Download The Structure and Function of the Auditory Nerve Brad May

The Structure and Function of the
Auditory Nerve
Brad May
Structure and Function of the Auditory and
Vestibular Systems (BME 580.626)
September 21, 2010
1
Objectives
Anatomy
Basic response patterns
Frequency coding
Loudness coding
Spectral coding
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The Organ of Corti
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Auditory nerve fibers link the Organ of Corti to the auditory brainstem. The cell bodies of the
fibers are located in the spiral ganglion.
Type 1 and II Fibers
4
The fibers are divided into two classes based on the anatomy of the projections. Type I fibers
innervate inner hair cells and encode sound.
Type 1 and II Fibers (cat)
Number of AN fibers: 50,000
90-95% of AN fibers are Type I
45,000 - 47,500 Type I fibers
2,500 - 5,000 Type II fibers
Number of Hair Cells: 12,500
9500 outer hair cells
3000 inner hair cells
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The majority of auditory nerve fibers are type I.
Type 1 and II Fibers
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Type I fibers innervate a single inner hair cells. Type II fibers innervate several outer hair cells.
Spontaneous Activity
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Type I fibers fire action potentials. Most fibers have spontaneous activity in the absence of sound
stimulation.
Sound-Driven Activity
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Activity is increased during sound presentations.
Frequency Tuning Curves
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Fibers show sound-driven activity over a narrow range of stimulus frequencies at low sound
levels.
Frequency Tuning Curves
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Different fibers respond best to different frequencies.
Frequency Tuning Curves
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The frequency tuning of the auditory nerve is determined by the mechanical tuning of the Organ
of Corti.
Tonotopic Organization
12
Like the basilar membrane, neural tuning is tonotopically organized.
Best Frequency
13
The best frequency of each fiber is determined by the location of its terminal projection in the
Organ of Corti.
Peri-Stimulus Time Histograms
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Physiological characterizations of the auditory nerve are usually based on responses to best
frequency tones. Temporal patterns are revealed by peri-stimulus time histograms.
Phase-Locking
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The action potentials of auditory nerve fibers synchronize to the phase of low-frequency tone.
They show sustained responses to high-frequency tones.
Period Histogram
15B
The period histogram provides a better indication of the quality of phase locking. Each spike is
plotted relative to its time in the stimulus period. This format is essentially a folding of the PSTH.
Inner Hair Cell Potentials
16
The upper limits are determined by the membrane capacitance of inner hair cell membranes.
Rate versus Time Coding
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The same neuron my represent low frequencies by the timing of action potentials and high
frequencies by a rate-place code.
Rate-Level Functions
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The magnitude of rate responses also represents stimulus level. Rate-level functions show a
dynamic range where changes in level are unambiguously encoded by changes in rate.
Best Frequency Thresholds
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The dynamic range of hearing is produced by the variations in threshold between spontaneous
rate classifications. Low levels are encoded by fibers with high spontaneous rates. High levels are
encoded by fibers with low spontaneous rates.
Stimulus Coding
The remainder of the lecture describes
the coding of complex sounds in the
auditory nerve.
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The Acoustic Basis of
Sound Localization
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Sound source location is indicated by the binaural time and level differences and
spectral cues introduced by the directional filtering properties of the outer ear.
Pinna Filtering Effects in
the Horizontal Plane
22
A movie steps through a series of HRTFs that were recording for fixed locations in the
horizontal plane.
Pinna Filtering Effects in
the Horizontal Plane
23
These head-related transfer functions (HRTFs) show how the sound spectrum at the
eardrum is modified by moving the sound in the horizontal plane. Note the clear
head-shadowing effect at low frequencies.
Pinna Filtering Effects in
the Vertical Plane
24
A movie steps through a series of HRTFs that were recording for fixed locations in the
vertical plane.
Pinna Filtering Effects in
the Vertical Plane
25
These head-related transfer functions (HRTFs) show how the sound spectrum at the
eardrum is modified by moving the sound in the vertical plane. Note the lack of headshadowing effect at low frequencies and the directionally dependent spectral shape at
high frequencies. The frequency location of the prominent mid-frequency notch
changes systematically with elevation.
The Traveling Wave
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Low-frequency sounds produce maximum membrane displacements in the apical
cochlea. High-frequency sounds produce peak displacements in the base.
Spectral Coding in the
Auditory Nerve
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As a result of traveling wave effects, each auditory-nerve fiber responds to a selective
frequency range of the HRTF spectrum.
Auditory Nerve
Responses to HRTFs
28
This movie describes the linear rate representation of spectral shape that is conveyed
by a single sharply tuned auditory-nerve fiber. As the spectrum changes near the
neuron’s best frequency,peak energy is encoded by peak discharge rates.
Auditory Nerve
Responses to HRTFs
29
This movie describes the linear rate representation of spectral shape that is conveyed
by a population of sharply tuned auditory-nerve fibers. Peak energy is the sound
spectrum is associated with peak discharge rates.
Auditory Nerve
Responses to HRTFs
30
This movie describes the linear rate representation of spectral shape that is conveyed
by a population of sharply tuned auditory-nerve fibers. Peak energy is the sound
spectrum is associated with peak discharge rates.
Hearing Loss
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Most forms of sensorineural hearing loss involve damage to outer hair cells. Without
the active mechanical influences of outer hair cells, neural responses of the auditory
nerve become less sensitive and more broadly tuned.
Hearing Loss
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Hearing aid amplification can restore sensitivity but not tuning. Individuals hear
sounds but cannot distinguish their meaning.
Suggested Reading
JO Pickles. Introduction to the physiology
of hearing. Academic Press, 1988.
MA Ruggero. Physiology and coding in the
auditory nerve. In: AN Popper and RR Faye
(eds), The Mammalian Auditory Pathway:
Neurophysiology. Springer-Verlag, 1992.
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