HISTO HOME
SSB Index
Nervous Tissue

Muscle Tissue

Skeletal Tissue
RESOURCE CENTER

Neurons and Support Cells

Please note that this guide is intended to complement, NOT to replace, textbook readings (i.e., Kandel et al.).


Histology textbooks are NOT recommended for the study of nervous tissue.  Most histology textbooks begin with relatively insignificant, and often misleading, details rather than emphasizing features important for understanding nervous tissue function.

Recommended are selected chapters in in Kandel, Schwartz and Jessell, Principles of Neural Science.

Kandel's classic text is remarkable.  The extended table of contents can be read, just as if it were a "capsule" textbook.  In about two dozen pages following immediately after the chapter listing, all of the subheadings from every chapter are presented, each as a complete sentence.  This extended table of contents offers a concise summary of major ideas.  Your study through this entire unit can be usefully guided by this summary.  You should, at a minimum, be fluent in the vocabulary of this summary so that every sentence here is meaningful to you.

Senior author Eric Kandel received the 2000 Nobel Prize for Physiology and Medicine for work on "signal transduction in the nervous system" (Kandel's Nobel Prize lecture).

Choose:  5th edition, 2013, 4th edition, 2000, or 3rd. edition, 1991. 


INTRODUCTORY COMMENTS

Please note that this guide is intended to complement, NOT to replace, textbook readings (i.e., Kandel et al.).

Nervous tissue has presented extraordinary challenges to science.  Historically, a basic appreciation of the cellular composition of nervous tissue did not come until decades after other tissues were fairly well understood. One reason is that none of the cell types which comprise nervous tissue can be properly visualized in routine histological preparations.  

Significance:  Do not expect examination of nervous system specimens to yield satisfying observations, at least not without an exceptional effort to understand why things look the way they do.

Neuron preparation by Ramón y Cajal

Historical note:  The most famous pioneer in the descriptive anatomy of nerve cells was Ramón y Cajal.  Cajal introduced the four principles which comprise the neuron doctrine (quotes are from Kandel 2006, In Search of Memory, pp. 65-66): 

  1. Cellularity: "The nerve cell is the fundamental structural and functional element of the brain."
  2. Synaptic communication: "The terminals of one neuron's axon communicate with the dendrites of another neuron only at specialized sites, later named synapses by Sherrington."
  3. Connection specificity: "Neurons do not form connections indiscriminately.  Rather each nerve cell forms synapses and communicates with certain nerve cells and not with others."
  4. Dynamic polarization: "Signals in a neural circuit travel in only one direction. . .  Information flows, from the dendrites of a given nerve cell to the cell body [then] along the axon to the presynaptic terminals and then across the synaptic cleft to the dendrites of the next cell, and so on."

Cajal's 1906 Nobel Prize lecture includes some elegant images of nerve cells in spinal cord and in cerebellar and cerebral cortex.  Click here to see a micrograph of one of Cajal's original nerve cell preparations. 

Axons, dendrites and synapses -- the most significant features of nerve cells -- cannot be readily seen without specialized techniques such as those used by Cajal.

No other tissue in the body is characterized by cells whose cytoplasmic processes reach out for great distances away from the cell nuclei.  This reality puts a special burden on the student as well as the researcher.  You cannot simply look at a slide or micrograph, not even an electron micrograph, and truly "see" the most interesting features of the nerve cell.  

Nerve cell processes are quite thin, often less than a micron (1µm) in diameter.  However, the length of axons and dendrites is wondrously great, far greater than ordinary cellular dimensions. Dendrites may extend several millimeters away from the cell body, into a volume the size of a pea.  Axon length may exceed a meter (for many sensory and motor axons), and commonly extends for several centimeters.

As a simple consequence of this cellular geometry, the cell body of a neuron may comprise less than one percent of the entire cell volume.  From this, you may deduce that the bulk of nervous tissue consists of nerve cell processes rather than nerve cell bodies.

The study of neuroanatomy consists largely of understanding the routes travelled by nerve cell axons.

Unfortunately, the organization of neural processes, most particularly the full length of axons and dendrites and the synaptic interactions between them, can seldom be visualized directly.  

In most other tissues of the body, what you can see in the microscope is directly informative.  Consider skin, where a routine section of epidermis reveals almost everything interesting about the size, shape and growth sequence of epidermal cells.  Electron microscopy of similar specimens simply adds more finely resolved detail.  But making any sense at all of nervous tissue requires that you "see" with concepts acquired over decades of research with many special techniques.  

When you examine microscope slides or micrographs of nervous tissue, patterns of functional connection cannot usually be seen.  Nevertheless, what you can observe should be interpreted in terms of neuronal functions and connectivity, including unseen axons, dendrites and synapses as well as associated supporting cells.  

Thus your job for comprehending nervous tissue is not just to look-and-learn, but to think rather deeply, to fit many different views and facts together.  Most of the listed vocabulary terms for neuronal and glial structures are well defined in standard textbooks.  You just have to make sense of it all.  

How to read this page.  You might read this page straight down, from top to bottom.  But it is written with hyperlinks to facilitate browsing.  You might more profitably check out each link, at least if it suggests a question in your mind, and use your browser's "back" arrow to return.  And return repeatedly to the outline at the top of this page to choose the topic that most closely engages your current curiosity.


BASIC STRUCTURE OF NERVE CELLS ("NEURONS")

"Swiftly the brain becomes an enchanted loom, where millions of flashing shuttles weave a dissolving pattern-always a meaningful pattern-though never an abiding one" (Sherrington, 1940).

