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Advance Cochlear Physiology

Organ of Corti

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Traveling Wave

Basilar Membraneperforms a frequencydecomposition

The gradation in widthand stiffness of thebasilar membranesystem is important tothe basic mechanicalpattern of theresponse to thestimulation

Cochlear Amplifier

Fifteen years of experimental studieshave established that the OHC is ableto generate an active force and thatthis can be produced at frequencies upto and exceeding 50 kHz

Gold, an American astrophysicist, wasthe first to suggest that mechanicalfiltering by the inner ear could beenhanced by force generation withinthe hearing organ

The concept that a source ofmechanical energy exists in thecochlea appeared validated in the late1970s,when it was discovered that theliving inner ear produces soundsreferred to as otoacoustic emissions.

Isolated OHCs were observed toelongate when hyperpolarized andshorten when depolarized

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Structure of OHCs Lateral Wall

Prestin is Required for Electromotility

of OHCs

OHCs perform electromechanical

transduction, whereby transmembrane

voltage drives cellular length changes at

audio frequencies

Dallos and colleagues identified the OHC

lateral membrane motor, a protein of 744

aminoacids residues that they called

prestin

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Evidence for Prestin Involvement

To test the hypotheses that prestin is the molecularmotor underlying outer hair cell (OHC) electromotility andthat OHC electromotility is the basis of the cochlearamplifier, a mutant mouse mouse was created in which

the prestin gene was targeted for deletion (prestin knockout). This resulted in loss of outer hair cell electromotilityand a 4060 dB loss of cochlear sensitivity

The demonstration of a human non-syndromic deafnessthat is linked to a prestin mutation demonstrate theproteins requirement for OHC electromotility, and thedevastating effects of its absence on the cochleaamplifier (Liu et al., 2003)

The Nonlinear Cochlea

Some of these important nonlinear

cochlear measures include:

The frequency dependent basilar membrane

response level compression

The frequency dependent neural twotone

suppression

Distortion components generated by the

cochlea,

Compressive Nonlinearity

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Tuning Curve (TC)

All neurons reveal

tuning

TC show the

minimum soundpressure level needed

to cause the neuron

to respond at each

frequency called

characteristic

frequency

Lateral Suppression

In this example, a fixed 4-kHz tone (suppressor) at 70 dB sound

pressure level (SPL) is presented simultaneously with a second

tone at 50 dB SPL. The frequency of the second tone is variedbetween 1.2 kHz and 8.8 kHz. The solid lines represent the basilar

membrane velocity profiles for corresponding pairs of tones. The

dashed lines are the responses generated by the 50 dB tones in

the absence of the suppressor. All supressor curves are nearly

superimposed, implying that the suppressor is scarcely affected by

the suppressed tones for this input amplitude ratio.

Loss of OHCs

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OAE

OAE