Cochlear Anatomy and PhysiologyCochlear Anatomy and Physiology

By: Okhovat, Hanif, MD, Junior ENT residentBy: Okhovat, Hanif, MD, Junior ENT resident

ESFAHAN ORL HNS departmentESFAHAN ORL HNS department

Ordibehesht, 1388Ordibehesht, 1388

Essentials of Anatomy:Essentials of Anatomy:

• Osseous LabyrinthOsseous Labyrinth• ModiolusModiolus • scala vestibuliscala vestibuli : Contains perilymph : Contains perilymph• Helicotrema Helicotrema • scala tympaniscala tympani: Contains perilymph: Contains perilymph• scala media(cochlear duct):scala media(cochlear duct): contains endolymph contains endolymph• perilymphperilymph: extracellular-like fluid, high Na+ and low K+: extracellular-like fluid, high Na+ and low K+• endolymphendolymph: intracellular-like fluid, high K+ and low Na+: intracellular-like fluid, high K+ and low Na+

Membranous labyrinth:Membranous labyrinth:(1) (1) Reissner's membraneReissner's membrane(2) the lateral wall: (2) the lateral wall: spiral ligamentspiral ligament, , stria vascularisstria vascularis (3) the (3) the basilar membrane: basilar membrane: the organ of Cortithe organ of Corti

the organ of Cortithe organ of Corti contains : supporting cells, contains : supporting cells, pillar cellspillar cells, , outer hair cellsouter hair cells, and the , and the inner hair cellsinner hair cells

• Tectorial MembraneTectorial Membrane : an acellular, extracellular matrix : an acellular, extracellular matrix

Sensory CellsSensory Cells

StereociliaStereocilia crucial for hair cell transduction.crucial for hair cell transduction.

Stereocilia: not true cilia Stereocilia: not true cilia long, stiff microvillilong, stiff microvilli

Stereocilia increase in length from the cochlear base Stereocilia increase in length from the cochlear base toward the apex and laterally across the rows of the toward the apex and laterally across the rows of the outer hair cells. outer hair cells.

Tip links occur between the tips of the shorter stereocilia Tip links occur between the tips of the shorter stereocilia and shafts of the longer stereociliaand shafts of the longer stereocilia

Mature cochlear hair cells, unlike vestibular hair cells, do Mature cochlear hair cells, unlike vestibular hair cells, do not contain a kinocilium. not contain a kinocilium.

The outer hair cell stereocilia bundle forms a The outer hair cell stereocilia bundle forms a VV -shaped -shaped or or WW -shaped pattern. -shaped pattern.

The inner hair cell stereocilia form a slightly curved The inner hair cell stereocilia form a slightly curved bundle of three or four rows of stereocilia. bundle of three or four rows of stereocilia.

The tips of the stereocilia of the longest row of the outer The tips of the stereocilia of the longest row of the outer but not the inner hair cells are attached to the but not the inner hair cells are attached to the undersurface of the tectorial membrane. undersurface of the tectorial membrane.

Outer hair cells:Outer hair cells:13,400 outer hair cells, cylindrical13,400 outer hair cells, cylindrical

insert into a cup-shaped insert into a cup-shaped Deiters' Deiters' cellcell body at their basal body at their basal

polepole

surrounded by the fluid-filled surrounded by the fluid-filled spaces of Nuelspaces of Nuel

The cells increase in length The cells increase in length toward the apex and laterally toward the apex and laterally

across the rowsacross the rows

Inner Hair CellsInner Hair Cells differ from the outer hair cells:differ from the outer hair cells:

a single row of flask-shaped a single row of flask-shaped cellscellsInstead of W arrangement of Instead of W arrangement of

stereocilia in OHC, stereocilia in OHC, IHC IHC stereocilia have a stereocilia have a

linear linear patternpattern

Chapter 150 – Chapter 150 –  COCHLEAR TRANSDUCTION AND COCHLEAR TRANSDUCTION AND THE MOLECULAR BASIS OF PERIPHERAL THE MOLECULAR BASIS OF PERIPHERAL

AUDITORY PATHOLOGYAUDITORY PATHOLOGY

Cochlear Mechanics:Cochlear Mechanics:

1)1) PassivePassive

2)2) ActiveActive

PASSIVE COCHLEAR MECHANICSPASSIVE COCHLEAR MECHANICS

Fundamental nature of cochlear mechanics is evident even in Fundamental nature of cochlear mechanics is evident even in cadaverscadavers

Not requiring adenosine triphosphate (ATP) Not requiring adenosine triphosphate (ATP)

Components of the principal elements underlying passive Components of the principal elements underlying passive mechanical transduction:mechanical transduction:1)Basilar membrane1)Basilar membrane : the central structure of the end organ : the central structure of the end organ both functionally and anatomically.both functionally and anatomically.2)scala vestibuli2)scala vestibuli and and scala tympani: scala tympani: filled with perilymphfilled with perilymph3)scala media: 3)scala media: filled with endolymphfilled with endolymph4)tectorial membrane4)tectorial membrane

Reissner's membraneReissner's membrane : important homeostatic role : important homeostatic role No role as an element of passive cochlear mechanics. No role as an element of passive cochlear mechanics.

sound pressure waves are delivered to the sound pressure waves are delivered to the scala vestibuli scala vestibuli through through oval window oval window

propagate through the incompressible cochlear fluids at a velocity propagate through the incompressible cochlear fluids at a velocity of 1.5 km/secof 1.5 km/sec

the spread of the pressure wave throughout the cochlea is nearly the spread of the pressure wave throughout the cochlea is nearly instantaneous. instantaneous.

the higher pressure (or lower, depending on the direction of motion the higher pressure (or lower, depending on the direction of motion of the stapedial footplate) in of the stapedial footplate) in scala vestibuliscala vestibuli relative to relative to scala scala tympanitympani produces a pressure differential across the cochlear produces a pressure differential across the cochlear partition that set the partition into motion partition that set the partition into motion

How does cochlea have TONOTOPIC properties?How does cochlea have TONOTOPIC properties?