 Nerve cells comprise the "enchanted loom" that is our brain. 

Here are three absolutely wonderful facts about nerve cells.  

  1. The first and most wonderful fact is that, working together, nerve cells can perceive and think and dream.  They are us.  This is magic of the highest sort.  (See Cells-R-Us for extended discussion.)
  2. The second wonderful fact is that nerve cells are much like other cells.  Each is essentially a bag of water, surrounded by a fatty membrane and containing an assortment of molecules.  There seems to be nothing about individual nerve cells that cannot be explained, at least in principle, by basic chemistry and biology.
  3. The third wonderful fact is that each nerve cell has a truly magnificent shape.  Somehow, this third fact bridges the gap between the mystery that is our mind and the chemistry that is our cells.  Somewhere in the shape of nerve cells, in the complexity of connections among billions of such cells, and in the intricate pattern of activity that plays upon those cells, our "self" emerges.
     

*** The following generalities all have exceptions. ***

Every nerve cell has three distinctive portions -- a cell body, one axon, and several dendrites.

Nerve cells come in extreme variety.  In every region of the brain are several different nerve cell types, each distinguished by its own characteristic soma size, dendritic shape, source of synaptic input, destination of axonal output, and chemistry.  Occasional nerve cell types may have characters which depart from the the typical description presented below.

Because of this immense variety of nerve cell types, there is no "one-size-fits-all" description.  So textbook descriptions of nerve cells tend to present overwhelmingly abundant detail.  Although details of nerve cell shape and connectivity are usually insignificant for clinical practice, they can be quite beautiful and are essential for understanding research on brain function.  It is also often necessary to learn some "irrelevant" detail in order to understand the particular examples used to demonstrate basic functional principles.

Nerve cell bodies look more or less like other body cells, although they do have certain characteristic features.  Extending out from each nerve cell body are long cytoplasmic processes, one axon and several dendrites.  (These processes usually cannot be distinguished in routine histological preparation.) 

A typical nerve cell body contains only a small fraction of the total cell volume; the rest is contained in the axon and dendrites.  The spaces between nerve cell bodies with a feltwork of these axonal and dendritic processes, called neuropil (which also includes glial cell processes). 

Top of page.

MYELIN

Myelin is a fatty covering which envelops many axons and permits action potentials to be propagated at a much greater velocity.

Myelin is formed by support cells (Schwann cells in the peripheral nerve system, oligodendroglia in the CNS) wrapping around the axons.  Myelin is not part of, nor produced by, the nerve cell whose axon it envelops.

In peripheral nerves, myelin consists of Schwann cell membrane wrapped around and around an axon, while most of the Schwann cell cytoplasm lies alongside the axon.  (See oligodendroglia for myelination of CNS axons.)

Myelination of a peripheral axon
Animation from Blue Histology, Copyright Lutz Slomianka 1998-2004

(The image should be animated, if you watch patiently.)

A Schwann cell is illustrated with brown cytoplasm.

The blue oval is the Schwann cell's nucleus.

Observe that as the growing Schwann cell spirals inward around the axon, it wraps its membrane into layers of myelin.

The myelin of one Schwann cell wraps about one or two millimeters of an axon.  To myelinate the entire length of the axon, many of these Schwann cell wrappings line up end-to-end along the axon.

The points between segments of myelin are called nodes of Ranvier.  The stretch of axon between nodes is called an internode.

The spacing of nodes is critical for propagation of action potentials.  Along myelinated axons, action potentials are regenerated only at the nodes.  Myelin provides insulation (and, more importantly, decreased capacitance) so that the ionic currents at one node can flow efficiently (and quickly) to the next node.  This is called saltatory conduction (saltation = jump).  In contrast, action potentials propagating along unmyelinated axons are regenerated at each point along the way, a much slower process.  [Hydrodynamic metaphor for saltatory conduction]

Clinical notes
Because the currents generated at one node are generally sufficient to depolarize axonal membrane two or three nodes away, local anesthesics (which block action potentials but do not prevent current flow) must be distributed across several nodes (several millimeters) in order to produce effective anesthesia. 

Because myelinated axons have voltage-dependent sodium channels only at nodes of Ranvier, demyelination (such as that which occurs in multiple sclerosis) effectively prevents the propagation of action potentials.

Many details of myelin cannot be well-appreciated by light microscopy.  For electron micrographs of myelin in peripheral nerves, see the online Electron Microscopic Atlas of Mammalian Tissues (the text is in German, but most figure labels can be deciphered fairly easily).

Top of page.

RECOGNIZING NERVE CELLS in histological preparations.

Although axons reach into all parts of the body, the vast majority of nerve cell bodies occur in the central nervous system (brain and spinal cord), in those regions described as gray matter.  Relatively few nerve cell bodies occur peripherally, in the ganglia (small clusters of nerve cells) of sympathetic and parasympathetic nervous systems.

Wherever they occur, nerve cell bodies have a distinctive appearance.

Because these features of nerve cell bodies are related to the heavy metabolic demands imposed by extensive processes, they are exaggerated (i.e., bigger nuclei, more cytoplasm) in those nerve cells which have the longest, largest diameter axons.

Nerve cells with the most extremely long, large diameter axons -- such as pyramidal cells of motor cortex and motor neurons of spinal cord -- are often illustrated as "typical" neurons simply because they are big and hence especially easy to visualize.  Cerebellar Purkinje cells comprise another "popular" type of nerve cell, also large but with a huge dendritic tree rather than an especially long axon.

