Sensory Physiology
221
sensitive receptor cells. Pressure differences across the cochlear
duct cause the basilar membrane to vibrate.
The region of maximal displacement of the vibrating bas-
ilar membrane varies with the frequency of the sound source.
The properties of the membrane nearest the middle ear are
such that this region vibrates most easily—that is, undergoes
the greatest movement—in response to high-frequency (high-
pitched) tones. As the frequency of the sound is lowered,
vibration waves travel farther along the membrane toward the
helicotrema. Progressively more distant regions of the basi-
lar membrane vibrate maximally in response to progressively
lower tones.
Hair Cells of the Organ of Corti
The receptor cells of the organ of Corti are called
hair cells.
These cells are mechanoreceptors that have hairlike
stereo-
cilia
protruding from one end (
Figure 7–37c
). The hair cells
transform the pressure waves in the cochlea into receptor
potentials. Movements of the basilar membrane stimulate the
hair cells because they are attached to the membrane.
The stereocilia of the hair cells are in contact with the
tec-
torial membrane
(see Figure 7–37c), which overlies the organ
of Corti. As pressure waves displace the basilar membrane, the
hair cells move in relation to the stationary tectorial membrane,
and, consequently, the stereocilia bend (
Figure 7–38
). When
the stereocilia are bent toward the tallest member of a bundle,
fi brous connections called
tip links
pull open mechanically-
gated cation channels, and the resulting charge infl
ux depolar-
izes the membrane. This opens voltage-gated calcium channels
near the base of the cell, which triggers neurotransmitter
release. Unlike other extracellular fl uids, the endolymph sur-
rounding the stereocilia has a high concentration of K
+
, so an
infl
ux of K
+
(rather than Na
+
) is what depolarizes the hair cell.
Bending the hair cells in the opposite direction slackens the
tip links, closing the channels and allowing the cell to rapidly
repolarize. Thus, as sound waves vibrate the basilar membrane,
the stereocilia are bent back and forth, the membrane potential
of the hair cells rapidly oscillates, and bursts of neurotransmit-
ter are released onto afferent neurons.
The neurotransmitter released from hair cells is gluta-
mate, which binds to and activates protein-binding sites on the
terminals of 10 or so afferent neurons. This causes the genera-
tion of action potentials in the neurons, the axons of which
join to form the cochlear branch of the
vestibulocochlear
nerve
(cranial nerve VIII). The greater the energy (loud-
ness) of the sound wave, the greater the frequency of action
potentials generated in the afferent nerve fi bers. Because of its
position on the basilar membrane, each hair cell responds to
a limited range of sound frequencies, with one particular fre-
quency stimulating it most strongly.
In addition to the protective refl
exes involving the tensor
tympani and stapedius muscles, efferent nerve fi bers from the
brainstem regulate the activity of certain hair cells and dampen
their response, which also protects them. Despite these pro-
tective mechanisms, the hair cells are easily damaged or even
destroyed by exposure to high-intensity noises such as ampli-
fi ed rock concerts, engines of jet planes, and construction
equipment. Lesser noise levels also cause damage if exposure
is chronic.
Tip links stretch
Stereocilia
Nucleus
Afferent neurons
Vesicles
Ca
2+
Ca
2+
(b)
(a)
(c)
K
+
K
+
+
+
Tip links slack
Figure 7–38
Mechanism for neurotransmitter release in the hair cell of the
auditory system. (a) Scanning electron micrograph shows the size
gradation in a bundle of stereocilia at the top of a single hair cell
(tectorial membrane removed). (b) Bending stereocilia in one
direction depolarizes the cell and stimulates neurotransmitter
release, while bending in the opposite direction (c) repolarizes the
cell and stops the release.
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