1) the width of the basilar membrane progressively increases toward 1) the width of the basilar membrane progressively increases toward apexapex the width at the base is 100 µm and 500 µm near the apex.the width at the base is 100 µm and 500 µm near the apex.

2) the number of cells lining the basilar membrane increases along the 2) the number of cells lining the basilar membrane increases along the basoapical length basoapical length

3) the length of OHCs and stereocilia increase toward apex. 3) the length of OHCs and stereocilia increase toward apex. 4) the thickness of the basilar membrane and the density of filaments 4) the thickness of the basilar membrane and the density of filaments

decrease from base to apex decrease from base to apex

the basilar membrane and organ of Corti complex is stiffer and less the basilar membrane and organ of Corti complex is stiffer and less massive at the base than at the apexmassive at the base than at the apex

The result is an end organ with resonant properties that are The result is an end organ with resonant properties that are

continuously distributed from base to apex continuously distributed from base to apex

Basilar membrane filters complex fluid waves into sinusoidal Basilar membrane filters complex fluid waves into sinusoidal constituents.constituents.

As a result, high-frequency acoustic events are preferentially As a result, high-frequency acoustic events are preferentially

transduced in the base, because it is stiff and less massive. The transduced in the base, because it is stiff and less massive. The inverse is true in the case of low-frequency acoustic events. inverse is true in the case of low-frequency acoustic events.

Other physical characteristics of the cochlea : Other physical characteristics of the cochlea : 1)the viscous friction between the basilar membrane and 1)the viscous friction between the basilar membrane and cochlear fluids produces a damping effect that limits its cochlear fluids produces a damping effect that limits its displacement.displacement.

2)The traveling wave propagates basoapically, 2)The traveling wave propagates basoapically, because the because the stiffness of the cochlear partition decreases longitudinally stiffness of the cochlear partition decreases longitudinally from the base to the apex as its mass increases. from the base to the apex as its mass increases.

The motion and direction of the wave that travels along the basilar The motion and direction of the wave that travels along the basilar membrane is independent of the location of the vibratory membrane is independent of the location of the vibratory source, because :source, because :

the stiffer and less massive elements forming the base of the stiffer and less massive elements forming the base of the organ of Corti are set into motion the organ of Corti are set into motion beforebefore the more the more

compliant and massive sections of the apical cochlea compliant and massive sections of the apical cochlea

It forms a frequency decomposition system: the space-frequency It forms a frequency decomposition system: the space-frequency or or tonotopic maptonotopic map

Near the characteristic place for a Near the characteristic place for a given stimulus frequency, the given stimulus frequency, the pressure wave returns to the pressure wave returns to the basal end of the cochlea basal end of the cochlea through the fluids of the scala through the fluids of the scala tympani, where the flexible tympani, where the flexible round window membrane is round window membrane is displaced in an equal but displaced in an equal but opposite direction of the stapes opposite direction of the stapes footplate footplate

Helicotrema:Helicotrema: allowing the fluids in the allowing the fluids in the scala scala vestibulivestibuli and and scala tympaniscala tympani to to communicate communicate directlydirectly

acts as an acoustic shunt across the acts as an acoustic shunt across the cochlear partitioncochlear partition

reduces the pressure difference reduces the pressure difference between the between the scalaescalae produced by produced by very-low-frequency stimulationvery-low-frequency stimulation

The size of the helicotrema determines The size of the helicotrema determines the cutoff frequency of the systemthe cutoff frequency of the system

large openings shunt pressure large openings shunt pressure waves waves extending to higher extending to higher frequencies than frequencies than do small do small helicotrema. helicotrema.

decrease damage to the cochlea decrease damage to the cochlea produced by intense low-produced by intense low-frequency pressure fluctuationsfrequency pressure fluctuations

Brace YourselfBrace Yourself

IT’S SHOW TIME!IT’S SHOW TIME!

ACTIVE COCHLEAR MECHANICSACTIVE COCHLEAR MECHANICS

Why bother with ‘Why bother with ‘active mechanicsactive mechanics’ theory?’ theory?