Neuron preparation by Ramón y Cajal

Special stains, like the silver-based Golgi stain, can reveal entire neurons or glial cells (at least as much as fits within the thickness of a single section) by impregnating them with opaque silver.  But this technique yields elegant results only by suppressing any staining of most neighboring cells, so neurons appear in splendid isolation when their essence is one of complex interaction.  Similarly, electron microscopy can display elegant synapses, but the narrow view offers few clues about the cells to which the pre- and post-synaptic profiles belong.  

Sections of central nervous tissue routinely show neuron cell bodies surrounded by a finely-textured fibrous material often called neuropil (which should not be confused with connective tissue).  This feltwork consists of axons and dendrites (and glial processes), with all the comings and goings that these processes entail.  Individual axons and dendrites can be distinguished only in fortuitous sections, and then only for a short length.   The so-called "molecular" layers of cerebral and cerebellar cortex consist of neuropil containing relatively few cell bodies (most of the cell bodies lie in deeper layers).

Note that a common artefact, resulting from tissue shrinkage, is for a clear "halo" to appear around cell bodies and blood vessels.  Although the presence of such halos can be misleading (there is no such space in intact, living nervous tissue), this consistent artefact serves to highlight or emphasize the locations for these structures.

Top of page.

SUPPORT CELLS of nervous tissue.

Schwann cells -- Support cells in peripheral nerves.

Many of the small, heterochromatic nuclei that can be seen within peripheral nerves belong to Schwann cells.  Some of the remaining nuclei belong to fibroblasts of the endoneurium, perineurium, and epineurium (i.e., connective tissue) that gives tensile strength to the nerve.  Perineurium also contains squamous perineural cells (perineural epithelium) which form a continuous layer that isolates the axons within from surrounding connective tissue. 

Fibroblast nuclei tend to be smaller and more densely heterochromatic than Schwann cell nuclei, but in most ordinary preparations that include peripheral nerves, it is impractical to distinguish these nuclei.

Note that none of the nuclei visible in peripheral nerves belong to nerve cells, since peripheral nerves do NOT contain nerve cell bodies, only axons of nerve cells whose cell bodies lie elsewhere.

Support cells in peripheral ganglia are sometimes called satellite cells.

Schwann cells can form tumors called schwannomas (see WebPath:  MRI, gross, dissection, microscopic low X, high X).

Top of page.

Glial cells -- Support cells of the CENTRAL NERVOUS SYSTEM

The most numerous cells within the central nervous system are glial cells.  The name "glia" means "glue" (filling the interstices of nervous tissue), reflecting old but enduring ignorance of their function (and the inadequacy of classical histology to offer much insight).  The small nuclei of glial cells may be readily observed in any section of central nervous tissue.  Unfortunately, like neurons, these cells are difficult to visualize satisfactorily.

Although glial cells vastly outnumber nerve cells (approx. 10:1 glia to neurons), nerve cells are so large, including the total volume of all their dendrites and axons, that most of the cellular volume of the brain consists of nerve cells.

Ignorance of glial function is beginning to dissipate.  For a recent review, see:

Barres, BA (2008)  The mystery and magic of glia: a perspective on their roles in health and disease. Neuron. 60 (3): 430-40 [PubMedID: 18995817]

"In this perspective, [Barres] review[s] recent evidence that glial cells are critical participants in every major aspect of brain development, function, and disease.  Far more active than once thought, glial cells powerfully control synapse formation, function, and blood flow.  They secrete many substances whose roles are not understood, and they are central players in CNS injury and disease. [Barres] argue[s] that until the roles of nonneuronal cells are more fully understood and considered, neurobiology as a whole will progress only slowly. "

"And please don't forget the glia!  Quite possibly the most important roles of glia have yet to be imagined."

The two most common types of glia, oligodendroglia and astroglia, both have extensive cytoplasmic processes and are intimately involved in the function of nervous tissue.  A third glial type, microglia, function similarly to macrophages.  

In most of our reference slides, both in the spinal smear and in sections of brain and spinal cord, only the nuclei of glial cells are clearly seen, with no indication of cytoplasmic shape.  The characteristic processes of glia can show up nicely in some of the Golgi-stained sections in your reference collection (variously cerebellum or cerebral cortex).   Even with electron microscopy, it is difficult to trace CNS myelin to the arms of the oligodendroglia from which it forms.  

Separately distinguishing among astroglia, oligodendroglia and microglia is a skill for specialists (i.e., pathologists), but with practice their nuclei can be recognized by relative size and texture, with astrocyte nuclei being somewhat larger and paler than the others.

Oligodendroglia (also called "oligodendrocytes" or just "oligos") typically have relatively few processes (hence their name; oligo = few), with each process ending in a sheet of myelin which wraps around a segment of an axon.  

Function of oligodendroglia:  Oligodendrocytes form myelin in the CNS and hence are responsible for normal propagation of action potentials.  Patchy loss of CNS myelin, as in multiple sclerosis  ( WebPath, WebPath w/ MRI ), can cause a variety of neurological problems.  

Myelin formation by oligodendroglia is slightly different than that by Schwann cells, each of which wraps myelin around a single axon.  Each of the several glial cell processes extends to and then myelinates a segment of one axon.  If the myelin of one oligodendrocyte process were unrolled, the process would be shaped rather like a wide-bladed shovel (the thin shovel blade would represent the membrane that rolls around the axon to form myelin and the shovel handle would represent the process which extends back to the glial cell body).  Each oligodendroglial cell has several such "shovels", forming myelin around several axons.  

