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
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
Innervation
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Summary
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