1)Traveling wave measured in cochleae 1)Traveling wave measured in cochleae from cadavers is from cadavers is so broadly tunedso broadly tuned as to as to be inconsistent with a living individual's be inconsistent with a living individual's ability to discriminate closely spaced ability to discriminate closely spaced frequenciesfrequencies

2)Sharp tuning and high sensitivity are lost 2)Sharp tuning and high sensitivity are lost in cochleae from cadavers in cochleae from cadavers

3)The peak of the traveling wave in living 3)The peak of the traveling wave in living animals is animals is very sharply tuned at low very sharply tuned at low levels of stimulationlevels of stimulation and exhibits and exhibits nonlinear growth as sound levels nonlinear growth as sound levels increaseincrease

4)Basilar membrane vibration relative to 4)Basilar membrane vibration relative to stapes displacement is as much as two stapes displacement is as much as two to three orders of magnitude greater to three orders of magnitude greater under low stimulus level conditions than under low stimulus level conditions than at higher levelsat higher levels

So, cochlear mechanics are nonlinearSo, cochlear mechanics are nonlinear

Active mechanics is a metabolically labile, energy-consuming Active mechanics is a metabolically labile, energy-consuming process process

the active mechanical event underlying amplification is highly the active mechanical event underlying amplification is highly localized. localized.

the traveling wave peaks at increasingly apical regions of the the traveling wave peaks at increasingly apical regions of the cochlear partition with increasing stimulus level, shifting the cochlear partition with increasing stimulus level, shifting the tonotopic map toward lower frequencies at any given place. tonotopic map toward lower frequencies at any given place.

Role of Outer Hair Cells in Active Role of Outer Hair Cells in Active MechanicsMechanics

Destruction of OHCs causes:Destruction of OHCs causes:1)diminished cochlear sensitivity1)diminished cochlear sensitivity2)Loss of frequency selectivity2)Loss of frequency selectivity3)loss of nonlinear operating 3)loss of nonlinear operating properties properties

only 5% of all auditory nerve fibers only 5% of all auditory nerve fibers innervate OHCs though there are at innervate OHCs though there are at least three times as many OHCs as least three times as many OHCs as IHCs IHCs

These findings suggested that the These findings suggested that the transmission of sensory information transmission of sensory information to CNS is not the primary function to CNS is not the primary function of OHCs. of OHCs.

when OHCs were lost, with IHCs appearing unaffected, some cochlear when OHCs were lost, with IHCs appearing unaffected, some cochlear functions were affected : functions were affected :

1)cochlear sensitivity reduced 1)cochlear sensitivity reduced

2)frequency selectivity reduced2)frequency selectivity reduced

3)input-output curves acquired a more linear property in the region of 3)input-output curves acquired a more linear property in the region of OHC damage. OHC damage.

4)two-tone suppression (i.e., the reduction in the response to one 4)two-tone suppression (i.e., the reduction in the response to one tone elicited by the presence of another tone) diminishedtone elicited by the presence of another tone) diminished

5)distortion tones (i.e., the appearance of additional tones not present 5)distortion tones (i.e., the appearance of additional tones not present in the stimulus produced by the interaction of two or more in the stimulus produced by the interaction of two or more

primary tones, such as 2f1-f2 or f2-f1, where f1 and f2 primary tones, such as 2f1-f2 or f2-f1, where f1 and f2 represent represent the primary tones with f1 < f2) are lostthe primary tones with f1 < f2) are lost

OAEOAE : evidence that an active, OHC-driven mechanism : evidence that an active, OHC-driven mechanism operates within the cochlea, which is capable of producing operates within the cochlea, which is capable of producing energy energy

OCB(olivocochlear bundle)OCB(olivocochlear bundle) activation altered activation altered cochlear cochlear outputoutput as well as as well as otoacoustic emissionsotoacoustic emissions and and basilar basilar membrane displacementmembrane displacement

Outer hair cell motility induced by electrical stimulation of Outer hair cell motility induced by electrical stimulation of the excised cochlea displaced both the reticular lamina the excised cochlea displaced both the reticular lamina and the basilar membrane. and the basilar membrane.

OHCs contract or elongate at very high rates (up to at least 70 kHz) OHCs contract or elongate at very high rates (up to at least 70 kHz) along the length of their longitudinal axes when depolarized or along the length of their longitudinal axes when depolarized or hyperpolarized, respectivelyhyperpolarized, respectively

OHCs amplify the displacement of the basilar membrane OHCs amplify the displacement of the basilar membrane Recently, the voltage-sensitive motor protein underlying Recently, the voltage-sensitive motor protein underlying the process the process

was identified and given the name was identified and given the name prestinprestin

intracellular Cl- ions act as extrinsic charged voltage sensors. intracellular Cl- ions act as extrinsic charged voltage sensors.

Salicylates, which are known to cause temporary hearing loss when Salicylates, which are known to cause temporary hearing loss when administered at high doses, are permeable to the plasma membrane administered at high doses, are permeable to the plasma membrane and act as competitive antagonists of Cl-, thereby reducing and act as competitive antagonists of Cl-, thereby reducing electromotility and, diminishing sensitivity. electromotility and, diminishing sensitivity.

Conversion of Basilar Conversion of Basilar Membrane Membrane Displacement to Displacement to Radial Shearing Radial Shearing ForcesForces

Reticular laminaReticular lamina: interdigitating heads of : interdigitating heads of the the inner and outer pillar cells, the inner and outer pillar cells, the apical surfaces of the phalangeal apical surfaces of the phalangeal processes of Deiters' cells, and the processes of Deiters' cells, and the apical surfaces of the hair cellsapical surfaces of the hair cells through which the stereocilia projectthrough which the stereocilia project

Reticular laminaReticular lamina is a relatively rigid is a relatively rigid structure that moves in concert with structure that moves in concert with basilar membrane motion. basilar membrane motion.

