Transcript PHY238Y Lecture 10 new

PHY238Y
Lecture 12
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The human inner ear
Physics of hearing (IV)
References:
Hallett et al.: Physics for the life sciences, 4th ed., Ch.2 (2.6)
Some of the pictures were taken from Hyper Physics:
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Hyper Physics: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/ear.html#c1
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Thanks to dr. Rod Nave for the permission to use the above resource
PHY238Y
Lecture 12
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The inner ear contains
specialized sense organs for
transducing sound into neural
impulses:
- the semicircular canals which
serve as the body's balance
organ
- the cochlea which serves as
the body's microphone,
converting sound pressure
impulses from the outer ear into
electrical impulses which are
passed on to the brain via the
auditory nerve.
PHY238Y
Lecture 12
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The semicircular canals are
the body's balance organs,
detecting acceleration in the three
perpendicular planes. These
accelerometers make use of hair
cells which detect movements of
the fluid in the canals caused by
angular acceleration about an axis
perpendicular to the plane of the
canal.
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The canals are connected to the
auditory nerve.
PHY238Y
Lecture 12
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Cochlea has three fluid filled
sections. :
The perilymph fluid in the
canals (almost identical to
spinal fluid)
the endolymph fluid in the
cochlear duct.
The fluids differ in terms of their
electrolytes and if the membranes
are ruptured so that there is mixing of
the fluids, the hearing is impaired.
The organ of Corti is the
sensor of pressure variations.
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PHY238Y
Lecture 12
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Cochlea is a coiled tube (10 mm
diameter; 35 mm long) bisected
by cochlear partition and
supporting the basilar
membrane
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Peak frequencies along the
basilar membrane: high
frequencies close to the base;
low frequencies at the apex
end.
PHY238Y
Lecture 12
Traveling waves of
different frequencies
along the basilar
membrane
(Fig. 6.5 from K. Bogdanov:
Physics in Biology)
PHY238Y
Lecture 12
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The basilar membrane of the inner ear plays a critical role in the
perception of pitch
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The theory that won the Nobel prize for Physiology or Medicine
(1961): Traveling Waves propagate along the basilar membrane
(Georg von Bekesy)
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Basilar membrane does not respond to vibrations in a simple way
but to sinusoidal vibrations and traveling waves:
- High frequencies peak near base
- Low frequencies peak near apex
PHY238Y
Lecture 12
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Organ of Corti is situated on the
basilar membrane in one of the
three compartments of the cochlea.
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It contains four rows of hair cells
which protrude from its surface.
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Above them is the tectorial
(tectoral) membrane which can
move in response to pressure
variations in the fluid - filled
tympanic and vestibular canals.
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Movement of tectorial membrane
relative to basilar membrane causes
hair cells to tilt. A lilt of ~ 0.3 nm
can be converted into a nervous
signal
PHY238Y
Lecture 12
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Taking electrical impulses
from the cochlea and the
semicircular canals, the
auditory nerve makes
connections with both
auditory areas of the brain.
PHY238Y
Lecture 12
Cochlear implants: electronic devices designed to provide sound
information in people with intact auditory nerve, but injured cochlea.
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22 electrodes are surgically inserted inside the inner ear;
They receive harmonics from a microphone + speech processor system
They send signals to the corresponding nerve terminals from the inner ear