460
UNIT THREE
f bers that make up part oF the vestibular branch oF the ves-
tibulocochlear nerve
(f
g. 12.20)
.
When the head or torso moves, the semicircular canals
move as well, but initially the fl
uid inside the membranous
canals tends to remain stationary because oF inertia. This
bends the cupula in one or more oF the canals in a direction
opposite that oF the head and torso movement, and the hairs
embedded in it also bend. The moving oF the hairs stimulates
the hair cells to signal their associated nerve f
bers, and as a
result, impulses travel to the brain
(f g. 12.21)
. The orienta-
tion oF the semicircular canals approximates the three ana-
tomical planes, so movements in diFFerent directions aFFect
diFFerent combinations oF semicircular canals. The brain
interprets impulses originating From these diFFerent combina-
tions as movements in diFFerent directions.
Parts oF the cerebellum are particularly important in inter-
preting impulses From the semicircular canals. Analysis oF
such inFormation allows the brain to predict the consequences
oF rapid body movements, and by modiFying signals to appro-
priate skeletal muscles, the cerebellum can maintain balance.
Other sensory structures aid in maintaining equilib-
rium. Various proprioceptors, particularly those associated
with the joints oF the neck, inForm the brain about the posi-
tion oF body parts. The eyes detect changes in posture that
result From body movements. Such visual inFormation is so
important that even iF the organs oF equilibrium are dam-
aged, keeping the eyes open and moving slowly is suFf cient
to maintain normal balance.
position. The brain responds by sending motor impulses to
skeletal muscles, which may contract or relax appropriately
to maintain balance.
The maculae also participate in the sense oF dynamic
equilibrium. ±or example, iF the head or body is thrust For-
ward or backward abruptly, the gelatinous mass oF the mac-
ulae lags slightly behind, and the hair cells are stimulated. In
this way, the maculae aid the brain in detecting movements
such as Falling and in maintaining posture while walking.
Dynamic Equilibrium
Each semicircular canal Follows a circular path about 6 milli-
meters in diameter. The three bony semicircular canals lie at
right angles to each other and occupy three diFFerent planes
in space. Two oF them, the
anterior canal
and the
posterior
canal,
stand vertically, whereas the third, the
lateral canal,
is
horizontal. Their orientations closely approximate the three
body planes (see chapter 1, p. 22).
Suspended in the perilymph oF each bony canal is a
membranous semicircular canal that ends in a swelling
called an
ampulla
(am-pul
ah). The ampullae communicate
with the utricle oF the vestibule.
An ampulla contains a septum that crosses the tube and
houses a sensory organ. Each oF these organs, called a
crista
ampullaris,
has a number oF sensory hair cells and support-
ing cells. As in the maculae, the hairs oF the hair cells extend
upward into a dome-shaped gelatinous mass called the
cup-
ula.
Also, the hair cells are connected at their bases to nerve
Ampullae of
semicircular
canals
Saccule
Utricle
Vestibulocochlear
nerve
Cochlea
Cochlear
duct
Maculae
Vestibule
FIGURE 12.17
The saccule and utricle, expanded portions of the membranous labyrinth, are in the bony chamber of the vestibule. (Compare with
F
gure 12.11.)
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