The tectorial membrane is rigid structure The tectorial membrane is rigid structure firmly attached to the spiral limbus firmly attached to the spiral limbus and overlies the reticular laminaand overlies the reticular lamina

. .

The The tallest row of stereociliatallest row of stereocilia protrudes from the apical surface of outer hair protrudes from the apical surface of outer hair cell and embeds within the tectorial membranecell and embeds within the tectorial membrane

the shearing motion between the reticular lamina and the tectorial membrane the shearing motion between the reticular lamina and the tectorial membrane

causes stereocilia to bend in the direction of the causes stereocilia to bend in the direction of the modiolusmodiolus or the or the spiral spiral limbuslimbus, depending on whether the basilar membrane is displaced toward , depending on whether the basilar membrane is displaced toward the the scala tympaniscala tympani or or scala vestibuliscala vestibuli, respectively., respectively.

In contrast to OHCs, the stereocilia of IHCs do not appear to firmly contact the In contrast to OHCs, the stereocilia of IHCs do not appear to firmly contact the tectorial membrane tectorial membrane

Therefore, during basilar membrane vibration, the mechanical stimulus to IHC Therefore, during basilar membrane vibration, the mechanical stimulus to IHC stereocilia is the flow of endolymph within the subtectorial space(the stereocilia is the flow of endolymph within the subtectorial space(the channel between the reticular lamina and tectorial membrane).channel between the reticular lamina and tectorial membrane).

displacement of the basilar membrane results in a radial shearing motion displacement of the basilar membrane results in a radial shearing motion between the reticular lamina and the tectorial membrane, a motion that between the reticular lamina and the tectorial membrane, a motion that serves as the mechanical trigger of transduction currentsserves as the mechanical trigger of transduction currents

Radial Displacement Patterns of the Radial Displacement Patterns of the Basilar MembraneBasilar Membrane

The foot of the inner pillar cell lies near The foot of the inner pillar cell lies near the bony spiral laminathe bony spiral lamina

The foot of the outer pillar cell lies over The foot of the outer pillar cell lies over the basilar membrane and is not the basilar membrane and is not supported by bone. supported by bone.

When the basilar membrane is displaced When the basilar membrane is displaced in a in a passive cochleapassive cochlea, movement , movement occurs maximally near the foot of the occurs maximally near the foot of the outer pillar cellouter pillar cell

When OHCs contract, the reticular When OHCs contract, the reticular lamina pivots at the apex of the lamina pivots at the apex of the tunnel of Corti, and the basilar tunnel of Corti, and the basilar membrane and reticular lamina are membrane and reticular lamina are pulled together, enhancing overall pulled together, enhancing overall basilar membrane displacement.basilar membrane displacement.

Greatest motion in active cochlea is in Greatest motion in active cochlea is in the domain of the OHC the domain of the OHC

HAIR CELL TRANSDUCTION and stereociliaHAIR CELL TRANSDUCTION and stereocilia

approximately 3500 IHCs and 12,000 OHCs approximately 3500 IHCs and 12,000 OHCs

Stereocilia: not true cilia Stereocilia: not true cilia more appropriately: specialized microvilli more appropriately: specialized microvilli

kinocilia are on the surface of immature auditory hair cells of mammals kinocilia are on the surface of immature auditory hair cells of mammals but are lost during the final stages of differentiation. but are lost during the final stages of differentiation.

Approximately 50 to 70 stereocilia protrude from each IHC and about 150 Approximately 50 to 70 stereocilia protrude from each IHC and about 150 from each OHC from each OHC

Toward Apex:Toward Apex:1)the number of stereocilia 1)the number of stereocilia on on OHCs decreases to less OHCs decreases to less

than halfthan half

2)the lengths of the 2)the lengths of the stereocilia stereocilia of both IHCs and of both IHCs and OHCs OHCs increaseincrease

3)The lengths of OHCs 3)The lengths of OHCs increase, increase, whereas the whereas the lengths of IHCs lengths of IHCs remain fairly consistentremain fairly consistent

Individual hair bundles are Individual hair bundles are

organized into three rows of organized into three rows of stereocilia stereocilia

Arranged linearly in the case of Arranged linearly in the case of IHCs IHCs

In the form of a "In the form of a "WW" in the case of " in the case of OHCs, with the base of the "OHCs, with the base of the "WW" " facing the lateral wall of the facing the lateral wall of the cochlea cochlea

Stereocilia within each row are Stereocilia within each row are approximately the same heightapproximately the same height

The longest row is located on the The longest row is located on the side of the cell that is proximal to side of the cell that is proximal to the spiral ligament the spiral ligament

The length of stereocilia decreases The length of stereocilia decreases in an orderly manner in rows that in an orderly manner in rows that are located progressively are located progressively proximal to the modiolus, proximal to the modiolus, producing a producing a stairstep stairstep configuration configuration

The stereocilium tapers near its The stereocilium tapers near its point of insertion into the surface point of insertion into the surface of the receptor cellof the receptor cell

The stereocilia pivots, as opposed The stereocilia pivots, as opposed to flexing, near the surface of to flexing, near the surface of the receptor cell, acting like a the receptor cell, acting like a stiff lever when mechanically stiff lever when mechanically distorted. distorted.

