448
UNIT THREE
PRACTICE
15
Where are the olfactory receptors located?
16
Trace the pathway of an olfactory impulse from a receptor to the
cerebrum.
Sense of Taste
Taste buds
are the special organs of taste. They resemble
orange sections and associate on the surface of the tongue
with tiny elevations called
papillae
(f
gs. 12.7
and
12.8)
.
Taste buds are also scattered in the roof of the mouth, the
linings of the cheeks, and the walls of the pharynx.
Taste Receptors
Each taste bud includes a group of modiF ed epithelial cells,
the
taste cells
(gustatory cells), that function as sensory
receptors. Each of our 10,000 taste buds houses 50 to 150
taste cells. The taste bud also includes epithelial supporting
cells. The entire structure is somewhat spherical, with an
opening, the
taste pore,
on its free surface. Tiny projections
(microvilli), called
taste hairs,
protrude from the outer ends
of the taste cells and jut out through the taste pore. These
taste hairs are the sensitive parts of the receptor cells.
Interwoven among and wrapped around the taste cells
is a network of nerve F
bers. The ends of these F bers closely
contact the receptor cell membranes. A stimulated receptor
cell triggers an impulse on a nearby nerve F ber, which trav-
els into the brain.
A chemical to be tasted must dissolve in saliva, the
watery fl uid surrounding the taste buds. The salivary glands
supply this fl uid. To demonstrate the importance of saliva,
blot your tongue and try to taste some dry food; then repeat
the test after moistening your tongue with saliva.
As is the case for smell, the sense of taste derives from
combinations of chemicals binding speciF
c receptor proteins
explains why odors can alter mood so easily. ±or example,
the scent of new-mown hay or rain on a summer’s morning
generally makes us feel good. The main interpreting areas
for the olfactory impulses (olfactory cortex) are deep within
the temporal lobes and at the bases of the frontal lobes, ante-
rior to the hypothalamus.
Olfactory Stimulation
Biologists are not certain how stimulated receptors encode
speciF c smells, but a leading hypothesis is that each odor
likely stimulates a distinct set of receptor cells that in turn
have distinct sets of receptor proteins. The brain then rec-
ognizes the particular combination as an
olfactory code.
±or
example, imagine a simpliF
ed system with ten types of odor
receptors. Banana might stimulate receptors 2, 4, and 7; gar-
lic, receptors 1, 5, and 9. Some investigators have proposed
seven primary odors, but others hypothesize that the num-
ber is much higher and may refl
ect the functioning of hun-
dreds of genes.
The olfactory organs are high in the nasal cavity above
the usual pathway of inhaled air, so snifF ng and forcing air
over the receptor areas may be necessary to smell a faint
odor. Olfactory receptors undergo sensory adaptation rather
rapidly, so the intensity of a smell drops about 50% within
a second following the stimulation. Within a minute, the
receptors may become almost insensitive to a given odor,
but even though they have adapted to one scent, their sensi-
tivity to other odors persists.
The olfactory receptor neurons are the only nerve cells in
direct contact with the outside environment. These neurons
are subject to damage because of their exposed positions.
±ortunately, basal cells along the basement membrane of the
olfactory epithelium regularly divide and yield differentiated
cells that replace lost olfactory receptor neurons. These are
the only damaged neurons that are regularly replaced.
FIGURE 12.6
Light micrograph of the olfactory epithelium (250×).
Olfactory epithelium
Olfactory cell cilia
Connective tissue
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