfactory bulb, an evagination of the CNS, which lies in a shallow sulcus on the inferior surface of the frontal lobe (Figure 16C). In the olfactory bulb lie the cell bodies of the second-order olfactory neurons, whose axons travel posteriorly in a discrete bundle, the olfactory tract (remember: nerves are outside CNS, tracts are within CNS), before disappearing into the substance of the cerebrum. From here, the olfactory system is difficult to trace, but it seems to be most intimately associated with a set of primitive cortical and subcortical structures which form part of the so-called limbic system or rhinencephalon (rhinos = nose, enkephalos = brain). (We will discuss the limbic system briefly later.) Notice that the olfactory system does not have the usual sensory ganglion, nor does it follow the usual sensory route through a thalamic relay nucleus to reach the cerebral cortex. Deficits in sense of smell due to neural damage nearly always reflect ipsilateral lesions in a nerve I, olfactory bulb, or olfactory tract--i.e., in the upper nasal cavity or near the base of the frontal lobe. Otherwise, olfaction is not of much use in neuroanatomical diagnosis, though olfactory hallucinations, usually very unpleasant, may accompany abnormal bursts of neuronal activity in the infero- medial temporal lobe--thus helping to localize such irritative lesions.
Each retina and optic nerve represents an evagination of the CNS, just as did each olfactory bulb and olfactory tract. Thus, the optic nerve, though structurally part of the PNS, is functionally a tract. The optic nerve is unlike other sensory nerves in another respect, which probably reflects its CNS origin. Since information from the photoreceptors passes through bipolar cells before reaching the dendrites/cell bodies of the so-called ganglion cells, whose axons run in the optic nerve, these latter axons are equivalent to at least second- order neurons, whereas other nerves contain only first-order neurons. The retina contains not only photoreceptors, bipolar cells, and ganglion cells, but also other interneurons that obtain information from the photoreceptors and, no doubt, influence the activity of the bipolar and ganglion cells. As each optic nerve travels back from the eyeball, it gradually angles medially till fibers from the two optic nerves merge together near the midline below the anterior thalamus, just in front of the stalk joining the (hypo-) thalamus to the nearby pituitary gland. The merger of fibers does not last more than a centimeter, then the fibers separate into two discrete bundles, one on either side, again. The area of merger is called the optic chiasm. Technically, the optic nerves end as fibers enter the chiasm, and the bundles leaving the chiasm are called the optic tracts. Since many fibers in each optic nerve cross in the optic chiasm and enter the contralateral optic tract, the nerves and tracts really are different structures. Remember the pupillary light reflexes? Well, some of the axons in each optic tract leave to course toward the midbrain. They form part of the afferent pathway influencing those preganglionic parasympathetics in the midbrain that innervate the pupillary constrictor muscle. The majority of fibers in each optic tract, however, end by synapsing in the ipsilateral thalamic relay nucleus for vision--the lateral geniculate body (LGB). The axons of the post-synaptic neurons with cell bodies in the LGB form the visual radiations, passing through the internal capsule and corona radiate to reach the primary visual cortex, which surrounds the calcarine fissure on the medial surface of the occipital lobe (Figure 16B). Notice by reference to Figure 16B and 16C that even though the optic system may be said to "ascend" from peripheral structures through the thalamus to cerebral cortex, as is the pattern for several other sensory systems we have described, its course is actually transverse, moving from receptors near the front of the head to cortex near the back.
Before examining the effects of lesions in the visual afferent system, we will need to introduce the concept of visual field. If you shut your right eye and look straight ahead with the left, what you see is in the visual field of your left eye. Although the objects you see with your left eye are all represented on your left retina, their positions are different. Imagine that you divide the visual field of your left eye into quadrants--upper temporal, lower temporal, upper nasal, and lower nasal. Light from an object above your head and to your far left (i.e., in the upper temporal quadrant of the visual field), as it travels toward your left eye, is aimed at and will be focused on the lower nasal quadrant of the retina. Similarly, light from an object in the lower temporal quadrant of the visual field is focused on the upper nasal retina; etc. In other words, the retinal representation of the visual field is upside down and reversed from left to right, just as if the entire visual field had been rotated 180 degrees. The same is true for the visual field of your right eye. When you look right at an object, as when reading, the light from the object is aimed at and becomes focused on the midpoint of each retina, which is called the macula. (See NEURO ANAT 18 to learn more about the anatomy and physiology of the retina.) Earlier we mentioned two aspects of visual function tested routinely--acuity and visual fields. From what has just been discussed, it should be clear that acuity tests are essentially tests of central or macular vision; whereas visual field testing examines the intactness of peripheral vision in all directions. The spatial transformation of information from visual field to retina is important to remember, for the cell bodies and axons in the rest of the visual afferent pathways are organized retinotopically, with those carrying information from the upper retina remaining superior to those from the lower retina. Similarly, cell bodies and axons carrying information from the left half of each retina go to the left cerebral hemisphere; while those from the right half of each retina go to the right hemisphere. For this latter segregation to occur, the axons from ganglion cells in the nasal half of each retina must cross the midline, and they do--in the optic chiasm (see Figure 17).