Hair Bundle Deflections and Receptor PotentialsHair Bundle Deflections and Receptor Potentials

Magnitude of the receptor potential is proportional to the degree Magnitude of the receptor potential is proportional to the degree of stereociliary deflection in the most sensitive region of its of stereociliary deflection in the most sensitive region of its operating rangeoperating range

Saturates with larger deflections in either direction. Saturates with larger deflections in either direction.

When the bundle is deflected toward the tallest stereocilia, the When the bundle is deflected toward the tallest stereocilia, the hair cell depolarizes, hair cell depolarizes, Deflections of the bundle in the direction opposite the tallest Deflections of the bundle in the direction opposite the tallest stereocilia hyperpolarize the cell stereocilia hyperpolarize the cell

Displacement-voltage curve is highly asymmetric relative to its Displacement-voltage curve is highly asymmetric relative to its resting stateresting state

responses saturate with smaller deflections directed responses saturate with smaller deflections directed away away from the taller stereocilia than in the excitatory or from the taller stereocilia than in the excitatory or depolarizing directiondepolarizing direction

Deflection of the hair bundle in a direction perpendicular to this Deflection of the hair bundle in a direction perpendicular to this axis produces no response. axis produces no response.

Hair Cell Transduction ChannelsHair Cell Transduction Channels

The latency of hair cell response to displacement: 40 μsecThe latency of hair cell response to displacement: 40 μsec

Activation time is inversely related to the magnitude of bundle Activation time is inversely related to the magnitude of bundle displacement displacement

enzymatic systems or second messengers are too slow to underlie enzymatic systems or second messengers are too slow to underlie the transducer responsethe transducer response

transduction channels operate in either an open or a closed state, transduction channels operate in either an open or a closed state, with the probability of being in one or the other state depending with the probability of being in one or the other state depending on the degree of hair bundle deflection.on the degree of hair bundle deflection.

the asymmetric displacement-voltage curve described previously is the asymmetric displacement-voltage curve described previously is explained by the fact that a small population of transduction explained by the fact that a small population of transduction channels, roughly 10% to 15%, is in the open state at rest. channels, roughly 10% to 15%, is in the open state at rest.

transduction channels are situated near the tips of the stereocilia transduction channels are situated near the tips of the stereocilia

Each stereocilia has one transduction channels and transduction Each stereocilia has one transduction channels and transduction channels are present in stereocilia of all heights within a bundle channels are present in stereocilia of all heights within a bundle

Gating of Transduction ChannelsGating of Transduction Channels

tip links stretch between the distal tip tip links stretch between the distal tip of each stereocilium with the side of each stereocilium with the side of the longer adjacent stereocilium of the longer adjacent stereocilium

transduction channels are located at transduction channels are located at each end of the tip link each end of the tip link

When the bundle is deflected toward When the bundle is deflected toward the taller stereocilia, tip links are the taller stereocilia, tip links are stretched and physically open stretched and physically open transduction channels, causing transduction channels, causing excitationexcitation

When the bundle is deflected away When the bundle is deflected away from the taller stereocilia, tip links from the taller stereocilia, tip links relax, causing the channel to close. relax, causing the channel to close.

Movement of the bundle Movement of the bundle perpendicular to the axis of perpendicular to the axis of symmetry produces little or no symmetry produces little or no responseresponse

Acoustic overstimulation can cause tip link Acoustic overstimulation can cause tip link breakagebreakage

Tip links broken as a result of exposure to BAPTA Tip links broken as a result of exposure to BAPTA regenerate within 24 hours, restoring regenerate within 24 hours, restoring mechanoelectrical transduction in the process.mechanoelectrical transduction in the process.

Perhaps a similar dynamic occurs in the case of Perhaps a similar dynamic occurs in the case of recovery from temporary threshold shifts recovery from temporary threshold shifts associated with acoustic overstimulation. associated with acoustic overstimulation.

Transduction Channel Ion SelectivityTransduction Channel Ion Selectivity

Hair cell transduction channels are Hair cell transduction channels are nonselectively permeablenonselectively permeable

K+ carries most of the receptor current K+ carries most of the receptor current

When transduction channels open, K+ When transduction channels open, K+ in the endolymph is flushed down a in the endolymph is flushed down a large electrochemical gradientlarge electrochemical gradient

This inward transducer current flows This inward transducer current flows across the across the basolateral membranebasolateral membrane, , producing the receptor potential producing the receptor potential and and voltage-gated Ca2+ channelsvoltage-gated Ca2+ channels are activated, resulting in an influx are activated, resulting in an influx of Ca2+ and release of of Ca2+ and release of neurotransmitter from the base of neurotransmitter from the base of the hair cell. the hair cell.

Osseous Labyrinth: Osseous Labyrinth: surrounds surrounds the internal membranous the internal membranous labyrinthlabyrinth

Modiolus: Modiolus: internal bony tubeinternal bony tube

communicates with the communicates with the endolymphatic endolymphatic compartments by compartments by way of way of the ductus reuniens.the ductus reuniens.