Astroglia or astrocytes extend branching cytoplasmic processes in all directions (yielding the star-like shape suggested by their name; astro = star).  Foot-processes of astrocytes line every surface where central nervous tissue contacts other body tissues, not only the obvious outer surface immediately underlying the pia mater (where they form the glia limitans) but also along every blood vessel and capillary which penetrates into the brain and spinal cord.  Other astrocyte foot processes approach nerve cells at any sites where the nerve cell membrane is not otherwise occupied by synapses or by oligodendroglia.  

Functions of astroglia:  There is growing awareness that astrocytes play several critical roles. 

Recent evidence shows that activity of individual astrocytes can correspond closely with that of associated neurons, and can also modulate local blood flow (Schummers, et al., Tuned responses of astrocytes and their influence on hemodynamic signals in the visual cortex, Science 320:1638-1643, 2008; doi:10.1126/science.1156120).

Additional functions and pathologies include all of the following (from Ransom, et al., New roles for astrocytes (stars at last) Trends in Neuroscience, 26:520-522, 2003; doi:10.1016/j.tins.2003.08.006]:

Recent research also implicates astroglia in the "glymphatic system" which allows recirculation of CSF and brain interstitial fluid along paravascular channels.  A 2013 report in Science 342:373 implicates this system in the function of sleep (Science news article). 

Microvascular control:  Local variation in blood flow through brain capillaries may be regulated by activity of pericytes, which in turn can respond to neural activity.  [Reference:  MacVicar & Salter, Neuroscience: Controlled capillaries, Nature 443, 642-643 (12 October 2006) | doi:10.1038/443642a.]

The Blood Brain Barrier 

"Blood-brain barrier" is the name given to a physiological property of CNS blood vessels.  In contrast to vessels in most other parts of the body, most molecules can NOT pass freely between blood to interstitial space.  The integrity of the blood-brain barrier is established by continuous capillary endothelium together with the absence of endothelial vesicular transcytosis.  The only substances which cross this barrier are those which can diffuse through endothelial plasma membranes or those for which specific endothelial membrane channels exist.

The blood-brain barrier is a concept with considerable clinical significance, not only because it limits the delivery of drugs to the central nervous system but also because pathological disturbance of the barrier can seriously impact brain function.  Read a more extensive description of the anatomy and physiology of the blood-brain barrier at the University of Arizona Health Science Center, Blood Brain Barrier.

Microglia are small cells, comprising about 10% of the total brain cell population, which represent the brain's immune system (i.e., macrophage-equivalents residing within the brain).  Microglia are also implicated in the maturation, plasticity, and remodelling of synaptic circuits (Science 333:1391, 9 September 2011, doi:10.1126/science.1212112; J. Neuroscience, 31:16064-69; doi: 10.1523/jneurosci.4158-11.2011)

As described by Kembermann and Neumann (Microglia: the enemy within? Science 302:1689, 5 December 2003, doi:10.1126/science.1092864), the brain exhibits "a robust innate immune response thanks to its microglia, which defend against invading microorganisms and clean up by engulfing the debris of dying cells.  In addition, the inflammatory mediators released by microglia during an innate immune response strongly influence neurons and their ability to process information."  Recent in vivo observations (Fetler and Amigorena, Brain under surveillance: the microglia patrol, Science 309:392-3, 15 July 2005, doi:10.1126/science.1114852) show microglia as surprisingly dynamic cells, continually extending and withdrawing fine motile cellular processes and contacting astrocytes, neurons, and blood vessels.

Recent research indicates that microglia (in mice) are "an ontogenically distinct population in the mononuclear phagocyte system," originating during embryonic development (Science, published online October 21, 2010; DOI: 10.1126/science.1194637)

Top of page.

Blood vessels in CNS

Central nervous tissue is highly vascular, so blood vessels should be a significant feature in any histological specimen of CNS.  Large vessels generally remain on the surface of the brain or spinal cord, so only smaller vessels penetrate into gray and white matter. 

Such small vessels may not be immediately recognizable as such.  As in other regions of the body, capillaries may be quite inconspicuous due to small size.  Even venules and arterioles may be small enough that the layers in their walls are not clearly visible.  Blood cells may be washed out during preparation.  Nevertheless, such vessels should be noticed, since they play a crucial role in brain function and pathology.  (Also see note on microvasculature, above.)

Blood vessels are generally the largest structural elements in neuropil and in white matter (i.e., even capillaries are larger in diameter than most CNS axons and dendrites). The thumbnails below link to several spinal cord specimens in which blood vessels may be observed.  Blood vessels appear similar in any region of the brain.

Note that a clear "halo" commonly appears around blood vessels (as well as neuronal and glial cell bodies). 
This an artifact of histological preparation, resulting from tissue shrinkage when the central nervous tissue is fixed.

 

Top of page.


Ependyma, choroid plexus and cerebro-spinal fluid

The ventricular system of the brain is lined by a simple cuboidal epithelium called ependyma, a remnant of the embryonic neuroectoderm which once formed the neural tube.  At certain sites (the posterior margin of the lateral ventricles, the midline of the 3rd ventricle, the roof of the 4th ventricle), this ependyma lies adjacent to overlying connective tissue.  Here the ependyma is extensively wrinkled, with blood vessels which are caught up in the folds, to form choroid plexus.

