COCHLEAR TRANSDUCTION

INTRODUCTION
three functional units within the cochlea
(1) mechanoelectrical transduction
The organ of Corti represents the "sensor" of the cochlea converting and
amplifying mechanical sound stimuli into electrical signals.
(2) endocochlear potential
The stria vascularis is the cochlea's "battery," generating the energy
mechanoelectrical transduction necessary for mechanoelectrical
transduction and influencing cochlear fluid homeostasis.
(3) electrical wires
The spiral ganglion contains the neurons featuring axons that transport the
electrical signals from the cochlea to the central nervous system

• stapes footplate vibrate
• compressional wave travels
• pressure in the scala vestibuli is higher
• pressure defference creates
• cochlear partition to vibrate
• traveling wave on the basilar membrane

• shearing force between the stereocilia of the hair cells
and the tectorial membrane
• deflection of the hair cell stereocilia
• If deflected in the direction of the tallest row
• tip links to stretch

• opening of gated potassium channels
• hair cell depolarization
• degranulation of neurotransmitter vesicles into
the synaptic terminal
• calcium influx through channels
• propagates an action potential along the auditory
nerve.

PASSIVE COCHLEAR MECHANICS
• Fundamental nature of cochlear mechanics is evident even in cadavers
• Not requiring adenosine triphosphate (ATP)
elements underlying passive mechanical transduction:
1)Basilar membrane
2)scala vestibuli and scala tympani
3)scala media
4)tectorial membrane
Reissner's membrane : important homeostatic role
No role as an element of passive cochlear mechanics.

sound pressure waves are delivered to the scala vestibuli through oval
window
the higher pressure (or lower, depending on the direction
of motion of the stapedial footplate) in scala vestibuli
relative to scala tympani produces a pressure differential
across the cochlear partition that set the partition into
motion

THE BASILAR MEMBRANE AND TONOTOPY
1) the width of the basilar membrane progressively
increases toward apex.
2) the number of cells lining the basilar membrane
increases along the basoapical length
3) the length of OHCs and stereocilia increase toward
apex.
4) the thickness of the basilar membrane and the
density of filaments decrease from base to apex

The basilar membrane and organ of Corti complex is
stiffer and less massive at the base than at the apex
As a result, high-frequency acoustic events are
preferentially transduced in the base, because it is stiff
and less massive. The inverse is true in the case of low-
frequency acoustic events

ACTIVE COCHLEAR MECHANICS
• The peak of the traveling wave in living animals is very
sharply tuned at low levels of stimulation and exhibits
nonlinear growth as sound levels increase
• Basilar membrane vibration relative to stapes
displacement is as much as two to three orders of
magnitude greater under low stimulus level conditions
than at higher levels

So, cochlear mechanics are nonlinear
Active mechanics is a metabolically labile
energy-consuming process
the active mechanical event underlying amplification
is highly localized

Role of Outer Hair Cells in Active
Mechanics
when OHCs were lost, with IHCs appearing unaffected, some
cochlear functions were affected :
1)cochlear sensitivity reduced
2)frequency selectivity reduced
3)input-output curves acquired a more linear property in the
region of OHC damage.

OHCs contract or elongate
• OHCs amplify the displacement of the basilar
membrane
• the voltage-sensitive motor protein underlying the
process was identified and given the name prestin
intracellular Cl- ions act as extrinsic charged voltage
sensors.

Conversion of Basilar Membrane
Displacement to Radial Shearing Forces
• displacement of the basilar membrane results in a
radial shearing motion between the reticular
lamina and the tectorial membrane, a motion that
serves as the mechanical trigger of transduction
currents

• The tallest row of stereocilia protrudes from the
apical surface of outer hair cell and embeds within
the tectorial membranethe shearing motion between
the reticular lamina and the tectorial membrane
causes stereocilia to bend in the direction of the
modiolus or the spiral limbus, depending on whether
the basilar membrane is displaced toward the scala
tympani or scala vestibuli, respectively

• In contrast to OHCs, the stereocilia of IHCs do not
appear to firmly contact the tectorial membrane
Therefore, during basilar membrane vibration, the
mechanical stimulus to IHC stereocilia is the flow of
endolymph within the subtectorial space.