Chapter 149 – Chapter 149 –  COCHLEAR ANATOMY AND CENTRAL AUDITORY PATHWAYSCOCHLEAR ANATOMY AND CENTRAL AUDITORY PATHWAYS

Membranous LabyrinthMembranous Labyrinth cochlear duct is divided into three regions: cochlear duct is divided into three regions:

(1) (1) Reissner's membraneReissner's membrane(2) the lateral wall ,which includes the (2) the lateral wall ,which includes the spiral ligamentspiral ligament, , stria vascularisstria vascularis, , spiral spiral

prominenceprominence, and the , and the external sulcusexternal sulcus(3) the (3) the basilar membranebasilar membrane and and osseous spiral laminaosseous spiral lamina, includes , includes Claudius' cellsClaudius' cells; ;

Boettcher's cellsBoettcher's cells; the ; the organ of Corti;inner sulcus;organ of Corti;inner sulcus; and and spiral limbusspiral limbus. .

the organ of Corti contains : the organ of Corti contains : Hensen's cellsHensen's cells, , Deiters' cellsDeiters' cells, , pillar cellspillar cells, , inner inner border cellsborder cells, , outer hair cellsouter hair cells, and , and the inner hair cellsthe inner hair cells

spiral limbusspiral limbus contains : the contains : the interdental cellsinterdental cells and overlying and overlying tectorial membranetectorial membraneRosenthal's canalRosenthal's canal contains : the contains : the spiral ganglionspiral ganglion and communicates with the and communicates with the

modiolusmodiolus

(1)Reissner's Membrane(1)Reissner's Membrane (vestibular membrane) (vestibular membrane)

a three-layered structure. a three-layered structure.

Attached medially to the modiolar edge of the spiral limbus and laterally to the spiral Attached medially to the modiolar edge of the spiral limbus and laterally to the spiral ligament at the apical edge of the stria vascularis.ligament at the apical edge of the stria vascularis.

The cells facing endolymph have a low-cuboidal shape, exhibit numerous apical microvilli, The cells facing endolymph have a low-cuboidal shape, exhibit numerous apical microvilli, and are sealed along their lateral borders by tight junctions. and are sealed along their lateral borders by tight junctions.

The cells facing perilymph have a squamous shape and are loosely joined together. The cells facing perilymph have a squamous shape and are loosely joined together. Reissner's membrane seems to be freely permeable to water, but it restricts paracellular Reissner's membrane seems to be freely permeable to water, but it restricts paracellular passage of molecules into the endolymphatic space by tight junctions.passage of molecules into the endolymphatic space by tight junctions.

hydrops : In certain pathologic conditions (i.e., Ménière's disease), Reissner's membrane hydrops : In certain pathologic conditions (i.e., Ménière's disease), Reissner's membrane bulges out into the space of the scala vestibuli.bulges out into the space of the scala vestibuli.

(2)Lateral Wall(2)Lateral Wall

A.A.Spiral LigamentSpiral Ligament

greatest portion of the lateral greatest portion of the lateral wall of the cochlear ductwall of the cochlear ductconsists of cells rich in ion-consists of cells rich in ion-transporting enzymes.transporting enzymes.lateral boundary: inner surface lateral boundary: inner surface of the otic capsuleof the otic capsulemedial boundary: formed by medial boundary: formed by the stria vascularis and the the stria vascularis and the spiral prominence. spiral prominence. extends into the scalae extends into the scalae vestibuli and tympani and vestibuli and tympani and forms a lateral route of forms a lateral route of communication between these communication between these two perilymphatic channels. two perilymphatic channels. The cells of the spiral The cells of the spiral ligament : recycling of K+ from ligament : recycling of K+ from endolymph through the hair endolymph through the hair cells and into to the stria cells and into to the stria vascularis(source of vascularis(source of endolymphatic K+)endolymphatic K+)Anchoring cells: exert tension Anchoring cells: exert tension on the basilar membrane,at on the basilar membrane,at the junction of the spiral the junction of the spiral ligament and the otic capsule.ligament and the otic capsule.

B.B.Stria Vascularis Stria Vascularis

forms the endolymphatic border forms the endolymphatic border of the cochlear duct. of the cochlear duct. extends from the attachment of extends from the attachment of Reissner's membrane to the Reissner's membrane to the spiral prominence. spiral prominence. lacks a basement membrane. lacks a basement membrane. three cell types (marginal, three cell types (marginal, intermediate, and basal cells) intermediate, and basal cells) and intraepithelial capillaries. and intraepithelial capillaries.

The marginal cells : The marginal cells : primary functional units of the primary functional units of the stria vascularis and produce the stria vascularis and produce the positive DC positive DC endocochlear potential and the endocochlear potential and the low-sodium, high-potassium ion low-sodium, high-potassium ion concentration of concentration of

cochlear endolymph. cochlear endolymph. The intermediate cells: The intermediate cells:

melanocytes, phagocytic melanocytes, phagocytic activity, contain carbonic activity, contain carbonic anhydrase enzyme.anhydrase enzyme.