Choroid plexus is the source for cerebrospinal fluid (CSF).  CSF is actively secreted by the ependymal cells of choroid plexus and (like aqueous humor in the eye) accumulates at a steady rate even if drainage points become occluded. 

In composition, CSF differs considerably from blood.  Although osmolarity and sodium concentrations are similar in blood and CSF, CSF has somewhat more chloride; less potassium, calcium, magnesium and glucose; much less protein, and practically no white blood cells.  For specific values as wells as alterations in disease, see Kandel et al., 4th edition, Appendix B, especially pp. 1295-1299.

CSF and brain interstitial fluid are exchanged through the so-called "glymphatic system" of paravascular channels.  A 2013 report in Science 342:373 implicates this system in the function of sleep (Science news article). 

The layout of choroid plexus is perhaps most easily appreciated embryologically -- click on the thumbnail for an image of embryonic choroid plexus.

Cerebrospinal fluid accumulates not only from the action of choroid plexus but also from the interstitial spaces of the brain.  It flows, under positive pressure developed by its active secretion, through the ventricular system, thence out through holes in the roof of the 4th ventricle into the subarachnoid space, finally draining through "arachnoid villi" into the venous sinuses of the cranial cavity.

Top of page.

Meninges:  dura mater, pia mater, and arachnoid

The central nervous system is enveloped by specialized layers of connective tissue.

Top of page.


SOME EXAMPLES of nervous tissue.
This section offers a guide for microscope lab (i.e., for viewing slides in your reference set).

Also see:

  • Link to LUMEN (Loyola University Medical Education Network), click on "Part 6:  Neural Tissue".
  • Link to Blue Histology.  Click on "Notes", then "Nervous Tissue".


Spinal cord smear

Using your reference slides, the best view of "whole" neurons is provided by the slide labelled "nerve cells, ox spinal cord".  (This is a slide of spinal smear, not a slice but a small amount of gray matter squished onto the slide.) 

The largest nerve cells in this preparation represent spinal motor neurons, the cells whose very long axons extend out peripheral nerves to the muscles.  From the nerve cell body extend several dendrites; these are broad at their base and contain Nissl but decrease in diameter and basophilia with increasing distance from the soma.  The full extent of the dendritic arborization is not visible, since the fine distal branches are hidden in the background texture of the slide.  

Each neuron also has a single axon, which can be readily identified only if it begins on the edge of the cell body (as opposed to the top or bottom, as viewed in the slide).  The axon, unlike the dendrite, has a uniform diameter and does not contain basophilic Nissl bodies.  It begins at the axon hillock, a specialized site on the cell body where the cytoplasm is clear (like the axoplasm, it lacks Nissl bodies).  The axon, even more so than the dendrites, disappears into the distance and cannot be followed to its end. 

In this same preparation, smaller cells with similar features represent spinal interneurons.  Scattered throughout this preparation are also very many cells whose nuclei are smaller than those of the neurons, oval with clumps of heterochromatin, and whose cytoplasm is inconspicuous.  These are the glial cells.  Numerous capillaries, narrow tubular profiles wandering across the slide, may also be seen.  


Spinal cord section

The spinal cord consists of ascending and descending axonal pathways (i.e., white matter) surrounding a central core of gray matter.  Use your preferred neuro text to rehearse the functions associated with the following regions in the spinal cord.

Some sections of spinal cord may include dorsal and ventral roots containing (respectively) sensory and motor axons. 


Cerebral cortex

The cerebral cortex forms the surface of gyri an sulci over each entire cerebral hemisphere.  Its composition is complex (after all, it is the seat of conscious perception and thought!), with many different types of nerve cells.  These include many local interneurons (stellate cells and granule cells) as well as the much larger and more conspicuous pyramidal cells, some of whose axons enter the underlying white matter and travel to other cortical areas or to other regions of the brain.

The cerebral cortex is traditionally (but rather arbitrarily) described as having six layers.  Although these layer cannot be readily distinguished (they are arbitrary, after all), they can be roughly approximated by looking for the following features.

Layer I (the "molecular layer") is the outermost layer.  This layer contains relatively few nerve cell bodies.  The odd name "molecular layer" derives from the fine texture of this layer, due to its composition largely of dendrites and fine axon terminals (and glia, of course).

Layer II (the "outer granular layer"), typically contains many very small cells (granule cells).

Layer III (the "outer pyramidal layer") contains cell bodies of small pyramidal cells.  Axons from these cells typically project to the upper layers of neighboring cortical regions.

Layer IV (the "inner granular layer") contains axonal ramifications of afferent fibers, such as sensory axons from the thalamus.  Axons from the lateral geniculate nucleus (the visual relay of the thalamus) are so numerous that the primary visual cortex which receives these axons (Brodmann's area 17, at the occipetal pole of each hemisphere) is sometimes called "striate cortex", because these axons conspicuously divide the cortex into layers that are visible to gross inspection.

Layer V (the "inner pyramidal layer") contains cell bodies of large pyramidal cells.  Axons from these cells typically project to more distant cortical regions, to other parts of the brain, or to lower centers (such as spinal motor neurons).  The larger size of these pyramidal cells (compared the the smaller cells of layer III) is associated with the greater length of their axons.  (Recall that cell bodies provide most of the basic cellular functions needed to maintain the axon, while the axonal surface membrane and axoplasmic volume may be many times greater than the surface and volume of the cell body.)

Layer VI (the "layer of pleiomorphic cells) typically contains cells of assorted size and shape (hence, "pleiomorphic").