Now we can discuss the localizing value of lesions in the afferent visual system. To begin with, it should be clear that deficits in acuity or visual fields due to a neural lesion already narrow the possible locations down to the retina, optic nerve, optic chiasm, optic tract, or cerebrum. Imagine you examine a patient and find that she cannot see objects placed to the left of her point of focus when using only her left eye. Where might her lesion be (assuming that non-neural possibilities have been ruled out)? Could it be in the left retina or left optic nerve? Yes, of course. If so, would you expect any deficit in the visual field of her right eye? No, as fibers from the right retina do not enter the left optic nerve. How about the optic chiasm? Yes, this would be a possibility. The patient's deficit is in the temporal half of her left visual field, which projects onto the nasal half of the left retina; and it is axons from the nasal half of each retina that cross in the optic chiasm. If the lesion were in the optic chiasm, would you expect any deficit in the visual field of her right eye? Yes, you would. What would the deficit be? An absence of the temporal half, right? In other words, the patient would be unable to see objects out to either side of her line of sight. She would have bilateral (i.e., affects visual fields of both eyes) temporal hemianopsia (hemi = half, an = without, opsis = sight). Could the lesion be in the left optic tract, left LGB, or beyond? No, it couldn't, because the axons from the nasal left retina crossed, so are found exclusively in the right optic tract and right hemisphere. If the lesion were in the right optic tract, right LGB, right optic radiations, or right primary visual cortex, would you expect any deficit in the visual field of her right eye? Yes, you would, for axons from ganglion cells in the temporal half of the right retina also travel in the right optic tract. Where then, would the deficit in the visual field of the right eye be located? In the temporal half? No, of course not, for that portion of the visual field would project to the nasal retina. The deficit would be in the nasal half of the visual field. In other words, the patient would be unable to see anything to the left of her line of sight with either eye. She would have a left (the deficient half of each visual field) homonymous (homo = same, having the same distribution in the two visual fields) hemianopsia. [Be aware that the phrase "left visual field" can be used in two different ways, which are differentiated by context for a person "in the know," but can confuse the beginning student. Basically, "left visual field" means the visual field of the left eye. However, "left visual field of each eye" is sometimes used as a short-hand for the left half of the visual field of each eye. Thus, a patient with left homonymous hemianopsia may be said to have deficits in the left visual fields. It is probably best to avoid the latter usage yourself, but you'll probably come across it. Obviously, the phrase "right visual field" has the same potential ambiguity.]
You may now find it interesting to consider the relationships between the part of the visual fields from which a cerebral hemisphere receives information, the part of the body from which it receives somatosensory information, and the part of the body whose voluntary movements it directs. As you look straight ahead, movements performed by your right arm and leg will tend to occur primarily in the right half of your visual fields. Information from those halves of your visual fields is transmitted to your left cerebral hemisphere, which is the same hemisphere that directs the voluntary movements being visually observed and receives information regarding the positions and movements of those limbs. Also, objects that may touch the right half of the body, causing such sensations as touch, heat, pain, and pressure, will also tend to appear in the right halves of the visual fields. Again, both the visual and somatosensory information is transmitted to the same hemisphere--the left. Thus, one can see how visual, somatosensory, and motor functions for one side of the body could be integrated within a single (the contralateral) hemisphere. Even if these relationships are coincidental, they may help you to remember some of the anatomy involved.
When considering a patient with a visual field deficit, the reasoning given above for distinguishing among lesions in an optic nerve (or retina), the optic chiasm, or in an optic tract (or beyond) is valid, whether the deficit involves entire temporal or nasal halves of visual fields or just portions of the halves. Thus, for example, a patient with deficits confined to the upper right quadrant of each visual field has partial destruction of either the left optic tract, left LGB, left visual radiations, or left primary visual cortex. Be sure that you understand why. In homonymous hemianopsias, there is often "sparing" of macular vision. This sparing is apparently due mainly to the disproportionately large percentage (about 90%) of the visual afferent pathways devoted to macular vision, so that most incomplete lesions miss some of these macular neurons. In summary, a monocular deficit points to a lesion anterior to the optic chiasm, a heteronymous (e.g., bilateral temporal) deficit suggests a chiasmal problem, and a homonymous deficit accompanies lesions of the contralateral optic tract, LGB, optic radiations, or primary visual cortex. [To learn more about the visual pathways, see NEURO ANAT 18 and pp. 167-175 of Adams and Victor, Principles of Neurology (2nd ed.).]
Before continuing our discussion of cerebral functions, try posing and solving some problems that combine findings from the special sensory systems with those of systems covered earlier. I'll give you two, as usual.
Where might her lesion be located? Why?
Where might his lesion be located? Why?
For purposes of neuroanatomical diagnosis, we can divide findings associated with cortical and subcortical lesions into three groups (see Table 3). The first group (A) are findings not necessarily indicative of cortical/subcortical pathology. We have discussed most of these findings already, for they are referable to lesions in primary projection areas of the cortex-areas that either receive or give rise to axons connecting them with a non-cortical structure, as in the thalamus, brainstem, or spinal cord. Afferent projection systems that we have already covered include the somatosensory (a combination of spinothalamic, dorsal columns, and trigeminal), auditory, and visual; while the efferent projections already covered include the upper motor neurons (corticospinals and corticobulbars). Notice on Figure 18A, B the relative positions of the primary projection areas. They are important not only for precise localization of the areas associated with Group-A-type findings, but also as background for considering Group-B-type findings. For review, the primary somatosensory area is just posterior to the central fissure (post- central gyrus and posterior part of paracentral lobule--parietal lobe), the primary motor area is just anterior to the central fissure (precentral gyrus and anterior part of paracentral lobule--frontal lobe), the primary auditory area is hidden in the lateral fissure, on the medial surface of the posterior temporal lobe (transverse gyri of Heschl), and the primary visual area surrounds the calcarine fissure (in cuneus, lingula, and lateral occipital gyri of occipital lobe). Notice, in particular, how the long, narrow motor and somatosensory areas are somatotopically organized, side-by-side. Thus, cortical lesions may be quite large and still not cause dysfunction throughout the contralateral half of the body, yet even small lesions in this region might be expected to cause both UMN and somatosensory problems with a similar distribution in the body.
