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Ear


REGIONS of the ear

The ear has three distinct regions -- outer ear, middle ear, and inner ear.


OVERVIEW of the inner ear

The inner ear has a complex structure.  The following basic concepts should help organize that complexity.


BONY LABYRINTH and MEMBRANOUS LABYRINTH

Image courtesy Alec Salt, Cochlear Fluids Laboratory, Washington University; used with permission.

The inner ear resides within a space called the bony labyrinth

Suspended within the bony labyrinth, and approximating its shape, is an interconnected set of membrane-lined chambers and passageways called the membranous labyrinth.


HAIR CELLS

Hair cells, the specialized mechanoreceptor cells of the auditory and vestibular systems, are found in several positions along the chambers and passageways of the membranous labyrinth. 


OTOLITH ORGANS (SACCULE and UTRICLE)

The saccule and utricle contain patches of hair cells called maculae ("macula" means "spot" or "patch").

Image courtesy Alec Salt, Cochlear Fluids Laboratory, Washington University; used with permission.


SEMICIRCULAR CANALS

Each semicircular canal of the bony labyrinth is a hollow passageway looping out from and back to the vestibule.  Within each of these passageways is a semicircular canal of the membranous labyrinth

Hair cells of the ampullae respond to angular acceleration (i.e., rotation of the head).

The planes of orientation of the semicircircular canals correspond to the planes of action of the extraocular muscles, allowing simple reflexes to coordinate eye movement with head rotation.

Image courtesy Alec Salt, Cochlear Fluids Laboratory, Washington University; used with permission.

COCHLEA

The cochlea houses an elaborate configuration of membranous labyrinth and hair cells, called the organ of Corti, designed for auditory reception.

The basic shape of the cochlea is that of a snail-shell, or tapering helix. 

The human cochlea is short and broad; micrographs at this website (and in many other references) show the cochlea of a laboratory rodent which is proportionately taller and narrower.

The spiraling tunnel (blue, in image at right) that forms the cochlea of the bony labyrinth is divided into three distinct channels by portions of the membranous labyrinth attached to bony ridges.  Each of these channels is called a "scala", meaning "ramp" or "incline" (think of a musical "scale").

The central column (the modiolus) of the helical cochlea contains axons serving the organ of Corti on their way to the auditory nerve. 
A bony ridge, the spiral lamina, extends out from the modiolus and provides support for the organ of Corti.  A tubular cavity within the spiral lamina contains the cell bodies of the axons of the auditory nerve.  Because this collection of nerve cell bodies has a helical shape paralleling the cochlear scalae, it is called the spiral ganglion.


ORGAN OF CORTI

The organ of Corti is an elaborate structure with more named parts than the rest of inner ear.  Several key features are listed below.  For additional detail, including the basic physiology of hearing, consult a textbook or click here, Anatomy and pathology of organ of Corti (Bohne laboratory, Washington University).

Clinical note:  A cochlear implant is inserted into the scala tympani, where it lies close to the organ of Corti and can artificially stimulate axons of the auditory nerve.  (more)

The organ of Corti is considerably more complex than this simple account implies, with, among other things, two functionally distinct classes of hair cells (inner and outer).  Synapses from the inner hair cells apparently supply most of the sensory information that goes to the brain, while the outer hair cells (the ones which are most readily recognized by light microscopy) have a curious mechanical function (for more, see for example How the Ear Works).


Image courtesy Alec Salt, Cochlear Fluids Laboratory, Washington University; used with permission.

ENDOLYMPH and PERILYMPH

The membranous labyrinth is filled with endolymph and surrounded by perilymph.

Endolymph (blue, in image at right) is a unique fluid, with high K+ concentration and very low Na+ concentration.  This endolymph provides the proper ionic environment for hair cell function.

Endolymph is secreted by cells of the stria vascularis, along the scala media of the cochlea.  The stria vascularis resembles a stratified cuboidal epithelium, but unlike any proper epithelium (and as the name vascularis indicates) this tissue contains capillaries among the cuboidal cells. 

For additional detail, see Cochlear Fluids Lab at Wash. U.

Perilymph (orange, in image at right) is similar to ordinary interstitial fluid.  Perilymph fills the spaces of the bony labyrinth surrounding the membranous labyrinth.

In the vestibular system (surrounding the saccule, utricle, and semicircular canals), perilymph simply provides a cushioning support for the membranous labyrinth.

In the cochlea, perilymph of the ascending scala vestibuli and the descending scala tympani conveys pressure waves (sound) across the scala media.  Pressure waves flex the basilar membrane and thereby stimulate hair cells of the organ of Corti.

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Comments and questions: dgking@siu.edu

SIUC / School of Medicine / Anatomy / David King

http://www.siumed.edu/~dking2/ssb/ear.htm
Last updated:  12 January 2010 / dgk