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Inner Ear Physiology
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Inner Ear Physiology
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Inner Ear Physiology
•  Transduction
•  Tympanic membrane
•  Acoustical/mechanical
•  Oval window
•  Mechanical/hydraulic
•  Basilar & tectorial membrane
•  Hydraulic/mechanical
•  Hair Cells (stereocilia)
•  Mechanical/electric
•  Hair Cells (base)
•  Electrical/chemical
•  Auditory Neuron
•  Chemical/electrical
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Block diagram of the cochlea
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Frontal view of unrolled cochlea
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Effect of stapes insertion in
cochlea
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Effect of outward motion of
stapes.
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Effect of outward movement of
stapes
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Theories of Hearing
•  Place Coding Theories
•  Resonance theory
(Helmholtz, 1885)
•  Traveling wave theory
(Georg von Bekesy 1928-1958- Nobel Prize 1961)
•  Many other theories including standing wave theory,
pressure pattern theory, frequency analytic theory, etc.
See Zemlin for details.
•  Temporal Coding Theories
•  Telephone theory (Rutherford, 1886)
•  Volley theory (Wever, 1949)
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Place Theory: Impedance characteristics
of the basilar membrane
Resistance
Mass
reactance
Stiffness
reactance
•  Apical end is 5x
wider and 100x
more flaccid than
basal end.
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Resonance Theory
The BM acts as a series of filters or tuned rods (analogy of
piano strings). It performs a frequency analysis (Fourier
Analysis) of the incoming signal.
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Traveling Wave Theory
•  Most popular TW theory developed by
Bekesy (1928-1958).
•  Used cochlear models and direct
observation of cochlea.
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Traveling wave
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Traveling wave for complex
tones
Basilar Membrane Traveling Wave
8 kHz + 2 kHz
16 kHz + 2 kHz + 1 kHz
16 kHz + 8 kHz + 4 kHz + 2 kHz
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Traveling wave and nonlinearities
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Traveling wave and upward
spread of masking
•  Excitation patterns and can
be used to represent traveling
wave.
•  Notice that low frequencies
can mask high frequencies
(e.g., 1000 masking 2000 Hz)
better than the high
frequencies mask lows (e.g.,
1000 masking 500 Hz).
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Telephone Theory
•  Problem is that the absolute refractory period is about 1
msec.
•  That is, a hair cell can fire only once per millisecond (1000 times
per second or 1000 Hz).
•  Therefore, it would not be possible to transmit sounds to the CNS
greater than 1000 Hz.
•  Another problem is that damage to certain parts of the
basilar membrane (basal end) helps confirm a place theory.
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Volley Theory
•  Modification of
Telephone theory.
•  Receptor cells fire in
groups: first one,
then another, then a
third
•  Resetting: the first
group of cells to fire
are resetting while the
second and third
groups fire and so on
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Combining Place and Volley Theories
Frequencies
< 300, 400 Hz
Mid Frequencies
> 4000, 5000 Hz
Predominate Comments
Theory
Volley
The traveling wave is too
broad to allow for specificity.
Both theories
apply
Traveling
Temporal coding not
Wave
supported in high frequencies
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Electrical potential of the cochlea
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Role of Hair Cells on Hearing Thresholds
•  Normal behavioral thresholds are due to
amplification of motion by OHCs
•  Slight to moderate hearing loss is due to a
breakdown of OHCs, and not IHCs
•  Severe hearing loss is due to the breakdown
of OHCs and IHCs
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Nerve Cell Anatomy
Synapses
Soma
Terminal
Boutons
Axon & Myelin
Dendrites
Neurotransmitter
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Auditory Neurons are Bipolar
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Block diagram of auditory neuron
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Sensitivity (Detection) vs. Specificity (Identification)
Afferent Innervation of Inner and Outer Hair Cells
Specificity: Many-to-one arrangement
Nerve
Fiber
IHC
Sensitivity: One-to-many arrangement
Nerve
Fiber
OHC
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Innervation
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Summary
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