C. Spiral Prominence C. Spiral Prominence

D. External(outer) Sulcus : contain D. External(outer) Sulcus : contain carbonic anhydrase.carbonic anhydrase.

(3)Basilar Membrane(3)Basilar Membrane from the lateral edge of the osseous spiral laminafrom the lateral edge of the osseous spiral laminabasilar crestbasilar crest : wedge shaped tissue, insertion of basilar membrane into the spiral ligament : wedge shaped tissue, insertion of basilar membrane into the spiral ligamentspiral length averages 31.5 mm. spiral length averages 31.5 mm. width increases toward the cochlear apex from 150 to 450 µm.width increases toward the cochlear apex from 150 to 450 µm.width divided into : 1)pars arcuata(medial) 2)pars pectinata(lateral)width divided into : 1)pars arcuata(medial) 2)pars pectinata(lateral)the thickness of the pars pectinata decreases toward the apex the thickness of the pars pectinata decreases toward the apex Changes in the width and thickness of the basilar membrane along its length are Changes in the width and thickness of the basilar membrane along its length are responsible for the propagation of the traveling wave and responsible for the propagation of the traveling wave and the frequency-specific maximal the frequency-specific maximal vibrations of the membrane.vibrations of the membrane.vas spirale: vas spirale: A spirally directed vessel ,beneath the inner tunnel of Corti, rudimentaryA spirally directed vessel ,beneath the inner tunnel of Corti, rudimentaryexchange of metabolites of the organ of Corti : by means of diffusion of perilymph through exchange of metabolites of the organ of Corti : by means of diffusion of perilymph through the basilar membrane and spaces of Nuel.the basilar membrane and spaces of Nuel.

C. Organ of CortiC. Organ of Corti primary function: convert mechanical vibrations to primary function: convert mechanical vibrations to neural impulses neural impulses recent evidence : hair cells are able to recent evidence : hair cells are able to mechanoelectrically tune the output of the organ mechanoelectrically tune the output of the organ of Corti. of Corti. lateral to medial components: lateral to medial components:

Hensen's cells, Hensen's cells, outer tunnel of Corti, outer tunnel of Corti, three to four rows of outer hair cells, three to four rows of outer hair cells, Deiters' cells with phalangeal Deiters' cells with phalangeal

processes, processes, spaces of Nuel, spaces of Nuel, outer pillar cells, outer pillar cells, inner tunnel of Corti, inner tunnel of Corti, inner pillar cells, inner pillar cells, inner hair cells, inner hair cells, inner phalangeal cells, inner phalangeal cells, inner border cells . inner border cells .

reticular laminareticular lamina : A rigid platelike structure formed : A rigid platelike structure formed by the apical processes of the supporting and by the apical processes of the supporting and sensory cells of the organ of Corti. sensory cells of the organ of Corti. Toward the cochlear apex, increases occur in : Toward the cochlear apex, increases occur in : 1)the radial area of the organ of Corti, 2)the 1)the radial area of the organ of Corti, 2)the length of the inner and outer hair cell bodies and length of the inner and outer hair cell bodies and their stereocilia, 3)the angle and width of the their stereocilia, 3)the angle and width of the apical surface of the organ of Corti relative to the apical surface of the organ of Corti relative to the basilar membrane, 4)the length of the pillar cell basilar membrane, 4)the length of the pillar cell head plates, 5)the height of Hensen's cells.head plates, 5)the height of Hensen's cells.

A. Claudius' Cells A. Claudius' Cells Claudius' cells form a tight cellular border between endolymph of the scala media and perilymph of the scala tympani.Claudius' cells form a tight cellular border between endolymph of the scala media and perilymph of the scala tympani.

B. Boettcher's Cells B. Boettcher's Cells Boettcher's cells occur most frequently in the cochlear base Boettcher's cells occur most frequently in the cochlear base decrease in number completely toward the apex. decrease in number completely toward the apex. to produce fibronectin and other matrix components for the basilar membrane. to produce fibronectin and other matrix components for the basilar membrane. contain carbonic anhydrase.contain carbonic anhydrase.

Supporting CellsSupporting Cells

Hensen's Cells Hensen's Cells not part of the reticular lamina. not part of the reticular lamina. tectal cells : Hensen’s cells that tectal cells : Hensen’s cells that do not reach the endolymphatic do not reach the endolymphatic space. space. outer tunnel of Corti : a fluid outer tunnel of Corti : a fluid filled space Between the last filled space Between the last row of outer hair cells and row of outer hair cells and Hensen's cells. Hensen's cells.

Deiters' Cells Deiters' Cells Deiters' cells support the outer hair cells at their base and apex.Deiters' cells support the outer hair cells at their base and apex.phalangeal process: a thin process containing microtubules and filaments,, phalangeal process: a thin process containing microtubules and filaments,,

branches off the Deiters' cell branches off the Deiters' cell body and extends obliquely to the apical process body and extends obliquely to the apical process of adjacent outer hair cells. of adjacent outer hair cells.

reticular laminareticular lamina : This dumbbell-shaped apical pole joins together four : This dumbbell-shaped apical pole joins together four different outer hair cells in a rigid different outer hair cells in a rigid platelike array called the reticular laminaplatelike array called the reticular lamina. .

spaces of Nuel: The fluid-filled spaces between the outer hair cell bodies spaces of Nuel: The fluid-filled spaces between the outer hair cell bodies and the phalangeal processes of and the phalangeal processes of the Deiters' cells.the Deiters' cells.

Pillar Cells Pillar Cells tunnel of Corti : The inner tunnel of Corti : The inner and outer pillar cells and outer pillar cells oppose each other and oppose each other and form tunnel of corti.form tunnel of corti.the reticular lamina. the reticular lamina. Each pillar cell has a Each pillar cell has a broad base. The basilar broad base. The basilar membrane, spaces of membrane, spaces of Nuel, and the inner Nuel, and the inner tunnel of Corti probably tunnel of Corti probably contain perilymphcontain perilymph

Inner Border and Phalangeal Cells Inner Border and Phalangeal Cells The inner border and phalangeal cells separate the inner The inner border and phalangeal cells separate the inner

sulcus cells from the medial surface of the inner hair cells. sulcus cells from the medial surface of the inner hair cells. The inner border cells form the most medial edge of the The inner border cells form the most medial edge of the

organ of Corti. organ of Corti. Basally, the inner phalangeal cells envelop unmyelinated Basally, the inner phalangeal cells envelop unmyelinated

nerve fibers associated with the inner hair cell.nerve fibers associated with the inner hair cell.