|Table 3.||Classification of findings possibly due to cortical/subcortical lesions, with examples of each.*|
|Group A.||Findings associated with
discrete areas, non-cerebral as well as cerebral.|
|Group B.||Findings associated with discrete areas, cerebral only.|
|B1:||Deficits in higher mental processes|
|Subgroup B1.2:||Findings not
strictly related to communication|
|B2:||Deficits not strictly related to higher mental processes|
|Subgroup B2.1:||Abnormal reflexes
|Subgroup B2.2:||Illusions and
|Group C.||Findings usually associated with
poorly localized or diffuse damage, including cerebral.|
Before going on to Group-B-type findings (Table 3), we will discuss briefly the problems with conjugate gaze and sphincteric control (incontinence of urine and feces) that can result from cerebral lesions. When discussing the functions of extraocular muscles earlier, it was pointed out that UMN lesions did not produce weakness of individual muscles, but rather weakness of conjugate gaze (eyes moving together) in various directions. This "yoking" of eye movements is due to interneurons connecting the LMNs innervating different extraocular muscles. We will now describe the anatomy underlying cerebral control of voluntary horizontal gaze, tested by asking the patient to look to either side (without the aid of following an object, such as your finger or penlight). The UMNs involved lie in the motor association area, near the primary motor area (Figure 18A). (This seems reasonable, doesn't it, since we are dealing with motor control.) The axons of UMNs descend ipsilaterally, through the corona radiate, internal capsule, and midbrain, but then decussate in the pons to end near the LMNs innervating the contralateral lateral rectus. (Remember which nerve these LMNs travel in? Remember at what level the nerve leaves the CNS? If not, go back and review the general rules regarding location of the cranial nerves and the LMNs traveling in them.) Since each cerebral hemisphere controls gaze to the opposite side, so-called aversive gaze, it should not surprise you that these UMN fibers end near LMNs innervating the contralateral lateral rectus. After all, that is the muscle that turns the contralateral eye away from the involved hemisphere. But how is the medial rectus of the ipsilateral eye influenced to contract, so that the two eyes move conjugately? You might expect UMNs from the same hemisphere to descend and end ipsilaterally near the LMNs innervating that medial rectus, but they don't! Instead, axons from cell bodies near the LMNs innervating the contralateral lateral rectus not only run to those LMNs but also decussate and ascend to the LMNs innervating the medial rectus of the eye ipsilateral to the involved hemisphere. (See pp. 177-178 of Adams and Victor, Principles of Neurology (2nd ed.), for schematic diagrams of these circuits.) The tract in which the axons ascend from near the cell bodies of LMNs in nerve VI to the cell bodies of LMNs in nerves III and IV is called the medial longitudinal fasciculus (MLF). We will come across it again later, as we discuss the internal structure of the brain stem one last time. From the foregoing anatomy, what would you predict to find if the aversive gaze center in, say, the left cerebral hemisphere were damaged? Right! The eyes would deviate to the left, and the patient would be unable to turn his eyes to the right voluntarily. You should be aware that such gaze paralysis is often temporary, lasting only a day or so, presumably because of compensation from non-damaged areas. Though we will not discuss cerebral control of vertical gaze, it should be mentioned that lesions of a single hemisphere do not cause problems with vertical gaze.
Involuntary sphincteric control, associated with normal urination and defecation, is really an autonomic function. During normal emptying of the urinary bladder or rectum, smooth muscle in the walls contracts at the same time that the smooth muscle around the exit (the involuntary sphincter) relaxes. These are primarily parasympathetic functions, for which the cell bodies of the preganglionic neurons lie in the intermediolateral cell column of the sacral spinal cord--usually at the S2-S4 levels. Descending regulatory pathways from higher levels, including the cerebrum, influence these preganglionic parasympathetics, helping to prevent premature contractions and to ensure sustained effective emptying. Bilateral cortical/subcortical lesions in the motor association area on the medial side of the frontal lobe (see Figure 18B) can lead to loss of regulatory control, with incontinence resulting. Below the cerebrum, also, lesions to the descending regulatory fibers must almost always be bilateral to cause loss of function. What can indicate a cerebral origin for urinary and fecal incontinence is an accompanying indifference to the problem. The combination of incontinence and indifference to it is called frontal lobe incontinence, because large lesions extending into the anterior frontal lobes seem to be responsible. (The resulting bladder problems are types of neurogenic bladder. To learn more about the anatomy, pathophysiology, and clinical findings associated with neurogenic bladder and fecal incontinence, see NEURO ANAT 20.)
Many of the Group-B-type findings, associated only with cortical and subcortical lesions, have long and/or strange-sounding names and are found only by doing special additional procedures. These findings are often fascinating and/or bizarre, yielding insights into the anatomical organization of so-called higher intellectual functions. We will confine ourselves here to comments on the general anatomical and functional organization of the cortex, particularly as it is related to neuroanatomical diagnosis. It would seem worthwhile to start by defining several terms frequently encountered when discussing cerebral dysfunctions. We'll begin with the two most common prefixes--dys- and a(n)-. Dys- means "problem with," while a(n)- means "complete lack of." Thus, dysgnosia (gnosis = recognition) would mean problem with recognition, whereas agnosia would mean complete lack of recognition. The agnosias are modality-dependent, so one can have visual agnosia, auditory agnosia, tactile agnosia, etc. To use the term agnosia with a patient, one must first be sure that the modality in question is intact. For example, a blind person would not be described as having visual agnosia; whereas a person who can see but cannot recognize objects by sight alone would have visual agnosia. Some agnosias are given special names. I will mention only a few, to give you the "flavor." Astereognosis, which we have already encountered, is a type of tactile agnosia, where a person is unable to name objects placed in the hand from feel alone. Agraphesthesia has also been described earlier. It, too, is a type of tactile agnosia, in which letters and numbers (i.e., familiar shapes) traced on the skin are not recognized. Asomatognosia (somato = body) is a combined visual-tactile agnosia, in which the patient does not recognize part of his body as belonging to himself.
The illusions and hallucinations are also often modality-specific. An illusion is an abnormal perception of a real object or event; whereas an hallucination is an abnormal perception for which no real external stimulus exists. Thus, an auditory illusion would have to do with abnormal alterations in perception of real sounds--e.g., sounds are heard as louder than normal or seem to be repeated. An auditory hallucination, on the other hand, may occur in the complete absence of sounds; the patient may near voices, music, sirens, etc., when the examiner hears nothing. As with the agnosias, hallucinations may be polymodal. For example, a patient may both see and hear a non-existent person. Illusions may also involve time, rather than a specific modality, so that a patient experiences the passage of time strangely, perhaps as standing still or racing by rapidly.
On the motor side, apraxias (praxis = practice) are inabilities to perform habitual learned motor acts well, even though the muscles involved have normal strength, there is no relevant sensory loss, and there is no general incoordination or lack of movement of that part of the body, as might be encountered with a lesion of the cerebellum or basal ganglia, respectively (see below). Apraxias are usefully named according to the part of the body involved-- e.g., facial apraxia or manual apraxia. The release of reflexes not normally present, such as sucking and grasping reflexes, may be viewed as related to the apraxias, as follows. A facial apraxia may prevent a patient from performing habitual learned movements, such as licking the lips or blowing out a match; whereas a released sucking reflex substitutes a non-learned motor pattern. Similarly, a manual apraxia may prevent the patient from using a comb, razor, fork, etc., appropriately; while a released manual grasping reflex substitutes a non-learned motor pattern.
