Chapter 15
The Special Senses
583
15
depicted in
Figure 15.36
, impulses travel initially to one of
two destinations: the
vestibular nuclei
in the brain stem or the
cerebellum
. Te vestibular nuclei, the major integrative center
for balance, also receive inputs from the visual and somatic
receptors, particularly from proprioceptors in neck muscles
that report on the position of the head. Tese nuclei integrate
this information and then send commands to brain stem mo-
tor centers that control the extrinsic eye muscles (cranial nerve
nuclei III, IV, and VI) and reflex movements of the neck, limb,
and trunk muscles (via the vestibulospinal tracts). Te ensuing
reflex movements of the eyes and body allow us to remain fo-
cused on the visual field and quickly adjust our body position to
maintain or regain balance.
Te cerebellum also integrates inputs from the eyes and so-
matic receptors (as well as from the cerebrum). It coordinates
skeletal muscle activity and regulates muscle tone to maintain
head position, posture, and balance, oFen in the face of rapidly
changing inputs. Its “specialty” is fine control of delicate pos-
tural movements and timing.
Notice that the vestibular apparatus
does not automatically
compensate
for forces acting on the body. Its job is to send warn-
ing signals to the central nervous system, which initiates the
appropriate compensations (righting) to keep your body bal-
anced, your weight evenly distributed, and your eyes focused on
what you were looking at when the disturbance occurred.
Cerebellum
Oculomotor control
(cranial nerve nuclei
III, IV, VI)
(eye movements)
Spinal motor control
(cranial nerve XI nuclei
and vestibulospinal tracts)
(neck, limb, and trunk
movements)
Visual
receptors
Somatic receptors
(skin, muscle
and joints)
Vestibular
nuclei
(brain stem)
Input: Information about the body’s position in space comes from three main sources
and is fed into two major processing areas in the central nervous system.
Output: Responses by the central nervous system provide fast reflexive control of the
muscles serving the eyes, neck, limbs, and trunk.
Vestibular
receptors
Central nervous
system processing
Figure 15.36
Neural pathways of the balance and orientation system.
Homeostatic Imbalance
15.13
Responses to equilibrium signals are totally reflexive, and usu-
ally we are aware of vestibular apparatus activity only when its
function is impaired. Equilibrium problems are usually ob-
vious and unpleasant. Nausea, dizziness, and loss of balance
are common and there may be nystagmus in the absence of
rotational stimuli.
Motion sickness
has been difficult to explain, but it appears
to be due to sensory input mismatches. ±or example, if you are
inside a ship during a storm, visual inputs indicate that your
body is fixed with reference to a stationary environment (your
cabin). But as rough seas toss the ship about, your vestibular
apparatus detects movement and sends impulses that disagree
with the visual information. Te brain thus receives conflicting
signals, and its “confusion” somehow leads to motion sickness.
Warning signals, which precede nausea and vomiting, in-
clude excessive salivation, pallor, rapid deep breathing, and
profuse sweating. Removal of the stimulus usually ends the
symptoms. Over-the-counter antimotion drugs, such as
meclizine (Bonine), depress vestibular inputs and help alleviate
the symptoms. Tey are most effective when taken “before the
fact.” Scopolamine-containing timed-release skin patches may
also head off motion sickness.
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