Variations in the detailed appearance ("cytoarchitecture") of the several cortical layers, as described a century ago by K. Brodmann, formed the original basis for recognizing regional differentiation of the cortex ("Brodmann's areas").  Now, of course, this cytoarchitectural differentiation is known to correspond with functional localization in the cortex.

See WebPath for cortical changes associated with Alzheimer's disease.


Cerebellar cortex

The cortex of the cerebellum consists of three very well-defined layers.  The most prominent nerve cells are Purkinje cells, whose cell bodies all lie in a discrete layer.

The inner granular layer is packed with nuclei of vastly many cerebellar granule cells.  These are among the smallest (and most numerous) neurons in the body.

The Purkinje cell layer contains large cell bodies of Purkinje cells, the sole output cells for the cortex.

The outer molecular layer consists principally of the dendrites of Purkinje cells and the axons of granule cells.  The odd name "molecular layer" derives from the fine texture of this layer, due to its composition largely of dendrites and fine axon terminals.  Nuclei in this layer belong mostly to glial cells.

The pattern of connections among various axons and dendrites in the cerebellum is extremely elegant and regular, and has been described in extensive detail.  Any thorough neuro text (e.g., Kandel et al., 4th ed., pp. 835 ff) should have a good account.


Peripheral ganglia

Both the paravertebral ganglia of the sympathetic nervous system and the scattered ganglia of the parasympathetic nervous system consist of small clusters of nerve cell bodies.  Parasympathetic ganglia may turn up in sections of various visceral organs, where they can be recognized by the classic appearance of nerve cell bodies.


Retina

Tissues of the eye are listed in a separate page.

 


Composition and appearance of PERIPHERAL NERVES

Like other "pieces" of the nervous system, peripheral nerves are a part of a functioning, highly organized whole.  Each "piece" must be understood in relation to the rest of the system.  

Examples of peripheral nerves are often fairly easy to find in sections of the skin.  Larger nerves also often run in parallel with blood vessels.

Images which include examples of peripheral nerves.
Click on a thumbnail below for enlargement.

Peripheral nerves consist of axons bundled together within an epineurium (connective tissue sheath).  Peripheral nerves are only meaningful in relation to their connections.  All of the axons which travel along peripheral nerves begin and end somewhere else.

Motor axons originate with cell bodies in the spinal cord's ventral horn or in the brainstem's motor nuclei or in peripheral sympathetic or parasympathetic ganglia.  Motor axons terminate at muscles (including smooth muscle along blood vessels) or glands.

Somatosensory axons begin with a peripheral receptor (e.g., a Meissner's corpuscle in skin or a muscle-spindle in a muscle).  These sensory axons then travel toward their cell bodies in a dorsal root ganglion or trigeminal ganglion, and finally terminate at synapses within the spinal cord or brain stem.  (Note that somatosensory axons are an exception to the rule that axons always conduct impulses away from the cell body.)

All the cellular nuclei which are obviously visible within a peripheral nerve belong not to nerve cells but to Schwann cells or to fibroblasts.  

In routine H&E slides, peripheral nerves resemble other fibrous tissues like smooth muscle or collagen.  All three are eosinophilic, and all contain scattered, elongated nuclei.

Several features may be used to distinguish nerves from smooth muscle or other fibrous tissue.

Note that the texture of peripheral nerves can differ from site to site, depending on axon size and especially on the proportion of myelinated to unmyelinated axons.  Nerves in the tongue, with many large myelinated axons, are much more obvious than are autonomic nerves in Auerbach's plexus of the gut, where most axons are smaller and unmyelinated.  

In peripheral nerve cross sections stained for myelin, the myelin is generally visible as a dark or black frame around each pale myelinated axon.  The typical round shape is often distorted by tissue preparation.  In longitudinal sections containing large myelinated axons, nodes of Ranvier can be easily seen where the myelin appears to be "pinched".  Seldom can a single axon be followed throughout an entire internode (i.e., the segment of myelin from one node to the next, about 1-2 mm); the axon is just not straight enough to remain within the plane of section.  Nevertheless, the length of each internode can be estimated by measuring the total length of all axons visible in a field of view and dividing by the number of nodes that appear.

In ordinary H&E stained cross sections of peripheral nerve, myelin is generally visible as a pale unstained halo around the axon.  Less-than-ideal fixation also often distorts the relationship, so the axon may not be centered within the halo

Many details of peripheral nerves cannot be well-appreciated by light microscopy.  For electron micrographs of peripheral nerves, see the online Electron Microscopic Atlas of Mammalian Tissues (the text is in German, but most figure labels can be deciphered fairly easily).

SENSORY and MOTOR NERVE ENDINGS associated with PERIPHERAL NERVES

For sensory receptors in skin, see skin innervation.

For sensory receptors associated with muscle, see muscle innervation.

For motor endings on skeletal muscle, see muscle innervation.


Composition of CENTRAL NERVOUS SYSTEM

The organization of the central nervous system is based upon interconnections across varying distances among billions of individual nerve cells.  The basic principle of neural organization is quite straightforward.  Nervous tissue consists of nerve cells communicating with other nerve cells.  This simple yet fundamental concept can easily become lost in the forest of details presented in standard textbooks.  Here, then, is a brief guide to nervous tissue, including the classification and nomenclature of nerve cells.  

Gray Matter and White Matter (Cortex, Nuclei, and Fiber Tracts)

Each nerve cell has a cell body in one place and an axon which travels some distance to synapse with the cell bodies and dendrites of other neurons. 

The microscopic appearances of gray matter and white matter may be conveniently contrasted in a section of spinal cord.

