1

Anatomy of the Inner Ear

2

Auditory transduction takes placein the inner ear

Transduction refers to thetransformation of energy from one

form to another. In the case of the ear,acoustic (mechanical) energy is

transformed to electrochemical energy

3

The Inner Ear

From www.unc.edu/courses/psyc21/3-26-99/sld008.htm

The inner ear consists of a membranous “labyrinth” encased in an osseouslabyrinth.

4

Parts of the inner ear

From Gelfand (1998)

The vestibule and semicircular canals are concerned with vestibular function(balance); the cochlea is concerned with hearing. The cochlea is a coiled tube.Notice that the oval window and round window open into the vestibule, at thebase of the cochlea.

5

http://depts.washington.edu/otoweb/inner_ear.html

The cochlea

The cochlear coil extends “up” from its base. It is coiled around the modiolus.

6

The cochlea uncoiled

From Pickles (1992)

Reissner’s membrane and the basilar membrane divide thethe cochlea longitudinally into three scalae. Movement of the the basilarmembrane by pressure changes induced by stapes footplate motion at the ovalwindow is a critical step in the transduction process

7

http://btnrh.boystown.org/cel/inside.htm/

Cross-section of the cochlearduct

If you cut the cochlear tube cross sectionally, you’d see something like this.Scala vestibuli on top, scala tympani on the bottom. Scala media is a triangularduct in the middle. The process of transduction occurs in the structures withinscala media, sitting on the basilar membrane -- these structures comprise theorgan of Corti. The side of the duct where the nerve fibers exit (left in thispicture) is the “inner” or “modiolar” side of the duct. The opposite side is the“outer” side.

8

the organ of Corti

From Gelfand (1998)

Notice that scala media is more or less triangluar, formed by Reissner’smebrane, basilar membrane and the structure called the stria vascularis.The fluid that fills scala tympani and scala vestibuli is called perilymph; thefluid that fills scala media is called endolymph.The organ of Corti rests on the basilar membrane within scala media.

9

A closer look at the organ ofCorti

From Pickles (1992)

Two types of cells in the organ of Corti are support cells and hair cells. Thehair cells are the “receptor” cells-- the ones that transduce sound. Support cellssuch as the Deiter’s cells support hair cells. The tops of the hair cells and pillarcells form the reticular lamina, which isolates the hair cells’ stereocilia fromtheir cell bodies. The tectorial membrane is loosely coupled to the reticularlamina.There are 4 rows of hair cells, one on the inner (modiolar) side of the tunnelformed by the pillar cells-- these are the inner hair cells; and 3 one the outerside of the Tunnel of Corti, these are the outer hair cells. Notice that theDeiter’s cells support the Outer hair cells at their base, but that the outer haircell walls are surrounded by fluid. The inner hair cell is surrounded by supportcells.

10

Reticular lamina

From Gelfand (1998), Lim (1986)

The reticular lamina is a solid surface at the tops of the hair cells, so the topsof the hair cells are in endolymph and the bottom of the hair cells are inperilymph.

11

Deiter’s cells

From Gelfand (1998)

Deiter’s cell processes “fill in the gaps” between the tops of the outer hair cellsto form the reticular lamina.

12

Arrangement of hair cells

From Yost (1993), Courtesy of Dr. Ivan Hunter-Duvar, Hosptial for Sick Children, Toronto

Outer hair cells, supported by Deiter’s cells, form “columns” between thebasilar membrane and the reticular lamina.

13

Stereocilia

From Gelfand (1998)

Stereocilia on inner (left) and outer (right) hair cells. Stereocilia are arrangedin curved or v-shaped rows that face toward the modiolus.

14

Arrangement of stereocilia

From Schneider et al. (2002)

Each row of stereocilia is taller than the next. The tip of each stereocilium islinked to the side of the stereocilium behind it by a tip link.

15

Another view...

