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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

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

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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 non-linearities

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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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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

Nerve Cell Anatomy

Axon & Myelin

Soma

Synapses

Dendrites

TerminalBoutons

Neurotransmitter

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Auditory Neurons are Bipolar

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Block diagram of auditory neuron

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

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Innervation

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Summary