Radial Displacement Patterns of the
Basilar Membrane
• The foot of the inner pillar cell lies near the bony spiral
lamina
• The foot of the outer pillar cell lies over the basilar
membrane and is not supported by bone.
• When the basilar membrane is displaced in a passive
cochlea, movement occurs maximally near the foot of the
outer pillar cell When OHCs contract, the reticular lamina
pivots at the apex of the tunnel of Corti, and the basilar
membrane and reticular lamina are pulled together,
enhancing overall basilar membrane displacement.Greatest
motion in active cochlea is in the domain of the OHC

Hair Bundle Deflections and Receptor
Potentials
•Magnitude of the receptor potential is proportional to the
degree of stereociliary deflection in the most sensitive region
of its operating range
•When the bundle is deflected toward the tallest stereocilia,
the hair cell depolarizes
•Deflections of the bundle in the direction opposite the tallest
stereocilia hyperpolarize the cell

Gating of Transduction Channels
• transduction channels are located at each end of the tip
link
•When the bundle is deflected toward the taller
stereocilia, tip links are stretched and physically open
transduction channels, causing excitation
•When the bundle is deflected away from the taller
stereocilia, tip links relax, causing the channel to close.

Hair cell transduction channels are
nonselectively permeable
• K+ carries most of the receptor current
• When transduction channels open, K+ in the endolymph is
flushed down a large electrochemical gradient
• This inward transducer current flows across the basolateral
membrane, producing the receptor potential and voltage-
gated Ca2+ channels are activated, resulting in an influx of
Ca2+ and release of neurotransmitter from the base of the
hair cell.

Hair Cell–ANF Synapses
• a large presynaptic dense body commonly referred to as a
ribbon. Each afferent dendrite receives input from a single
“ribbon synapse”
• When depolarized, Ca channels open and allow calcium to
rapidly enter the cell, and intracellular Ca+ activates the
protein otoferlin, which, in turn, triggers the fusion of
synaptic vesicles with the presynaptic membrane and
subsequent vesicular exocytosis at the hair cell-afferent
bouton synapse

• Each inner hair cell is contacted by several type I spiral
ganglion cells (SGCs), each peripheral terminal ending in a
single synaptic bouton, whereas type II SGCs innervate
multiple outer hair cells.
• Type I SGCs constitute 95% of the afferent innervation of
the cochlea and both peripheral and central processes are
myelinated, except peripheral to the habenula perforata,
whereas type II SGCs are unmyelinated.

ENDOLYMPH HOMEOSTASIS
• extrusion and recycling of K+ after the depolarization
of sensory cells are necessary components of
mechanoelectrical transduction

CENTRAL AUDITORY
PROCESSING

Central auditory system
• It is the elaborate neural network that governs sound
representation.
• Most of auditory system is afferent
• auditory receptors are selectively excited by sound
frequency.
• All other perceptual dimensions must be computed by
integrating coincident activity across neural populations.

Pathway
• Eight (auditory ) nerve
• Cochlear nucleus
• Olivary nucleus(superior)
• Lateral lemniscus
• Inferior colliculus
• Medial geniculate body
• Auditory cortex

Cochlear nucleus
•First relay station for all ascending auditory
information that originates in the ear, and it is located
in the pontomedullary junction of the dorsolateral
brainstem
•The cochlear nucleus contains a number of cell types,
each of these has unique somatic and dendritic
characteristics.
•These cell types display varying response properties to
auditory stimulation and project to different targets

• The temporal and spectral features of sound that
originate in the ear are processed in the cochlear
nucleus
• These pathways project to the auditory brainstem,
midbrain, and cortex and integrate information from
the ear to determine the identity, intensity, and
location of the sound source

Superior Olivary Complex
• relay station for auditory information from both ears
• information from both cochlear nuclei is integrated in
the SOC
• important role in sound localization by analyzing
interaural time and amplitude differences
• allows for sound localization but also helps enhance
auditory perception

Lateral Lemniscus
•formed by the three fiber tracts from the
cochlear nucleus
•lateral lemniscus sends out multiple branches;
some terminate in the SOC

Inferior Colliculus
• processes frequency-specific information
• receives auditory inputs from the lateral lemniscus,
the cochlear nucleus, and the SOC, in addition, it
receives projections from the somatosensory, visual,
and vestibular systems
• integrates information from both auditory and
nonauditory source
• sends fibers to the medial geniculate body of the
thalamus

Medial Geniculate Body
•the auditory processing influenced by an
abundance of inputs from the auditory cortex
•important role in sound localization and
processing of complex vocal communications,
such as human speech.

Auditory Cortex
• primary auditory cortex is tonotopically tuned, with
high frequencies being represented more medially
and low frequencies more laterally.
• Involved with integrating and processing complex
auditory signals, which includes language
comprehension

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