1 Inner Ear Physiology 2 3 Transduction Tympanic membrane Acoustical/mechanical Oval window...
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Transcript of 1 Inner Ear Physiology 2 3 Transduction Tympanic membrane Acoustical/mechanical Oval window...
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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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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
Massreactance
Stiffnessreactance
Resistance • 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 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 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 Predominate Theory
Comments
< 300, 400 Hz Volley The traveling wave is too broad to allow for specificity.
Mid Frequencies Both theories apply
> 4000, 5000 Hz Traveling Wave
Temporal coding not supported in high frequencies
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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
Sensitivity (Detection) vs. Specificity (Identification)
Specificity: Many-to-one arrangement
Sensitivity: One-to-many arrangement
IHC
OHC
NerveFiber
NerveFiber
Afferent Innervation of Inner and Outer Hair Cells