Post on 10-Feb-2018
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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