If a cerebral problem involves a language function, it is called a dysphasia (phanai = to speak) or aphasia. (Be sure not to confuse dysphasia with the similar-sounding word "dysphagia" (phagein = to eat), which means difficulty in swallowing.) Because of the importance of language functions, some of the disorders have special names. For example, a visual verbal agnosia is called alexia (i.e., inability to read) and a manual verbal apraxia is called agraphia (i.e., inability to write). The dysphasias can be divided roughly into receptive (fluent, sensory) and expressive (non-fluent, motor) types. With a receptive dysphasia, the patient is unable to understand verbal information transmitted in one or more modalities, though each modality involved is perceived by him. Thus, the receptive dysphasias are closely related to the agnosias. A patient with a strictly receptive dysphasia will be able to talk and write fluently (hence the name fluent aphasia), but the verbal content of the output will bear no relation to input obtained through the affected modality(ies). With a strictly motor dysphasia, on the other hand, the patient can comprehend verbal information presented in the visual, auditory, and tactile modalities, but has difficulty expressing himself verbally--in speech and/or in writing. In other words, his verbal output is non-fluent. Since the muscles involved in the impaired verbal output may still function normally in non-verbal motor acts, the motor dysphasias are closely related to the apraxias.
When considering motor dysphasias involving speech alone, it is important to distinguish them from two other sets of related findings--dysarthrias and dysphonias--which bring to mind rather different anatomical and etiologic differential diagnoses. A dysarthria (arthron = joint, i.e., disjointed speech) is a problem with articulation of speech; whereas a dysphonia (phone = voice, sound) is a problem with production of speech (vocal vibration). Since vocal vibrations are dependent on appropriate respiration (e.g., diaphragmatic action) and laryngeal action, dysphonias could involve LMNs traveling in spinal nerves C3-C5 and cranial nerve X, respectively. (Though we have not discussed individual spinal nerves separately, the chief respiratory muscle--the diaphragm--is innervated bilaterally by axons leaving the spinal cord in nerves C3-C5 (primarily C4), so problems with respiratory flow might suggest involvement of these neurons, or their cell bodies in the cervical cord.) Articulation, on the other hand, involves primarily movements of the lips, tongue, palate, and pharynx, so with what nerves would you associate a dysarthria? Muscles in the lips are supplied by cranial nerve VII, those in the palate and pharynx by IX and X, and those in the tongue by XII. (If you couldn't assign these muscle groups to the appropriate cranial nerves from memory, it's probably time to review the topic again briefly.) From the topics already covered, you should already be able to specify some neural locations at which damage might cause a dysarthria or dysphonia. Where are they? Well, what about damage to LMNs, either in the periphery or centrally at the level where the associated nerve(s) join the CNS? If the patient had a selective problem pronouncing, e.g., m's, b's, and p's--i.e., labial consonants (labium = lip)--, you might suspect cranial nerve VII or the pontomedullary level of brainstem. If he had a selective problem with, e.g., l's and t's--i.e., lingual consonants (lingua = tongue)--, you might suspect cranial nerve XII or the medullary level of the brainstem. If the selective problem involved e.g., g's, ng's, and nk's--i.e., guttural consonants (guttur = throat)--, involving actions of both tongue and soft palate, you might suspect cranial nerves IX, X or the medulla. If the patient had difficulty producing vowels appropriately, in the absence of problems with respiratory flow, cranial nerve X and medulla should again come to mind.
Notice that with the exception of the cervical cord, the CNS level of concern for LMN speech problems is primarily medullary. The name given to such LMN involvement in the brainstem, which often leads to dysphagia (problem with swallowing) as well, is bulbar palsy. Where else might you expect damage to produce a dysarthria or dysphonia? Of course, the corticobulbar fibers (UMNs)! Would you expect the damage to be one-sided or bilateral? Remember, most muscles supplied by cranial nerves are associated with UMNs from both hemispheres, so bilateral damage would be necessary. This is even true for the lips and tongue, which include some muscles for which LMNs receive primarily contralateral UMN input! Unilateral weakness of these muscles does not usually cause problems with speech. Even in the case of the diaphragm, which is associated with corticospinals, bilateral damage might be required to produce significant dysphonia, since it extends to the rib cage on both sides of the body and, therefore, receives innervation from both halves of the spinal cord. At what levels might the damage occur? Well, since most of the LMNs involved are spread throughout the medulla, certainly lesions in the pons, medulla, and cerebrum should be considered. If a problem with weakness of muscles innervated by cranial nerves is due to bilateral damage to the corticobulbar fibers, it is called a pseudobulbar palsy (pseudes = false). Do you understand why? Given your knowledge of LMN and UMN pathways and findings, can you imagine ways to distinguish between these two types of palsy? How about presence of marked atrophy or fasciculations in the involved muscles? What would that suggest? What if the problems are all on one side? Could this happen with an UMN lesion? Highly unlikely. Why? How about deep tendon reflexes, such as the jaw jerk--mediated by cranial nerve V? If they were hyperactive, what would that suggest? A lesion above the mid-pons, right? If you noticed problems referable to a cranial nerve entering one level of the brainstem, e.g., inability to pronounce labial consonants (nerve VII), but no problem with muscles innervated by LMNs lower in the brainstem, e.g., lingual muscles used to pronounce l's and t's, would that be a useful clue? Yes, it might be, for the corticobulbars on each side travel together until they prepare to decussate. Thus, you might expect that bilateral lesions wiping out the corticobulbars to one set of LMNs would probably damage the other corticobulbar fibers, serving LMNs lower in the brainstem, as well. Another clue may come from the presence of inappropriate and excessive laughing and/or crying. I won't attempt to explain its occurrence, but such pathological display of emotion often accompanies pseudobulbar palsies.
