461
CHAPTER TWELVE
Nervous System III
Motion sickness is a disturbance of the inner ear’s sensation of bal-
ance. Nine out of ten people have experienced this nausea and vom-
iting, usually when riding in a car or on a boat. Astronauts suffer a
form of motion sickness called space adaptation syndrome.
Motion sickness is thought to result when visual information
contradicts the inner ear’s sensation that one is motionless. Consider
a woman riding in a car. Her inner ears tell her that she is not moving,
but the passing scenery tells her eyes that she is moving. The problem
is compounded if she tries to read. The brain reacts to these seem-
ingly contradictory sensations by signaling a “vomiting center” in the
medulla oblongata.
PRACTICE
26
Distinguish between the senses of static and dynamic equilibrium.
27
Which structures provide the sense of static equilibrium? Of
dynamic equilibrium?
28
How does sensory information from other receptors help
maintain equilibrium?
Macula
of utricle
Hair cells
Sensory nerve fiber
Supporting cells
Otoliths
Hairs of
hair cells bend
Gelatinous
material sags
Gravitational
force
FIGURE 12.19
Scanning electron micrograph of hairs of hair cells,
such as those in the utricle and saccule (8,000×).
(a) Head upright
(b) Head bent forward
FIGURE 12.18
The maculae respond to changes in head position. (
a
) Macula of the utricle with the head in an upright position. (
b
) Macula of the
utricle with the head bent forward.
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