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Chapter 7
Single receptor cells, however, respond in varying degrees to
substances that fall into more than one taste category. This
property is analogous to the overlapping sensitivities of pho-
toreceptors to different wavelengths. Awareness of the specifi c
taste of a substance depends also upon the pattern of fi ring
in other types of sensory neurons. For example, sensations of
pain (hot spices), texture, and temperature contribute to taste.
The pathways for taste in the central nervous system
project to the parietal cortex, near the “mouth” region of the
somatosensory cortex (see Figure 7–14).
Smell
Eighty percent of the fl
avor of food is actually contributed by
the sense of smell, or
olfaction.
This is illustrated by the com-
mon experience of fi nding that food lacks taste when a head cold
blocks your nasal passages. The odor of a substance is directly
related to its chemical structure. We can recognize and identify
thousands of different odors with great accuracy. Thus, neural
circuits that deal with olfaction must encode information about
different chemical structures, store (learn) the different code
patterns that represent the different structures, and at a later
time recognize a particular neural code to identify the odor.
The olfactory receptor neurons, the fi rst cells in the path-
ways that give rise to the sense of smell, lie in a small patch of
epithelium called the
olfactory epithelium,
in the upper part
of the nasal cavity (
Figure 7–45a
). Olfactory receptor neu-
rons live for only about two months, so they are constantly
being replaced by new cells produced from stem cells in the
olfactory epithelium. The mature cells are specialized affer-
ent neurons that have a single, enlarged dendrite that extends
to the surface of the epithelium. Several long, nonmotile cilia
extend from the tip of the dendrite and lie along the sur-
face of the olfactory epithelium (
Figure 7–45b
) where they
are bathed in mucus. The cilia contain the receptor proteins
(binding sites) for olfactory stimuli. The axons of the neurons
form the olfactory nerve, which is cranial nerve I.
For us to detect an odorous substance (an
odorant
), mol-
ecules of the substance must fi rst diffuse into the air and pass
into the nose to the region of the olfactory epithelium. Once
there, they dissolve in the mucus that covers the epithelium and
then bind to specifi c odorant receptors on the cilia. Proteins
in the mucus may interact with the odorant molecules, trans-
port them to the receptors, and facilitate their binding to the
receptors. Stimulated odorant receptors activate a G-protein-
mediated pathway that increases cAMP, which in turn opens
nonselective cation channels and depolarizes the cell.
Although there are many thousands of olfactory recep-
tor cells, each contains only one of the 1000 or so differ-
ent plasma membrane odorant receptor types. In turn, each
of these types responds only to a specifi c chemically related
group of odorant molecules. Each odorant has characteristic
chemical groups that distinguish it from other odorants, and
each of these groups activates a different plasma membrane
odorant receptor type. Thus, the identity of a particular odor-
ant is determined by the activation of a precise combination of
plasma membrane receptors, each of which is contained in a
distinct group of olfactory receptor cells.
The axons of the olfactory receptor cells synapse in the
brain structures known as
olfactory bulbs,
which lie on the
undersurface of the frontal lobes. Axons from olfactory recep-
tor cells that share a common receptor specifi city synapse
together on certain olfactory bulb neurons, thereby maintain-
ing the specifi city of the original stimulus. In other words,
specifi c odorant receptor cells activate only certain olfactory
Olfactory bulb
Afferent nerve fibers
(olfactory nerve)
Olfactory
epithelium
Nose
Upper lip
Inner
chamber
of nose
Hard
palate
Olfactory
nerve
Axon
Mucus
layer
Olfactory
epithelium
Cilia
Supporting
cell
Olfactory
receptor cell
Stem cell
(a)
(b)
Figure 7–45
(a) Location and (b) enlargement of a portion of the olfactory epithelium showing the structure of the olfactory receptor cells. In addition to
these cells, the olfactory epithelium contains stem cells, which give rise to new receptors and supporting cells.
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