Though we have not covered the basal ganglia and cerebellum yet, you should be aware that lesions in these extrapyramidal systems can also cause dysarthrias/dysphonias. We will not pursue the topic further, but these various anatomical types of speech problem can often be distinguished by other data, such as yielded by more detailed analysis of the speech characteristics themselves. (To learn more about dysarthrias and dysphonias, see pp. 334-336 of Adams and Victor, Principles of Neurology (2nd ed.).)
Besides the motor and sensory dysphasias, there are also conduction dysphasias. In them, the patients are able to comprehend verbal information and produce fluent speech and writing, but the verbal information given to them is incapable of influencing their verbal output adequately. Hence the modifier conduction, for it's as if the verbal information can simply not reach the areas responsible for verbal output. Another common term--global dysphasia is used to describe the patient who can neither comprehend verbal information nor produce fluent verbal output. (We will also not discuss the dysphasias any further, but to learn more about them, see pp. 327-334, 336-337 in Adams and Victor, Principles of Neurology (2nd ed.).)
By this point, I can imagine that your head is swimming with visual verbal agnosias, manual verbal apraxias, asomatognosias, etc. Take heart! By discussing the general organization of the cerebral hemispheres, much of which you have already learned, we will now try to convince you that the various cerebral findings can also be similarly organized, and in a manner that corresponds nicely to cerebral anatomy. First of all, don't let yourself be inundated with names of structures that you feel you must memorize. The cortical surface, for example, is covered with a large number of gyri (plural of gyrus, meaning raised area of cortex between two crevices: sulcus being a shallow one, fissure being a deep one), each gyrus (and sulcus) having a special name. You will probably use a few of the more functionally important ones as landmarks and end up learning their names, but don't make a special effort to do so unless you have plenty of time and want to--they're simply not important to memorize. The cortex has also been divided into a large number of different regions on the basis of histologic criteria. There are various histologic maps, but that most commonly referred to clinically is the map of Brodmann areas. These, too, are not important to memorize, though you'll probably end up learning a few functionally important Brodmann areas, simply because you'll come across them repeatedly.
The next structures we will consider are the axons making up the massive subcortical white matter. These, too, need not cause much concern, as long as you remember their general organization. First of all, there are three types of fibers (axons) in the subcortical white matter--association, commissural, and projection fibers. Association fibers join together two different cortical areas on the same hemisphere, commissural fibers join together bilaterally symmetrical cortical areas on the two hemispheres, and projection fibers join an area of cortex to a non-cortical structure, such as the thalamus, brainstem, or spinal cord. As a generalization, the following rules are helpful. (1) Every area of cortex gives rise to association, commissural, and projection fibers. (2) Every cortical area is linked by association fibers to every other cortical area on the same hemisphere. (3) Commissural fibers cross the midline, front to back, in an order corresponding to the location of the cortical areas they link, front to back. (The corpus callosum is the name given to the massive commissure that contains the vast majority of all subcortical commissural fibers. As you might expect from rule (3), fibers linking the two frontal lobes travel in the anterior of the corpus callosum, while those linking the two occipital lobes travel in the posterior of the corpus callosum.) (4) The projection fibers, as they descend and come gradually together, maintain their relative anterior-posterior relationships to each other. Thus, when they have descended far enough to come together as a discrete bundle (called the internal capsule, sound familiar?), having left behind the criss-crossing web of association and commissural fibers, those projection fibers from the anterior part of the frontal lobe lie in the anterior part of the internal capsule, those from the occipital lobe lie in the most posterior part.
Since you have already studied the primary projection systems subserving somatosensation (combination of spinothalamics, dorsal columns, and trigeminal systems), motor function (UMNs--corticospinals and corticobulbars), hearing, and vision, and you know where their primary cortical areas are located (Figure 18), where in the internal capsule do you think the fibers for each system run? If you guessed that the somatosensory and motor radiations run near the middle of the internal capsule, with motor fibers slightly anterior to somatosensory (though with overlap), you are right! How about visual radiations? Since the primary visual area is in the occipital lobe, it should also be no surprise that these fibers are far posterior in the internal capsule. Now for the auditory radiations. At first, their position doesn't seem to fit, for they are quite a bit more posterior than the somatosensory radiations, even though the temporal lobe containing the primary auditory area is about as anterior as the primary somatosensory area. Here it is helpful to digress briefly into comparative anatomy. The "stem" of the temporal lobe lies posterior, just in front of the occipital lobe and inferior to the posterior parietal lobe. In most mammals, the temporal lobe is quite small, not extending very far forward, so that the lateral fissure is quite short and lies posteriorly. Apparently, in the course of relatively recent human evolution, there has been a marked expansion of the temporal lobe, possibly associated with new and expanding higher intellectual functions involving audition, so that the human temporal lobe can be thought of as a mushroom extending anteriorly from its more posterior stem. As one examines the anatomy of the human temporal lobe, it looks as if a variety of subcortical structures, white (e.g., association, commissural, and projection fibers), gray (e.g., extensions of the basal ganglia), and hollow (temporal horn of lateral ventricle), were pulled along. If one bears this in mind, then the rather posterior position of the auditory radiations in the internal capsule does fit the general pattern. You might also find it interesting to note the following. If one considers the temporal lobe to stem from below the posterior parietal lobe, then an imaginary plane through the two central fissures seems to cut each hemisphere into an anterior "motor'` portion and a posterior "sensory" portion. (Note the similarity to the spinal cord, which also has the motor portion of gray matter placed anteriorly and the sensory portion of gray matter placed posteriorly!)
The various bundles of subcortical fibers, especially those of association fibers, also have a variety of names, which are largely unimportant. As with the cortical areas, those bundles that are functionally important will come up repeatedly and will become learned, but don't feel compelled to memorize the names as a separate exercise.
Now that I've told you what not to bother learning, what should you remember? Basically, you need to remember where the primary and association areas for somatosensation, audition, vision, and motor function are, and where association fibers must run to connect these various cortical areas (Figure 18). You also need to be familiar with the concept of dominant hemisphere. In each of us, language function is centered in one hemisphere--referred to as the dominant hemisphere. If you are right-handed, chances are about 99 to 1 that your left hemisphere is dominant. If you are left-handed, chances are about 2 to 1 that your left hemisphere is dominant. Other functions may also be lateralized (i.e., represented primarily in one hemisphere), but not necessarily in the dominant hemisphere. For example, music appreciation, unlike recognition of verbal sounds, seems to be centered in the non-dominant hemisphere; and appreciation of spatial relationships seems to be centered in the non-dominant parietal lobe in most patients. We will not discuss lateralization of higher functions any further, but be aware that when we discuss areas associated with various dysphasias, we are referring only to those on the dominant hemisphere.
