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The Auditory and Vestibular System
Chapter 15
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The Auditory and Vestibular System
Auditory System - sense of hearing
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Used to detect sound and interpret nuances
of sound
Important in communication and survival
Able to evoke sensations and emotions
Vestibular system - sense of balance
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Informs nervous system about the relative
position and movement of the head
Subconsciously controls muscle to reorient
body and eye position.
The Nature of Sound
Hearing is a response
to vibrating air
molecules.
Sounds are audible
variations in air
pressure
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Moving objects
compress air as they
move forward and
decrease the density of
air as they move away.
Waves move at 343 m/s
or 767 mph.
The Nature of Sound
Pitch is determined by the frequency of vibration
Frequency = the number of compressions per second.
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One cycle is the distance between the waves of compression
Frequency is expressed in hertz (Hz)
 Hearing range is 20 to 20,000 Hz.
Most sensitive to frequencies ranging from 1,500 to 4,000 Hz.
 Decreases with age or exposure to loud sounds.
Loudness = difference in pressure between compressed
and rarefied patches of air
Range is tremendous
 loudest sound without ear damage is a trillion times greater than
the faintest sound we can hear
 Intensity is expressed in decibels (dB)
 120 to 140 dB causing pain in most people.
Real world sounds rarely consist of simple periodic sound waves at
one frequency or intensity.
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The Structure of the Auditory System
Outer Ear
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Auricle or Pinna
External Auditory
Meatus
Tympanic Membrane
Middle Ear
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Ear ossicles
Inner Ear
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Vestibule
Cochlea
Middle Ear
Ear Ossicles
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Malleus, Incus, Stapes
Act as a lever system to
amplify sound.
Eustachian tubes
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Equalizes pressure in
middle ear
Attenuation Reflex
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Tensor Tympani and
Stapedius Muscle
Reduces hearing saturation
levels
Protects the inner ear.
Reduces low-frequency
background noise.
The Inner Ear
Vestibule
Semicircular Canals
Cochlea
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Chambers
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Scala Vestibuli
Scala Media
Scala Tympani
Membranes
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Basilar membrane
Reissner’s (vestibular)
membrane
Tectorial membrane
Stria vascularis
Cochlear Structures
Oval Window
Round window
Helicotrema
Basilar membrane
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widens toward the Apex of
cochlea.
Perilymph
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Fills Scala Vestiblia and Scala
Tympani
Low K+, High Na+
Endolymph
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Fills Scala media
High K+, Low Na+
Produces an endocochlear
potential that enhances auditory
transduction
Basilar Membrane
Structural properties determine how the membrane
responds to sound.
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Wider at apex than at the base by 5 times.
Stiffest at base and most flexible at apex.
Movement of stapes causes endolymph to flow causing a
traveling wave in the membrane
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The distance the wave travels depends on the frequency of the
wave.
Different locations of the basilar membrane are maximally
deformed at different frequencies creating a placed code.
Response of the Basilar Membrane to Sound
High frequency
waves dissipate near
the narrow, stiff base.
Low frequency waves
dissipate near the
wide flexible apex.
A place code where
maximum amplitude
deflection occurs is
responsible for the
neural coding of
pitch.
Organ of Corti
Hair cells
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Three rows of outer
hair cells and one
inner.
Stereocilia are
embedded in the
reticular lamina and
tectorial membrane.
Have no axons
Interact with bipolar
spiral ganglion cells
that form the cochlear
(auditory) nerve.
Transduction by Hair Cells.
Vibration in the basilar membrane results in the bending of
stereocilia
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Stereocilia are cross linked by filaments and move together as a unit.
Bending of steriocilia causes changes in the polarization of
the hair cells.
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Displacement of only 0.3 nm can be detected (diameter of a large
molecule).
Loud noises that saturate hair cells move stereocilia by only 20 nm.
Bending depolarizes
or hyperpolarizes
hair cells depending
on the direction the
stereocilia are
pulled.
Hair cell receptor
potentials closely
follow the air
pressure changes
during a lowfrequency sound.
Depolarization of Hair Cells.
K+ channels on the
stereocilia are linked by
elastic filaments.
Displacement of cilia opens
or closes K+ channels
K+ entering cell causes
depolarization.
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Note: K+ entry generally
causes hyperpolarization.
Endolymph has a high
concentration of K+.
Depolarization causes
voltage gated Ca++
channels to open
Ca++ triggers the release
of neurotransmitter.
High K+
Low Na+
High Na+
Low K+
The Innervation of Hair Cells
Auditory nerves are
bipolar with nuclei in the
spiral ganglia.
95% of spiral ganglia
neurons communicate
with inner hair cells.
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Each inner hair cells feeds
about 10 spiral ganglia
cells
Most detection of sound
occurs on the inner hair
cell.
One spiral ganglia cell
will connect to multiple
outer hair cells.
Amplification by Outer Hair Cells
Motor proteins in the outer
hair cells can shorten hair
cells.
Shortening of hair cells
increases the bending of the
basilar membrane.
Amplification of basilar
membrane vibration causes
the stereocilia on the inner
hair cells to bend more.
Central Auditory Processes
Spiral ganglion neurons travel
through the vestibulo-cochlear
nerve to the medulla and
branch to enter both the
dorsal and ventral cochlear
nucleus.
Neural signals travel through
numerous pathways.
The ventral cochlear nucleus
projects to the olivary bodies
on both sides of the brain then
through the lateral lemniscus.
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The dorsal path bipasses the
superior olive.
All paths converge at the
Inferior Colliculus then go on
to the Medial Geniculate
Nucleus then into the Auditory
Cortex.
Central Auditory Processes
Inferior colliculus communicates with
the superior colliculus to integrate with
visual input.
There is an extensive feedback system
in the auditory system
Other than the cochlear nuclei, auditory
nuclei receive input from both ears.
Response Properties of Spiral
Ganglion Cells
Spiral ganglion cells
are frequency tuned.
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Each cell responds
at a characteristic
frequency.
Response properties
of nuclei are diverse
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Cochlear nuclei specialized for
varying time with
frequency
MGN- Vocalization
Superior Olive Sound localization
Encoding Sound Intensity
and Frequency
Sounds are diverse and complex
Our brain must analyze the important
ones and ignore the noise
Sound is differentiated based upon
intensity, frequency and point of origin
Each of these features is represented
differently in the auditory pathway.
Stimulus Frequency and Tonotopy