nerve (X). These cranial nerves conduct the impulses into
the medulla oblongata. From there, the impulses ascend to
the thalamus and are directed to the gustatory cortex of the
cerebrum, located in the parietal lobe along a deep portion
of the lateral sulcus. Clinical Application 12.3 and
discuss disorders of smell and taste.
Why is saliva necessary to taste?
Name the f
ve primary taste sensations.
What characteristic oF taste receptors helps maintain a sense oF
taste with age?
Trace a sensory impulse From a taste receptor to the cerebral
To Chapter 11, Cranial Nerves, pages 414–417 and
Sense of Hearing
The organ of hearing, the
has outer (external), middle,
and inner (internal) sections. In addition to making hearing
possible, the ear provides the sense of equilibrium.
Outer (External) Ear
The outer ear consists of all of the structures that face the
outside. These include an outer, funnel-like structure called
(pinna) and an
-shaped tube, the
(external auditory canal) that leads
inward for about 2.5 centimeters
. The meatus ter-
minates with the
The external acoustic meatus passes into the temporal
bone. Near this opening, hairs guard the tube. The opening
and tube are lined with skin that has many modi± ed sweat
which secrete wax (ceru-
men). The hairs and wax help keep large foreign objects,
such as insects, out of the ear.
The transmission of vibrations through matter produces
sound. Just as the sounds of some musical instruments are pro-
duced by vibrating strings or reeds, the sounds of the human
voice are caused by vibrating vocal folds in the larynx. The
auricle of the ear helps collect sound waves traveling through
air and directs them into the external acoustic meatus.
proportional to the concentration of the hydrogen ions in the
substance being tasted. Ionized inorganic salts mainly stimu-
The quality of the sensation that each salt
produces depends upon the type of positively charged ion,
such as Na
from table salt, that it releases into solution.
A variety of chemicals stimulates
many organic compounds. Inorganic salts of magnesium and
calcium produce bitter sensations, too. Extreme sensitivity to
bitter tastes is inherited—this is why diet colas taste sweet
to some people but are bitter to others. Twenty-± ve types
of bitter receptors have been identi± ed. Quite a few of them
avors unique to fermented foods.
One group of bitter compounds of particular interest are
which include a number of poisons such as
strychnine, nicotine, and morphine. Spitting out bitter sub-
stances may be a protective mechanism to avoid ingesting
poisonous alkaloids in foods.
The taste sensation called
has long been recog-
nized in Japan but has only recently come to the attention of
western taste researchers. Umami is described as “savory,”
“pungent,” “meaty,” or “delicious.” Umami arises from the
binding of certain amino acids, including glutamate and
aspartate, to speci± c receptors. The ﬂ avor enhancer mono-
sodium glutamate (MSG), used in many prepared foods, also
stimulates umami receptors.
Taste receptors, like olfactory receptors, rapidly undergo
sensory adaptation. The resulting loss of taste can be avoided
by moving bits of food over the surface of the tongue to stim-
ulate different receptors at different moments.
Although taste cells are close to the surface of the tongue
and are therefore exposed to environmental wear and tear,
the sense of taste is not as likely to diminish with age as is
the sense of smell. This is because taste cells are modi±
epithelial cells and divide continually. A taste cell functions
for only about three days before it is replaced.
The sense oF taste re±
ects what happens to Food as it is chewed. Most
Foods are chemically complex, so they stimulate di²
In an experiment to track the act oF tasting, chemists collected sam-
ples oF air From participants’ nostrils as they bit into juicy red toma-
toes. An analytical technique called mass spectrometry revealed that
chewing activates a sequence oF chemical reactions in the tomato as
its tissues are torn, releasing f
rst aromatic hydrocarbons; then aFter
a thirty-second delay, products oF Fatty acid breakdown; and, f
several alcohols. This gradual release oF stimulating molecules is why
we experience a series oF ±
avors as we savor a Food.
Taste Nerve Pathways
Sensory impulses from taste receptor cells in the anterior
two-thirds of the tongue travel on ± bers of the facial nerve
(VII); impulses from receptors in the posterior one-third
of the tongue and the back of the mouth pass along the
glossopharyngeal nerve (IX); and impulses from receptors at
the base of the tongue and the pharynx travel on the vagus
Types of Smell and
Loss oF sensation