Now let's tie the anatomy and the Group-B-type findings all together! As a generalization, the following rules are true. (1) Agnosia for a particular modality is associated with damage near the association area for that modality. This is also true for illusions or hallucinations utilizing that modality. In fact, the agnosia may be seen as absence of normal perception, while the illusions and hallucinations can be viewed as presence of substitute (?) abnormal perceptions. If a patient were found to have a combination of auditory agnosia and auditory illusions, for example, what cortical area would come to mind? Temporal lobe? Yes. What if it turned out to be damage near the auditory association area of the dominant hemisphere? The patient would have an auditory verbal agnosia--i.e., an auditory sensory (Wernicke's) dysphasia. (2) Apraxias are associated with damage to the motor association area. Not surprisingly, damage to the part of the association area anterior to the primary area for the face is associated with facial apraxias; and damage somewhat more superior, near the primary area for the hand, is associated with manual apraxias. Presence of abnormal reflexes, such as the sucking and grasping reflexes, as well as the "release" of urination and defecation, leading to incontinence, is also associated with damage to the motor association area. As in the case of agnosias and illusions/hallucinations, apraxias may be viewed as absence of normal motor coordinations, while the reflexes can be viewed as presence of substitute (more primitive?) abnormal motor coordinations. If a patient were found to have a manual apraxia, what cortical area would come to mind? Of course, the frontal lobe, rather inferior on the lateral surface, in front of the "hand" portion of the primary motor area. And what if a patient had damage in the motor association area of his dominant hemisphere, just anterior to the "face" portion of the primary motor area? He would have a facial verbal apraxia--i.e., a facial motor (Broca's) aphasia. (3) Problems with higher mental processes that involve more than one modality are associated with damage in areas between association areas for those modalities, presumably where association fibers from the involved modalities come together. Thus, if damage occurs in the parietal lobe, between the combined primary/association area for somatosensation and the association area for vision, the patient may experience loss of recognition for the contralateral half of his body, apparently a complex tactile-visual agnosia. If the lesion is in the inferior parietal lobe of the dominant hemisphere, where association fibers from both auditory and visual association areas probably come together as the information is passed along to the verbal motor areas anteriorly, the patient may experience a combined auditory-visual sensory dysphasia--i.e., auditory dysphasia and dyslexia. (When thinking of the pathways taken by association fibers joining the auditory areas to the motor areas, remember that they cannot jump across the lateral fissure, but must run back to the "stem" of the temporal lobe (see above) before going forward again above the fissure.) (To learn more about cortical functions, see NEURO ANAT 9, NEURO PSYC 4, and pp. 296-320, 323-337 in Adams and Victor, Principles of Neurology (2nd ed.). To learn more about some of the tests for cortical functions, see NEURO HxPx 5, NEURO HxPx 9, and pp. 57-59, 86-89, and 127-136 of Barrow's Guide to Neurological Assessment.)
We will not discuss Group-C-type findings, as they are generally of little use in localizing discrete neural lesions. However, they should be remembered as not inconsistent with diffuse cerebral damage.
Before going on, you might wish to pose and solve some clinical problems that integrate what you have learned so far. I'll start you out with two.
Where might his lesion be located? Why?
Where might his lesion be located? Why?
28 March 1991
|1.||Throughout the text, the modules referred to are now named differently. Ignore.|
|2.||p. 21:||In the brainstem, there is only one "spinothalamic tract," as stated. However, this tract is probably a continuation of only the "lateral spinothalamic tract," whereas the "anterior spinothalamic tract" (i.e., the fibers mediating "touch") probably run up through the brainstem with the "medial lemniscus."|
|3.||p. 22:||It is really not certain that there is a separate "anterior spinothalamic tract," conveying "light touch." Whether there is or not, the important point is that "light touch" from each side of the body is conveyed up both sides of the spinal cord by the posterior columns ipsilaterally and by spinothalamic fibers contralaterally.|
|4.||p. 23:||The first-order fibers in the dorsal columns travel not just to the junction of the spinal cord with the medulla but into the caudal medulla, where the nucleus gracilis and the nucleus cuneatus lie.|
|5.||p. 24:||Light touch will not be spared in lesions of the medial lemniscus. It will be spared in CNS lesions of the dorsal-columns system only if they occur above the ventral posterolateral nucleus (of the thalamus) or below the medial lemniscus.|
|6.||p. 47:||The descending subsystem of the trigeminal probably does not handle much of light touch. Lesions in this subsystem usually spare light touch.|
|7.||p. 53:||Müller's muscle is found not only in the upper eyelid but also in the lower eyelid.|
|8.||p. 53:||Although decreased ability to vasoconstrict arterioles over the ipsilateral face and anterior scalp does result from damage to the cranial sympathetic system, it is not usually listed as part of Horner's syndrome.|
|Problem 1||a.||At T1 level or above. Why?|
|b.||Probably in the CNS. Why?|
|c.||Most likely on the right side. Why?|
|d.||If on the right side, most likely in the lateral spinothalamic or spinothalamic tract. Why?|
|e.||No matter at what level in the lateral spinothalamic tract or the spinothalamic tract on the right side, the lesion does not cut across the entire tract. How do you know?|
|f.||If the lesion is not in the lateral spinothalamic or spinothalamic tract, could it be in the anterior funiculus on the right side from levels C3-T1? Yes, it could. How?|
|g.||Could it be in the posterior horn on the left side from levels C5-T1 ? Yes, it could. How?|
|h.||Could it be above the thalamus on the right side? No, it couldn't. Why not?|
|Problem 2||a.||In the CNS. Why?|
|b.||On the left side. Why?|
|c.||Probably at about the T8 level. Why?|
|d.||If at the T8 level, it interrupts the entire left lateral spinothalamic tract. How do you know?|
