451
CHAPTER TWELVE
Nervous System III
I
magine a spicy slice of pizza or freshly brewed
coF
ee, and your mouth waters in anticipation.
But for millions of people, the senses of smell
and taste are dulled, distorted, or gone. Many
more of us get some idea of their plight when a
cold temporarily sti±
es these senses.
Compared to the loss of hearing or sight,
being unable to taste or smell normally may
seem more an oddity than an illness. People with
such ailments would probably disagree. In some
situations, a poor or absent sense of smell can be
dangerous, such as in a house on ²
re.
The direct connection between the outside
environment and the brain makes the sense of
smell vulnerable to damage. Smell and taste dis-
orders can be triggered by colds and ±
u, allergies,
nasal polyps, swollen mucous membranes inside
the nose, a head injury, chemical exposure, a
nutritional or metabolic problem, or a disease. In
many cases, a cause cannot be identi²
ed.
Drugs can alter taste and smell in many ways,
aF
ecting cell turnover, the neural conduction sys-
tem, the status of receptors, and changes in nutri-
tional status. Consider what happened to twelve
hikers touring Peru and Bolivia. A day before a
long hike, three of them had begun taking aceta-
zolamide (Diamox), a drug that prevents acute
mountain sickness. The night after the climb, the
group went out for beer. To three of the people,
the brew tasted unbearably bitter, and a drink of
cola to wash away the taste was equally oF
ensive.
At fault: acetazolamide.
Drugs containing sulfur atoms squelch taste.
They include the anti-inflammatory drug pen-
icillamine, the antihypertensive drug captopril
(Capoten), and transdermal (patch) nitroglycerin
to treat chest pain. The antibiotic tetracycline and
the antiprotozoan metronidazole (³lagyl) impart
a metallic taste. Cancer chemotherapy and radia-
tion treatment often alter taste and smell.
Exposure to toxic chemicals can aF
ect taste
and smell, too. One woman, suddenly found that
once-pleasant smells had become oF
ensive. Her
doctor traced her problem to inhaling a paint
stripper. Hydrocarbon solvents in the product—
toluene, methanol, and methylene chloride—were
responsible for her
cacosmia,
the association of an
odor of decay with normally inoF
ensive stimuli.
12.3
CLINICAL APPLICATION
Smell and Taste Disorders
Middle Ear
The
middle ear,
or the
tympanic cavity,
is an air-f lled space
in the temporal bone that separates the outer and inner ears.
It is bounded by the tympanic membrane laterally and the
inner ear medially and houses three small bones called
audi-
tory ossicles
(aw
di-to
re os
i-klz).
The three auditory ossicles, called the
malleus,
the
incus,
and the
stapes,
are attached to the wall oF the tympanic
AFter entering the meatus, the sound waves pass to the
end oF the tube and alter the pressure on the tympanic mem-
brane. The tympanic membrane is a semitransparent mem-
brane covered by a thin layer oF skin on its outer surFace
and by mucous membrane on the inside. It has an oval mar-
gin and is cone-shaped, with the apex oF the cone directed
inward. The tympanic membrane moves back and Forth in
response to sound waves, reproducing the vibrations oF the
sound-wave source.
Vestibulocochlear
nerve
Tympanic cavity
Round window
Pharynx
Auditory tube
Tympanic
membrane
Auricle
External acoustic
meatus
Round window
Oval window (under stapes)
Cochlea
Malleus
Semicircular
canals
Incus
Stapes
FIGURE 12.9
Major parts of the ear.
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