From Gelfand (1998)

16

Innervation of the organ of Corti

Nerve fibershttp://btnrh.boystown.org/cel/inside.htm/

Nerve fibers exit the organ of Corti on the modiolar side.

17

Neuron review

From Gelfand (1998)

In the auditory nerve, the dendrites contact the hair cells. The cell bodies formwhat is called the spiral ganglion, and the axons for the auditory nerve thatconnects the ear to the brainstem. The “contact” points between the dendritesand the hair cells or between the axons of one neuron and the dendrites ofanbother are called synapses. Synaposes have specialized structures andsubstances that allow communication between recpetors and neurons orbetween neurons.

18

Action potentials

From Kandel et al. (1991)

When a neuron’s intracellular electrical potential is changed enough by releaseof neurotransmitter at a synapse (or in some other way), an abrupt change inelectrical potential, an “action potential”, occurs. Action potentials aretransmitted along the axon to another synapse, where neurotransmitter isreleased and an action potential may be generated in the neuron on the otherside of the synapse (postsynaptic neuron).

19

The spiral ganglion

From Pickles (1992)

The cell bodies of the neurons that form the auditory nerve are located withinthe cochlear modiolus. The collection of cell bodies is called the spiralganglion.

20

Pattern of afferent innervation

From Gelfand (1998)

Different types of nerve fibers innervate IHCs and OHCs. Type I fibersinnervate IHCs; Type II neuorns innervate OHCs.

21

Pattern of afferent innervation

From Pickles (1992)

Nearly all of the nerve fibers that carry messages from the ear to the braininnervate inner hair cells. Notice that the many nerve fibers that contact oneinner hair cell do not branch to other inner hair cells. Each IHC has its own“private” set of fibers. The Type II nerve fibers innervate many OHCs. and theOHCs they innervate are basal to the point at which the nerve fiber enters thecochlea.

22

But there aredifferencesamong the

fibersinnervatingan inner hair

cell

From Gelfand (1998)

Thin fibers attach toward modiolar side, thick fibers toward outer side of IHC.

23

Pattern of efferent innervation

From Gelfaqnd (1998)

Neurons from the brainstem also contact hair cells. These neurons carryinformation from the brain to the ear and are called efferent neurons. The vastmajority of efferents innervate OHCs, and the contacts on OHCs differ fromthose on IHCs. Efferents form large calyx-shaped contacts on the OHC cellbody; efferents form small bouton-like contacts on the afferent nerve fibersthat contact IHCs.

24

Sources of efferent cochlearinnervation

From Gelfand (1998)

The nuclei shown here are in a part of the brainstem called the superior olivarycomplex. Fibers from both sides of the brain innervate both IHCs and OHCs,but the fibers innervating the two types of HC originate in different places.One recent study suggests that the SOC receives input from auditory cortex--so fairly high level processing. The fiber tract containing the efferent fibers isknown as the olivocochlear bundle (OCB). The tract from the same side of thebrain is called the uncrossed OCB and the tract from the opposite side of thebrain is called the crossed OCB.

25

Conclusions• The cochlea contains an array of highly

specialized cells arranged in a highlyspecialized manner.

• There are structural differences betweenIHCs and OHCs that suggest that they differin function

• The cochlea not only sends a message to thebrain, but it may also receive messagesfrom the brain via efferent innervation.

26

Text sources• Gelfand, S.A. (1998) Hearing: An introduction to

psychological and physiological acoustics. New York:Marcel Dekker.

• Kandel, E.R., Schwartz, J.H., & Jessell, T.M. (1991)Principles of neural science. Norwalk CT: Appleton &Lange.

• Pickles, J.O. (1988) An introduction to the physiology ofhearing. Berkeley: Academic Press.

• Schneider, M.E., Belyantseva, I.A., Azevedo, R.B. &Kachar, B. (2002) Structural cell biology: Rapid renewalof auditory hair bundles. Nature, 418:837-838.

• Yost, W.A. (1994) Fundamentals of hearing: anintroduction. San Diego: Academic Press.