|e.||Could it be in the left lateral spinothalamic tract at the C3 level? Yes, it could. How?|
|f.||Could it be in the left spinothalamic tract at the pontine level? Yes, it could. How?|
|g.||Could it be in the left somatosensory radiations at the level of the internal capsule? No, it couldn't. Why not?|
|Problem 3||a.||In the CNS. Why?|
|b.||At or above T4. Why?|
|c.||Probably in the anterior white commissure. Why?|
|d.||Probably at about levels C8-T4. Why?|
|e.||If the lesion were elsewhere, it would probably have to be in two different places. Why?|
|f.||The two places would probably involve both lateral spinothalamic tracts or both spinothalamic tracts. Why?|
|g.||If the lateral spinothalamic or spinothalamic tracts were involved, the lesions would not cut across the entire tracts. How do you know?|
|Problem 4||a.||In the CNS. Why?|
|b.||Probably interrupting the right lateral spinothalamic tract at about the T6 level. Why?|
|c.||Possibly involving also the right posterior horn at the T6-T10 levels, damaging axons of second-order neurons of the lateral spinothalamic system there. Why?|
|d.||Possibly involving the anterior white commissure at the T6-T10 levels. Why?|
|e.||Do you need lesions as indicated in both c. and d. above? No. Why not?|
|f.||Do you think c. or d. is the more likely alternative? I favor d. Why?|
|g.||Could the lesion involve the right spinothalamic tract in the medulla? Yes, it could. How?|
|h.||Do you think b. is more likely than g.? So do I. Why?|
|i.||Could the lesion involve the right somatosensory radiations in the corona radiate? No, it couldn't. Why not?|
|j.||Could the lesion involve the left lateral spinothalamic tract at the C5 level? Yes, it could. How?|
|k.||If there were a lesion in the left lateral spinothalamic tract at the C5 level, would it interrupt the entire tract? No. How do you know?|
|l.||If there were a lesion in the left lateral spinothalamic tract at the C5 level, what previously mentioned lesion- sites would no longer be necessary? Those in c. and d. Why?|
|Problem 5||a.||In the CNS. Why?|
|b.||Probably at about the L1 spinal cord level. Why?|
|c.||Probably involving the left posterior funiculus. Why?|
|d.||Could the lesion possibly be in the right half of the CNS? Yes, it could. How?|
|e.||If the lesion were in the right half of the CNS, could it be in the thoracic cord? No, it couldn't. Why not?|
|f.||If the lesion were in the right half of the CNS, could it be in the pons? Yes, it could. Why?|
|g.||If the lesion were in the cervical cord, would the entire left posterior funiculus be severed? No. How do you know?|
|h.||Could the lesion possibly be in the right ventral posterolateral nucleus of the thalamus? No, it couldn't. Why not?|
|i.||Could the lesion be in the right somatosensory radiations at the level of the internal capsule? No, it couldn't. Why not? (There are at least two reasons.)|
|NOTE: The following item goes beyond the workbook.|
|j.||If the lesion in b.-c. above had a vascular etiology, what artery would probably be involved? Branches of the left posterior spinal artery. Why? Would the entire distribution of the left posterior spinal artery be affected at that level? No, it wouldn't. How do you know?|
|Problem 6||a.||Not in the PNS. Why not?|
|b.||Not in the spinal cord. Why not?|
|c.||Not in the brain stem. Why not?|
|d.||Not in the thalamus. Why not?|
|e.||On the right side of the CNS. Why?|
|f.||Could the lesion be in the right internal capsule? Yes, it could. Why?|
|g.||Could the lesion be in the right postcentral gyrus and posterior half of the paracentral lobule? Yes, it could. Why?|
|h.||Which lesion is bigger, that in f. or that in g.? That in g. Why?|
|NOTE: The following item goes beyond the workbook.|
|i.||If the etiology of the lesion were vascular, would f. or g. be the more likely site of the lesion? f. would be the more likely site. Why? (Hint: One reason involves the number of arteries supplying each site.)|
|Problem 7||a.||Probably not in the PNS. Why not?|
|b.||If any peripheral neural structures were involved, what would they be? Sensory (posterior, dorsal) roots at the T10 level bilaterally. Why?|
|c.||Probably at the T10 level bilaterally (for loss of position sense, vibratory sense, and two-point discrimination). Why?|
|d.||Probably involves both posterior funiculi completely at the T10 level. Why?|
|e.||Does the lesion in d. explain all findings? No. Which ones remain?|
|f.||If we consider the area of loss of pain and temperature sensation as due to a lesion in the anterior white commissure, at what levels is the lesion? T8-T10. Why? (Why not just T10?)|
|g.||Can we explain the loss of pain and temperature sensation with a lesion at the T10 level only? Yes, we can. How?|
|h.||Destruction of what structures at the T10 level would cause loss of pain and temperature in the T10 dermatome bilaterally? The dorsal horns or sensory roots. Why?|
|i.||Could the bilateral loss of position sense, vibratory sense, and two-point discrimination be explained easily by a single lesion in the upper thoracic or in the cervical cord? Yes, it could. Why?|
|j.||If bilateral loss of position sense, vibratory sense, and two-point discrimination could easily be due to a single lesion in the upper thoracic or in the cervical cord, is that a likely level for the lesion? No, it isn't. Why not?|
|k.||Can the area of loss of pain and temperature sensation be explained by a single small (few millimeters) lesion somewhere above T8? No, it can't. Why not?|
|l.||Could the findings be due to lesions above the thalamus? No, they couldn't. Why not? (There are at least three reasons, some more compelling than others.)|
|NOTE: The remaining items go beyond the workbook.|
|m.||Could the bilateral loss of position sense, vibratory sense, and two-point discrimination be explained easily by a single lesion in the medulla? Yes, it could. Why?|
|n.||Could the bilateral loss of position sense, vibratory sense, and two-point discrimination be explained easily by a single lesion in the midbrain? No, it couldn't. Why not? (There are at least two reasons.)|
|o.||If the bilateral loss of position sense, vibratory sense, and two-point discrimination were due to a thrombo- embolic problem in the upper thoracic cord, could the lesion be explained as due to involvement of just a single vessel? No, it couldn't. Why not?|
|p.||If the bilateral loss of position sense, vibratory sense, and two-point discrimination were due to a thrombo- embolic problem in the medulla, could the lesion be explained as due to involvement of just a single vessel? No, it couldn't. Why not?|