Inner SulcusInner Sulcus spirally directed open channelspirally directed open channelbounded by the lateral edge of bounded by the lateral edge of

the spiral limbus, by the the spiral limbus, by the medial edge of the organ of medial edge of the organ of Corti, and apically by the Corti, and apically by the tectorial membrane. tectorial membrane.

Spiral LimbusSpiral Limbus spirally directed shelf of vascularized connective tissue that overlies the spirally directed shelf of vascularized connective tissue that overlies the medial region of the osseous spiral lamina. medial region of the osseous spiral lamina. Reissner's membrane is attached at its most medial edge. Reissner's membrane is attached at its most medial edge. Its lateral edge forms a wedge-shaped prominence called the Its lateral edge forms a wedge-shaped prominence called the tooth of tooth of HuschkeHuschke The endolymphatic surface of the spiral limbus is covered by a thin The endolymphatic surface of the spiral limbus is covered by a thin extracellular matrix called the limbal portion of the tectorial membrane. extracellular matrix called the limbal portion of the tectorial membrane. Directly beneath the limbal portion of the tectorial membrane lie interdental Directly beneath the limbal portion of the tectorial membrane lie interdental or T cells. They are or T cells. They are TT -shaped cells whose apical surface forms broad plates -shaped cells whose apical surface forms broad plates across the apical surface of the spiral limbus. across the apical surface of the spiral limbus.

Tectorial MembraneTectorial Membrane an acellular, extracellular matrix an acellular, extracellular matrix overlies the spiral limbus, inner sulcus, overlies the spiral limbus, inner sulcus,

and the organ of Corti. and the organ of Corti. fibrous materials, hydrated by fibrous materials, hydrated by

endolymph.endolymph.divided into six regions: limbal layer, divided into six regions: limbal layer,

fibrous matrix, marginal band, cover fibrous matrix, marginal band, cover net, Hensen's stripe, and Hardesty's net, Hensen's stripe, and Hardesty's or Kimura's membrane. or Kimura's membrane.

main protein of the tectorial main protein of the tectorial membrane: Type II collagenmembrane: Type II collagen

the cover net : On the endolymphatic the cover net : On the endolymphatic surface, radial strands of material surface, radial strands of material extend out toward the margin of the extend out toward the margin of the membrane in an oblique apical membrane in an oblique apical direction. direction.

the marginal bandthe marginal band : The lateral margin : The lateral margin of the membrane forms a of the membrane forms a discontinuous net of materialdiscontinuous net of material

Hardesty's membraneHardesty's membrane : That portion of : That portion of the tectorial membrane that faces the tectorial membrane that faces the organ of Corti, a thin, poorly the organ of Corti, a thin, poorly defined region of material which defined region of material which overlies the outer hair cells overlies the outer hair cells

Osseous Spiral LaminaOsseous Spiral Lamina spirally directed shelf of bonespirally directed shelf of boneextends from the modiolus to the medial extends from the modiolus to the medial

attachment of the basilar attachment of the basilar membrane membrane The interior of the spiral lamina forms a The interior of the spiral lamina forms a channel and a pathway for the channel and a pathway for the nerve fibers that travel to and from nerve fibers that travel to and from the the organ of Corti. organ of Corti. broad in the cochlear base and narrows broad in the cochlear base and narrows toward the apex. toward the apex. At its lateral margin, the bone becomes At its lateral margin, the bone becomes thinner and perforated by channels thinner and perforated by channels called the called the habenulae perforatahabenulae perforata, , where nerve fibers lose myelination where nerve fibers lose myelination and and enter the organ of Cortienter the organ of Corti

VASCULAR SUPPLY VASCULAR SUPPLY The labyrinthine artery : enters the internal auditory The labyrinthine artery : enters the internal auditory

meatus with cranial nerve (CN) VIII as a branch of the meatus with cranial nerve (CN) VIII as a branch of the anterior inferior cerebellar artery. anterior inferior cerebellar artery.

The labyrinthine artery branches to form The labyrinthine artery branches to form common cochlear artery and common cochlear artery and anterior vestibular artery. anterior vestibular artery.

the common cochlear artery gives off two branches: the common cochlear artery gives off two branches: the spiral modiolar artery: travels apicallythe spiral modiolar artery: travels apicallythe vestibulocochlear artery: vascularizes the lower the vestibulocochlear artery: vascularizes the lower basal basal cochlear turn. cochlear turn.

The spiral modiolar artery gives off two coiled branches The spiral modiolar artery gives off two coiled branches (i.e., the radiating arterioles) that travel apically over (i.e., the radiating arterioles) that travel apically over the scala vestibuli and basally to vascularize the the scala vestibuli and basally to vascularize the spiral ganglion and spiral limbus. spiral ganglion and spiral limbus.

The cochlea is drained by the spiral modiolar veinThe cochlea is drained by the spiral modiolar vein