Transcript tympanic membrane - Jacobs University Mathematics

Structure of the Ear
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Goldstein, pp. 343 – 360
CWE, pp. 187 – 204
Levine, pp. 339 – 346, 350 – 357, 363 – 366
Purves et al., Chapter 12
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Outer ear
Middle
Inner ear
ear
Auditory channel, 3cm
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Functions of the external or outer ear
– Gathering sound energy and focusing it on
the eardrum (tympanic membrane)
– Selectively boosting sound pressure 30- to
100-fold for frequencies around 3000 Hertz
– Pinna and concha filter different sound
frequencies in order to provide cues about
the elevation of the sound source
(localization)
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Middle ear
Ossicles amplify the
vibrations in two ways:
1. Concentrating the
vibration of the large
tympanic membrane
onto the much
smaller stapes
2. Create a lever action
55 mm2
3.2 mm2 ! pressure can be increased by factor 17
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Lever principle
Length of incus
Length of malleus
Fresultant = Fapplied(D1/D2)
! the force of the incoming auditory signal is increased
by the ossicles by a factor of about 1.3
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Reduction in sound level
– Two sets of muscles in the middle ear contract and reduce
the magnitude of vibration transmitted through the middle
ear.
– Tensor tympani ! attached to the malleus ! pulls tympanic
membrane ! increase in stiffness ! reducing the magnitude of
vibration from incoming sounds
– Stapedius muscles ! connect to stapes ! retract from its normal
postion ! reducing the amount of movement of the stapes
– Muscles contract reflexively in response to very loud
noises and can cause reductions in sensitivity by as much
as 30 dB.
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The inner ear
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Inner ear
35 mm long, 2mm Ø
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– Scala vestibuli and scala timpaniare filled with
incompressible fluid, the perilymph.
– Both channels behave as one hydrodynamic system,
because they are connected at the far end, or apex, by a
small hole in the partition called helicotrema.
– The lower section is sealed off with an elastic membrane at
the round window.
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– The partition separating both scalae , called scala
media, is filled with another fluid, the endolymph.
– Its boundaries are the basilar membrane which
holds the sensory organ proper (organ of corti),
the Reissner's membrane, which serves to
separate endolymph from perilymph, and the rigid
lateral wall of the cochela.
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Cross-section of the cochlea
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Cross-section of the organ of
Corti
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Inner and outer hair cells
about 40 cilia
about 140 cilia
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– All of the hairs in a
bundle tend to move, or
bend, as a unit.
– Movement of the bundle
of cilia toward the tallest
one increases the firing
rate of the cochlear nerve
axon attached to the hair
cell, while movement
away from the tallest one
decreases it.
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– About 30 000 nerve fibers, whose cell bodies are
located in the spiral ganglion, form connections
with the bases of hair cells of each cochlea.
– About 95% of them make connections with the
inner hair cells at the same place where the fibers
enter the cochlea.
– There about 15 fibers per inner hair cell in the
middle of the cochlea and about 3 to 4 per inner
hair cell at the base and apex.
– The remaining 5 % of the fibers from the spiral
ganglion each make synaptic contact with about
10 outer hair cells located closer to the base than
the fibers' point of entry into the cochlea.
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Frequency Analysis in the Cochlea and
Auditory Nerve
– Goldstein, pp. 351 – 360
– CWE, pp. 195 – 204 (parts)
– Levine, pp. 350 – 360 (parts)
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Georg von Békésy
1961 Nobel Laureate
in Medicine
for his discoveries of
the physical
mechanism of
stimulation within the
cochlea.
Born in 1899,
Budapest, Hungary
Died in 1972,
Honolulu, Hawaii
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Békésy’s place theory of hearing
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Envelope
Base
Apex
Displacement of the cochlear membrane in response to a 200 Hz tone. The solid curves
represent the patterns of displacement at four instances; the darker the line the later in
time the configuration occurred. The dashed curve is the envelope of maximum
displacement.
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Envelopes of vibration patterns on the basilar membrane for pure tones of different frequencies.
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Tonotopic map of the cochlea
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