224
Chapter 7
nausea and lightheadedness. Such disruptions can result from
strokes, irritation of the labyrinths by infection, loose particles
of calcium carbonate in the semicircular canals, or inhibition
of vestibular inputs caused by excess alcohol consumption. A
similar phenomenon can occur when there is a mismatch in
information from the various sensory systems. For example,
many amusement parks feature widescreen virtual thrill rides
in which your eyes take you on a dizzying helicopter ride,
while your vestibular system signals that you are not mov-
ing at all.
Motion sickness
also involves the vestibular system,
occurring when you experience unfamiliar patterns of linear
and rotational acceleration and adaptation to them has not yet
occurred.
Chemical Senses
Recall that receptors sensitive to specifi c chemicals are
che-
moreceptors.
Some of these respond to chemical changes in
the internal environment; two examples are the oxygen and
hydrogen ion receptors in certain large blood vessels (Chapter
13). Others respond to external chemical changes. In this
category are the receptors for taste and smell, which affect a
person’s appetite, saliva fl ow, gastric secretions, and avoidance
of harmful substances.
Taste
The specialized sense organs for taste (also called
gustation
)
are the 10,000 or so
taste buds
found in the mouth and throat,
the vast majority on the upper surface and sides of the tongue.
Taste buds are small groups of cells arranged like orange slices
around a hollow pore, and are found in the walls of visible
structures called
lingual papillae
(
Figure 7–44
). Some of
the cells serve mainly as support cells, but others are special-
ized epithelial cells that act as receptors for various chemicals
in the food we eat. Small, hairlike projections increase the sur-
face area of taste receptor cells, and contain integral membrane
proteins that transduce the presence of a given chemical into
a receptor potential. At the bottom of taste buds are
basal
cells,
which divide and differentiate to continually replace
taste receptor cells damaged in the harsh environment of the
mouth. To enter the pores of the taste buds and come into
contact with taste-receptor cells, food molecules must be dis-
solved in liquid—either ingested or provided by secretions of
the salivary glands. Try placing sugar or salt on your tongue
after thoroughly drying it; no taste sensation occurs.
Many different chemicals can generate the sensation
of taste by differentially activating a few basic types of taste
receptors. Taste modalities generally fall into fi ve different cat-
egories according to the receptor type most strongly activated;
sweet, sour, salty, bitter, and
umami
(pronounced “oo-MOM-
mee”). This latter category is named after the Japanese word
for “delicious.” This taste is associated with the taste of gluta-
mate and similar amino acids, and is sometimes described as
conveying the sense of fl avorfulness. Glutamate (or monoso-
dium glutamate, MSG) is a common additive used to enhance
the fl
avor of foods in traditional Asian cuisine. In addition to
these known taste receptors, there are likely others yet to be dis-
covered. For example, recent experiments suggest that a fatty
Figure 7–43
Effect of head position on otolith organ of the utricle. (a) Upright
position, hair cells are not bent. (b) Gravity bends the hair cells
when the head tilts forward.
(a)
(b
Vestibular
nerve
Hair cell
Supporting cell
however, depend exclusively on input from the vestibular sys-
tem despite the fact that the vestibular organs are sometimes
called the sense organs of balance.
The third use of vestibular information is in provid-
ing conscious awareness of the position and acceleration of
the body, perception of the space surrounding the body, and
memory of spatial information.
Information about hair cell stimulation is relayed from
the vestibular apparatus to nuclei within the brainstem via the
vestibular branch of the vestibulocochlear nerve. It is trans-
mitted via a multineuronal pathway through the thalamus to
a system of vestibular centers in the parietal lobe. Descending
projections are also sent from the brainstem nuclei to the spi-
nal cord to infl uence postural refl
exes. Vestibular information
is integrated with information from the joints, tendons, and
skin, leading to the sense of posture
(proprioception)
and
movement.
Unexpected inputs from the vestibular system and other
sensory systems can induce
vertigo,
defi ned as an illusion of
movement—usually spinning—accompanied by feelings of
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