The Peripheral Auditory System

George PollakSection of Neurobiology

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Organ of Corti

Basilar membrane

• Hair cells, the transducers of the auditory system, and how they work.

1

stereocillia ofinner hair cells

stereocillia ofouter hair cells

stereocilia on one hair cell

9

a

trap doorofionicchannel

tip linkstereocilia

A B C D

GE F

A model for mechoelectrical transduction by hair cells. In the absence ofany stimulation, at any instant each transduction channel at a stereocilium'stip may be either closed (A and F) or open (E). The greater probability isthat the channel is closed. When the hair bundle is deflected with a positivestimulus, the spring or tip link, exerts a force on the trap door and opensthe channel (B,C and D). The influx of K+ ions into the hair cell causesit to depolarize. Pushing the hair bundle in the opposite direction compressesthe spring (tip link), ensuring that the channel remains closed (G). Thisprevents the influx of K+ ions and causes the cell to hyperpolarize.

aa

Endolymph

PerilymphFig. 1

apical surfaceof hair cell

basal surfaceof hair cell

stereociliakinocilium

tight junctionssynaptic ribbonsafferent nerves

Hi K+

Lo Na+

Hi Na+

Lo K+

Hi K+

Lo Na+

Ek= 58 logKout

Kin

= 0mV

Potential difference betweenEndolymph and cell interior

Ek= 58 logKout

Kin

= ~-70mV

Potential difference betweenPerilymph and cell interior

endolymph

perilymph

aaa

K+

K+

Ca++ K+

InhibitionK+

K+Ca++

ExcitationRest

Rest: Small amount of K+ leaks into cell from rattling channels of stereocilia. The leakingK+ depolarizes cell thereby opening voltage sensitive Ca++ channels resulting in spontaneousrelease of transmitter and excitation of afferent nerve fibers (not shown). The depolarizationcauses K+ to leave cell in basal region.Excitation: Stereocilia are bent thereby opening channels which results in larger entry of K+into cell. The influx of K+ causes additional depolarization thereby opening more voltage gatedCa++ channels. The additional Ca++ channels that are now open cause a larger influx of Ca++in the base. The influx of Ca++ causes more transmitter to be released and thus a greaterexcitation of the afferent fibers (not shown).Inhibition: Stereocilia are bent in opposite direction thereby closing channels. This results in less influx of K+ at channels in the tips of stereocilia. K+ efflux occurs at base and is notreplenished by influx at stereocilia. Consequently, the hair cell hyperpolarizes thereby closingvoltage sensitive Ca++ channels at the base resulting in a smaller release, or even no release oftransmitter. Discharge of afferent fiber is therefore reduced to a rate even below the restinglevel.

-45 mV

Hi K+

low Na+

Hi K+

low Na+

Hi K+

low Na+Hi Na+

low K+

Hi Na+

low K+

small leakage of K+ into cell

-45 mV

K+ into cell

-70 mV

No K+ into cell

Next, we are going to build a cochlea

Basilar membrane

Stapes

Sound is changed froma pressure wave in the air

into mechanical movements on the basilar membrane

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round window

Traveling waves on basilar membrane

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oval window

The structure of the basilar membrane causes it

to perform a frequency to place transformation

Basilar Membrane

has continuously changing dimensions along its length

Baseresponds maximally to high frequencies

Apexresponds maximally to low frequencies

StiffNarrow and thick

flexiblewide and thin

Basilar membrane converts frequency to a place of maximal response

Frequency-to-Place Transformation in the Cochlea

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The motion on the basilar membrane

causes shearing of the cilia onhair cells and thereby causes

the hair cells to depolarize and hyperpolarize in

response to sound

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Organ of Corti

Basilar membrane

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Organ of Corti

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basilar membranebasilar membrane

shearing of stereocillia

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Why are there two types of hair cells?

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98% of the fibers that project into the central auditory systemare innervated by inner hair cells!!

98%

What are the outer hairs doing?Answer: they act as amplifiersof the mechanical motion of

the basilar membrane generatedby sound

hyperpolarization-----

depolarization

release of transmitter

Hi K+K+

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Evoked mechanical responses of isolated cochlear outer hair cells.

Electromotility: OHC can change length in response to voltage change

Direct evidence of an active mechanical process in the organ of Corti

depolarized hyperpolarized

+++

___

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42

Dancing hair cell

Outer hair cells are the only cells in the body that express prestin. Even inner hair cells do NOT have prestin.

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Show movie of how Prestin works

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Sound stimuli

Basilar membrane

motion

Hair bundle deflection

Membrane potential change

Change in length of hair cells

IHC Sensory signal transmission

OHC

Positive feedback loop

Normal response with cochlear

amplifier

response without cochlear

amplifier

base

Apex

base

Apex

Basilar membrane

How motion of basilar membranegenerates tuning curves in

auditory nerve fibers and therebyimparts frequency selectivity

to auditory nerve fibers

6 kHz

7 kHz

8 kHz

9 kHz

10 kHz

10

6 7 8 9 10 11

Frequency ( kHz)

5

20

30

40

50

60

Inte

nsi

ty (

dB

SP

L)

base apex

50 dB SPL

6 kHz

7 kHz

8 kHz

9 kHz

10 kHz

10

6 7 8 9 10 11

Frequency ( kHz)

5

20

30

40

50

60

Inte

nsi

ty (

dB

SP

L)

Tuning Curve The most basic feature of an auditory neuron

best frequency

30 dB SPL

base apex

frequencylow high

Sou

nd in

tens

ity

low

high

tuning curves in normal animalstuning curves in animals with no outer hair cells or in animals without prestin gene

How is the tonotopic organizationthat was first established on the

basilar membranepreserved in in the

central auditory system?

Inferior

colliculus

Inferior

colliculus

Medial geniculate

Medial geniculate

Superior olive

Superior olive

Cochlear nucleus

Cochlear nucleus

Auditory cortex

Cochlea

Cochl

ea

Auditory nerve

Auditory nerve

Flow of Information Along the Central Auditory Pathway

Inferior

colliculus

Inferior

colliculus

Medial geniculate

Medial geniculate

Superior olive

Superior olive

Cochlear nucleus

Cochlear nucleus

Auditory cortex

Cochlea

Cochl

ea

Auditory nerve

Auditory nerve

cochlear nucleuscochlear nucleus superior olive

Inferior colliculus Inferior colliculus

medial geniculate medial geniculate

auditory cortex

The Frequency Representation on the Cochlea is Preserved in Every Nucleus of the Central Auditory System, and thus the Auditory

System is Tonotopically Organized

Inferior

colliculus

Inferior

colliculus

Medial geniculate

Medial geniculate

Superior olive

Superior olive

Cochlear nucleus

Cochlear nucleus

Auditory cortex

Cochlea

Cochl

ea

Auditory nerve

Auditory nerve

The Frequency Representation on the Cochlea is Preserved in Every Nucleus of the Central Auditory System, and thus the Auditory

System is Tonotopically Organized

The Frequency Representation on the Cochlea is Preserved in Every Nucleus of the Central Auditory System, and thus the Auditory System is Tonotopically Organized