Various stains have various effects on gray matter.  Note that a popular neuroanatomical stain (Weigert's), used to highlight different brain regions, colors myelin black.  Thus, paradoxically, in many pictures of the brain, white matter appears black while gray matter appears pale.  

Gray matter:  Where cell bodies and dendrites are common, the gross color of fixed (dead) brain tissue is gray.  Hence we have the term gray matter.  Note that gray matter is not just a place where cell bodies and dendrites happen to be.  Gray matter is the cell bodies and dendrites.

Note that gray matter necessarily contains both the beginnings and endings of axons, even though the greater portion of many axons' length is contained within the fiber tracts of white matter.  Gray matter is gray not because it lacks myelin, but because it contains lots of other stuff besides myelinated axons.  

White matter:  Axons from many different neurons often gather together in large numbers at some distance from their cell bodies.  In such regions, the relatively large amount of myelin confers a white color, hence, white matter.  Myelin is largely fat, which is white in both living and fixed condition.  

Note that although white matter consists of myelinated axons (and unmyelinated axons as well), myelinated axons are not excluded from gray matter.  Myelinated axons must begin and end somewhere, and that place is with cell bodies and dendrites of gray matter.  Gray matter just has a lot of other stuff in it besides myelinated axons.  

Also note that in many neuroanatomical images, white matter has been stained black.  

Sensory Neurons, Motor Neurons, and Interneurons

Sensory neurons convey sensory information into the central nervous system.  Primary sensory neurons receive their information directly through sense receptors rather than dendrites.  Second, third and higher order sensory neurons relay information to sequentially higher levels in the brain.  

Motor neurons (or motoneurons) convey information out from the central nervous system to muscles or glands.  Lower motor neurons, located in the ventral horn of the spinal cord or in motor nuclei of the brainstem, send their motor axons out peripheral nerves.  Upper motor neurons, pyramidal cells located in the motor cortex, relay information to the lower motor neurons.  

All other neurons are interneurons.  They interconnect neurons with other neurons.  Nearly all the nerve cells in the central nervous system are interneurons.  Their axons arise in one region of the CNS where the cell body resides and end somewhere else (sometimes several other places).  Second, third and higher order sensory neurons can be considered as ascending interneurons; upper motor neurons can be considered as descending interneurons.  

Synaptic Relays

Information from primary sensory neurons does not reach the highest levels (the cerebral cortex) directly.  Rather it is relayed at least twice (once in the spinal cord or brain stem, again in the thalamus).  At each relay, incoming (afferent, presynaptic) axons terminate by synapsing onto the dendrites of the next neurons in the series.  The outgoing axons of these neurons then relay the information to the next level.  At each relay site, some information processing and distribution can occur, so the information can be altered as it travels upward.  Similarly, muscle commands are relayed downward from motor cortex and other motor centers to the "final common pathway", the lower motor neurons of cranial nerve nuclei and the anterior horn of the spinal cord.  

Because each relay occurs at synapses onto dendrites and cell bodies of the next neurons in the pathway, each relay is associated with gray matter.  Conversely, every gray matter region (nucleus or cortex) is associated with relaying information from one set of axons (the afferent axons that enter the region in question) to another (the efferent axons that leave the region).  

Sometimes it is sufficient just to know the beginning and ending points of an entire pathway.  Other times knowing how far the neurons of each relay extend will be necessary to determine the site or effects of a lesion.  

Afferent and Efferent Axons

All gray matter regions of the brain, both cortex and nuclei, are associated with afferent ("input") and efferent ("output") axons.  Afferent axons enter the region from somewhere else (i.e., the cell body is located some distance away.) Efferent axons arise from cell bodies within the region and leave the region to go somewhere else.  Thus every long-distance axon is both efferent (with respect to its source, the location of its cell body) and afferent (with respect to its destination).  

The terms "afferent" and "efferent" are relational terms. Neither can be used precisely without specifying a region of reference (e.g., "cortical afferents" provide input to cortex; they may be efferents from thalamus or efferents from some other cortical area.)

Ascending and Descending Pathways

"Ascending" and "descending" refer to directions along the neural axis.  These terms may often correlate with "afferent" and "efferent", at least when the reference is high, like cortex.   (In fact, "afferent" and "efferent" are sometimes used as synonyms of "ascending" and "descending", respectively.  But they also have a relational meaning, defined above.) "Ascending" and "descending" are also closely associated with "sensory" and "motor", respectively.  But both sensory and motor information can be passed up, down, and sidewise, so these words should not be carelessly interchanged.  

Long-Axon And Short-Axon Neurons

Gray matter typically contains both many short-axon neurons and a smaller number of long-axon neurons.  

The largest and most conspicuous cell bodies in a particular region of gray matter are sometimes referred to as the principal cells of that region.  These cells generally have very long axons which leave the local region to go elsewhere, usually traveling within some white matter fiber tract.  The axons of these projection neurons may extend for appreciable distances, from a few centimeters to well over a meter.  Long-axon neurons are responsible for communicating with other brain regions.  Every parcel of gray matter has a class of long-axon neurons; otherwise information would come in but never go out.  The axons of a region's long-axon neurons are by definition identical with the region's efferent axons.  

The study of neuroanatomy is largely the study of the axonal projections of long-axon cells.  

The axons of the short-axon neurons do not leave the immediate neighborhood.  Short-axon neurons are also called intrinsic neurons or local interneurons.