|q.||If damage to a vertebra caused the findings, what vertebra would you suspect? About T8. Why? (Why not T10?)|
|r.||Is the etiology for this problem a biological agent? No. Why not? (What if he was hit by a car?)|
|Problem 8||a.||Is it possible to get weakness, hypotonia, and hyporeflexia with an upper-motor-neuron lesion? Yes, it is. How?|
|b.||Could this patient's findings be caused by an upper-motor-neuron lesion combined with spinal shock? Probably not. Why not?|
|c.||If this is a lower-motor-neuron problem, which side of the body is the lesion on? The right side. Why?|
|d.||If the lower-motor-neuron lesion is in the spinal cord, could it be at just the L2 level? No, it must involve at least the L2-S 1 levels. Why?|
|e.||If the lower-motor-neuron lesion is in the spinal cord, what structures could it include? The anterior horn and/or the white matter between anterior and lateral funiculi. Why?|
|f.||If the lesion is in the PNS, would you think it more likely to be in the motor roots, in the lumbosacral plexus, or distal to the plexus? In the motor roots or lumbosacral plexus. Why?|
|g.||If there were also sensory findings in the right leg, would you think the lesion more likely to be in the motor roots, in the lumbosacral plexus, or distal to the plexus? In the lumbosacral plexus (or perhaps distal to the plexus). Why?|
|h.||Would you expect to find atrophy in the right leg? Yes. Why? (There are at least two reasons.)|
|i.||Would you expect to find fasciculations in muscles of the right leg? Perhaps. Why? Why not?|
|NOTE: The remaining items go beyond the workbook.|
|j.||If the lesion were in the spinal cord and the etiology were thrombo-embolic, you would suspect involvement of branches of what vessel? The anterior spinal artery. Why not the right anterior spinal artery? (Trick question.)|
|k.||If the lesion were due to blockage of branches of the anterior spinal artery to the right half of the spinal cord at about L2-S1, what sensory problems would be most likely? Absence of pin-prick and temperature sensation in the left half of the perineum and in the left leg. Why?|
|l.||To what etiologic class do some diseases that tend to attack only motor neurons belong? Biological agents. Can you come up with potential mechanisms to explain this cellular selectivity?|
|m.||Among the diseases of motor neurons caused by biological agents, do any have a particularly high incidence at certain times of the year? Yes. In the days before polio vaccines, outbreaks of poliomyelitis tended to occur in late summer, at least in the temperate USA. Can you come up with some potential mechanisms to explain such seasonal peaking?|
|NOTE: Read the next item only if you're feeling very confident and willing to be confused.|
|n.||Is there any area in which an upper- motor-neuron lesion can cause a long-lasting (far beyond CNS shock) flaccid paralysis? As a matter of fact, there is, though such lesions are rare. If a lesion is limited to Brodmann area 4 (cerebral cortex) or to fibers coming only from Brodmann area 4, the resulting paralysis is flaccid. How can this be? (Don't let this information bother you. It is rarely of clinical consequence. If you wish to "get to the bottom of this," corner Dr. Clough, Dr. Anthoney, or some other appropriate resource person.)|
|Problem 9||a.||Could the lesion be in the PNS? Highly unlikely. Why?|
|b.||Could this patient's findings be caused by an upper-motor-neuron lesion combined with spinal shock? Probably not. Why?|
|c.||If this is a lower-motor-neuron problem, which side of the body is the lesion on? The left side. Why?|
|d.||What findings could a lower-motor- neuron lesion not explain? The increased ankle jerk and patellar response on the left, also the Babinski on the left. What about the absent superficial abdominal reflexes on the left?|
|e.||To explain as many findings as possible, at what levels of the cord would lower motor neurons have to be involved. C5-T1 and T8-T12. Does this seem likely to you?|
|f.||Could damage to only the left lateral corticospinal tract at the CS level give you all of this patient's findings? No. Why not?|
|g.||Could damage to only the left lateral corticospinal tract at the T8 level explain all of the patient's findings except for those in the left upper extremity? Yes.|
|h.||Could damage to only the corticospinal tract on the left above the spinal cord explain all of the patient's findings except for those in the left upper extremity? Yes. How?|
|i.||What's the most likely place for a lesion (single) that explains all of the patients' findings? About C5-C7 in the left half of the spinal cord. Why?|
|j.||What structures are probably involved at the C5-C7 level? The left lateral corticospinal tract and the left anterior horn. Why is the anterior horn more likely than the white matter between the anterior and lateral funiculi on the left?|
|k.||Could the upper motor neuron influencing the left upper extremity be damaged? Yes; in fact, they probably are. If so, why does the patient not have hypertonia, hyperreflexia, and a Hoffman sign in the left upper extremity?|
|l.||Could this lesion have started in the lateral corticospinal tract in the left half of the spinal cord at the C6-C8 level? Yes, it could have. If it did, what do you think your initial findings would have been one month after onset? Would you have seen fasciculations?|
|m.||Could this lesion have started in the left anterior horn at the C6-C8 level? Yes, it could have. If it did, what do you think your initial findings would have been one month after onset? Would you have seen marked atrophy?|
|n.||Do you think the lesion is continuing to enlarge? Probably so. Why? Because of the fasciculations present. Why?|
|NOTE: The remaining items go beyond the workbook.|
|o.||Is the etiology of this patient's problem vascular? Probably not. Is it biological, chemical, or physical? Probably not. Why not?|
|p.||What sorts of etiologies frequently present as slowly progressive? Growth disorders and degenerative diseases. Metabolic, congenital, and nutritional problems also can. Can you give an example of each that might have caused this patient's problem?|
|q.||If the etiology of this patient's problem had been vascular, you would expect involvement of what vessel? Branches of the anterior spinal artery. Why?|
|r.||If the lesion were due to blockage of branches of the anterior spinal artery in the left half of the spinal cord at about C5-T1, what sensory problems would be most likely? Absence of pin-prick and temperature sensation on the right side, from the foot on up to the clavicle, but including only the ulnar half of the right upper extremity. Why?|