CHAPTER 21
Ear
417
Oval window
Round window
Cochlea
(unrolled)
Scala vestibuli
Base
Apex
Basilar membrane
Spiral lamina
Scala tympani
Spiral ligament
Helicotrema
Vestibule
A
Tectorial membrane
Basilar membrane
Scala vestibuli
(perilymph)
Scala media
(endolymph)
Original position
B
C
Outer hair
cell
Inner hair
cell
Phalangeal
cell process
Phalangeal
cells
Cuticular
plate
Reticular
lamina
Afferent axon
Efferent axon
Figure 21-6A.
Sound transduction.
As shown in this diagram of a straightened cochlea,
sound
waves
are transmitted into the
perilymph
(
blue
) of the
scala vestibuli
by
movements of the footplate of the
stapes
on the
oval
window
.
The membranous
round window
provides pressure relief for the
sound waves within the closed chamber of the
bony
labyrinth
.
The
basilar membrane
(
gray
) is a thin sheet of F brous connective
tissue that supports the
organ of Corti
(±ig. 21-5). The basilar
membrane is narrower at the
base
of the cochlea (about 0.21 mm)
than at the
apex
of the cochlea (about 0.36 mm). It is also stiffer
at the base of the cochlea than at the apex. These properties cause
the basilar membrane to vibrate preferentially (
resonate
) near the
base when stimulated at high frequencies (
red arrows
) and near
the apex (
blue arrows
) when stimulated at low frequencies. This
arrangement creates a
tonotopic map
along the organ of Corti and
is one of the ways in which the cochlea encodes sound waves of
different frequencies into trains of nerve impulses that can be pro-
cessed by the nervous system to produce the sensation of
pitch
.
Figure 21-6B.
Stereocilia displacement.
When a
sound
wave
increases the pressure in the perilymph of the
scala vestibuli, the pressure in the
endolymph
of the
scala
media
increases simultaneously, because the vestibular membrane is
very thin and delicate. This increased pressure moves the tectorial
membrane and basilar membrane downward (
large red arrow
)
and away from their original positions (indicated by the
ghost
outline
). Because the centers of rotation of the
tectorial
membrane
and
basilar
membrane
are different, the downward movement of
the two membranes induces a transverse displacement of the tips
of the hair cells (
small red arrows
). This bending of the
stereocilia
opens K
+
channels and causes a change in the
membrane
potential
of the hair cells. The potential change (
depolarization
) induces the
release of transmitter molecules and produces
action
potentials
in
the afferent nerve F bers of the
cochlear
nerve
.
Figure 21-6C.
Auditory hair cells.
There are several differences between the
inner
and
outer
hair
cells
. The stereocilia of inner hair cells are arranged in a straight
line, whereas the stereocilia of the outer hair cells are arranged
in a “V” or “W” pattern (see ±ig. 21-7A,B). Inner hair cells are
completely surrounded by inner phalangeal cells; only the lower
third of outer hair cells is cupped by the cell bodies of outer pha-
langeal cells. About 95% of the sensory nerve F bers in the audi-
tory nerve contact inner hair cells. A single afferent axon typically
contacts only one inner hair cell, and each inner hair cell has syn-
aptic contact with at least 10 afferent axons (
green
). By contrast, a
single afferent axon may branch and contact as many as 10 outer
hair cells. In addition, there are efferent nerve F bers (
orange
) that
originate in auditory centers in the brainstem and make synaptic
contacts on hair cells or on afferent nerve endings. These efferent
F bers play a role in tuning the excitability of the hair cells. The
majority of the efferent endings are on outer hair cells.
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