In most regions, long-axon cells are much better understood than intrinsic cells.  Long axons provide opportunity for researchers to record and interfere with neuronal output.  Cells with short axons are much more difficult to manipulate.  

Neuron Names

Neurohistology is burdened by a profusion of names for different neuronal cell types.  Every nerve cell can be classified according to its place within the general organization of nervous tissue (above).  But every nerve cell also belongs to a unique population with a particular role in the information processing of the brain.  

Unlike most other basic cell types in the body (e.g., epidermal cells or fibroblasts), nerve cells are not all equivalent to one another.  Neurons in one region are structurally and functionally different from those in other regions, with different sources of input, different destinations for output, different patterns of dendritic branching, different neurotransmitters, etc.  The result is a tremendous abundance of nerve cell types, with specific names for each.  A few examples follow.  

 

Neuron preparation by Ramón y Cajal

Pyramidal cells are the efferent (long-axon) cells of the cerebral cortex.  The name refers to the shape of the cell body as seen in standard sections perpendicular to the cortical surface.  The apex of the pyramid points toward the cortical surface.  A large apical dendrite extends further upward toward that surface, while other dendrites arise from the corners and sides of the pyramid.  The axon extends down into white matter (the internal capsule) from the base of the pyramid.  Most pyramidal cells project association fibers to other cortical regions and/or to deeper nuclei of the brain.  (Classic illustrations of these cells were done by Ramón y Cajal).

The giant Betz cells are extremely large pyramidal cells of the motor (precentral) cortex.  These pyramidal cells comprise some of the upper motor neurons.  Axons from these cells descend in the corticospinal tract, or pyramidal tract, to synapse with lower motor neurons.  Their exceptionally large size is presumably associated with their need to sustain extremely long axons.

Stellate cells are intrinsic neurons named for their star-like shape, which results from dendrites arising in many directions.  The name is descriptive but relatively non-specific.  Thus stellate cells of the cerebral cortex are not the same as stellate cells of the cerebellar cortex.  

Horizontal cells are intrinsic neurons whose dendrites and local axons tend to be confined within a layer parallel to a surface.  Again the name is descriptive rather than specific.  Thus horizontal cells in the cerebral cortex are not the same as horizontal cells in the retina.  

Granule cells are intrinsic neurons which are both very small and very numerous, like grains of sand. Once again, this name is descriptive but non-specific unless additionally qualified.  For example, cerebellar granule cells are a very specific class of short-axon cells in one layer of the cerebellar cortex (a layer called, unimaginatively, the "granule cell layer").  

Purkinje cells are the efferent (long-axon) cells of the cerebellar cortex.  Purkinje cells have such impressive dendritic arborization, that images of these cells (often from the classic illustrations of Ramon e Cajal) are often featured in textbooks.  The synaptic input of Purkinje cells comes largely from the cerebellar parallel fibers (which are the axons of the extremely numerous local interneurons, the cerebellar granule cells).  Purkinje cells also receive potent synapses directly from one class of cerebellar afferent fibers, the "climbing fibers".  (Another class of cerebellar afferents, the "mossy fibers", provide input to the intrinsic granule cells.  In addition to Purkinje cells and granule cells, the cerebellar cortex also contains stellate cells, basket cells, and Golgi cells.)  [See your favorite neuro text (e.g., Kandel et al., 4th ed., pp. 835 ff) for elegant details of neural circuitry in the cerebellar cortex.]

Spinal motor neurons have the largest nerve cell bodies in the ventral horn.  These cells are also called "lower motor neurons", or just "motor neurons" (since upper moter neurons are properly called interneurons).  Axons of these cells extend through ventral (anterior) roots into peripheral nerves, and hence to motor end plates on muscle fibers.  Each muscle fiber of skeletal muscle is innervated by a single spinal motor neuron.  Each spinal motor neuron innervates one or more muscle fibers; the number of muscle fibers per axons is related to the fineness of motor control -- e.g., one-to-one for eye muscles, many-to-one for large postural muscles.  A single spinal motor neuron and all the muscle fibers innervated by its axon are called a motor unit.

Spinal motor neurons are lost in amyotrophic lateral sclerosis (ALS) -- for more, see:

The above are only examples.  Every region of the central nervous system contains many distinct neuronal cell types, most of which are appreciated only by the research specialist.  But a few examples are worth knowing, both because they are a part of familiar scientific knowledge and because they illustrate the complexity of neural organization.  Note that in every region the long-axon cells are directly associated with significant neuroanatomical pathways (output), while detailed knowledge of the intrinsic cells generally has little clinical importance (at least in our current state of ignorance).  


NERVOUS TISSUE PATHOLOGY.

Chapter 18 in the 3rd edition of Kandel et al. (1991) is an excellent resource for understanding the responses of nervous tissue to injury.  The newer, 4th edition of Kandel et al. (2000) offers a much sketchier account in Chapter 55 (pp. 1108 ff).

Washington University hosts an excellent web resource for neuromuscular pathology.
Within this site, you can find:

WebPath also offers some examples of nervous system pathology, see WebPath CNS Pathology Index and WebPath CNS Degenerative Diseases.
For example:

Online slides of the nervous system -- normal  |  pathology

These specimens at the Virtual Slidebox (University of Iowa Department of Pathology) may be examined with full range of magnification and movement.  Requires Java and fast internet connection


Comments and questions: dgking@siu.edu

SIUC / School of Medicine / Anatomy / David King

http://www.siumed.edu/~dking2/ssb/neuron.htm
Last updated:  26 November 2013 / dgk