CHAPTER 14
Smell & Taste 223ODORANT-BINDING PROTEINS
In contrast to the low threshold for olfactory stimulation when
the olfactory epithelium is intact, single olfactory receptors that
have been patch-clamped have a relatively high threshold and a
long latency. In addition, lipophilic odor-producing molecules
must traverse the hydrophilic mucus in the nose to reach the re-
ceptors. These facts led to the suggestion that the olfactory mu-
cus might contain one or more
odorant-binding proteins
(OBP)
that concentrate the odorants and transfer them to the re-
ceptors. An 18-kDa OBP that is unique to the nasal cavity has
been isolated, and other related proteins probably exist. The pro-
tein has considerable homology to other proteins in the body
that are known to be carriers for small lipophilic molecules. A
similar binding protein appears to be associated with taste.
VOMERONASAL ORGAN
In rodents and various other mammals, the nasal cavity con-
tains another patch of olfactory epithelium located along the
nasal septum in a well-developed
vomeronasal organ.
This
structure is concerned with the perception of odors that act as
pheromones.
Vomeronasal sensory neurons project to the
ac-
cessory olfactory bulb
and from there primarily to areas in
the amygdala and hypothalamus that are concerned with re-
production and ingestive behavior. Vomeronasal input has
major effects on these functions. An example is pregnancy
block in mice; the pheromones of a male from a different
strain prevent pregnancy as a result of mating with that male,
but mating with a mouse of the same strain does not produce
blockade. The vomeronasal organ has about 100 G protein-
coupled odorant receptors that differ in structure from those
in the rest of the olfactory epithelium.
The organ is not well developed in humans, but an anatomi-
cally separate and biochemically unique area of olfactory epithe-
lium occurs in a pit in the anterior third of the nasal septum,
which appears to be a homologous structure. There is evidence
for the existence of pheromones in humans, and there is a close
relationship between smell and sexual function. Perfume adver-
tisements bear witness to this. The sense of smell is said to be
more acute in women than in men, and in women it is most
acute at the time of ovulation. Smell, and to a lesser extent, taste,
have a unique ability to trigger long-term memories, a fact noted
by novelists and documented by experimental psychologists.
SNIFFING
The portion of the nasal cavity containing the olfactory recep-
tors is poorly ventilated in humans. Most of the air normally
moves smoothly over the turbinates with each respiratory cy-
cle, although eddy currents pass some air over the olfactory
epithelium. These eddy currents are probably set up by con-
vection as cool air strikes the warm mucosal surfaces. The
amount of air reaching this region is greatly increased by sniff-
ing, an action that includes contraction of the lower part of the
nares on the septum, deflecting the airstream upward. Sniffing
is a semireflex response that usually occurs when a new odor
attracts attention.ROLE OF PAIN FIBERS IN THE NOSE
Naked endings of many trigeminal pain fibers are found in the
olfactory epithelium. They are stimulated by irritating sub-
stances and leads to the characteristic “odor” of such substanc-
es as peppermint, menthol, and chlorine. Activation of these
endings by nasal irritants also initiates sneezing, lacrimation,
respiratory inhibition, and other reflexes.ADAPTATION
It is common knowledge that when one is continuously ex-
posed to even the most disagreeable odor, perception of the
odor decreases and eventually ceases. This sometimes benefi-
cent phenomenon is due to the fairly rapid adaptation, or de-
sensitization, that occurs in the olfactory system. It is
mediated by Ca
2+
acting via calmodulin on
cyclic nucleotide-
gated (CNG)
ion channels. When the CNG A4 subunit is
knocked out, adaptation is slowed.TASTE
TASTE BUDS
The specialized sense organ for taste (gustation) consists of ap-
proximately 10,000
taste buds,
which are ovoid bodies measur-
ing 50–70
μ
m. There are four morphologically distinct types of
cells within each taste bud: basal cells, dark cells, light cells, and
intermediate cells (Figure 14–6). The latter three cell types are
all referred to as
Type I, II, and III taste cells.
They are the sen-
sory neurons that respond to taste stimuli or
tastants.
The three
cell types may represent various stages of differentiation of de-
veloping taste cells, with the light cells being the most mature.
Alternatively, each cell type might represent different cell lin-
eages. The apical ends of taste cells have microvilli that project
into the taste pore, a small opening on the dorsal surface of the
tongue where tastes cells are exposed to the oral contents. Each
taste bud is innervated by about 50 nerve fibers, and conversely,
each nerve fiber receives input from an average of five taste
buds. The basal cells arise from the epithelial cells surrounding
the taste bud. They differentiate into new taste cells, and the old
cells are continuously replaced with a half-time of about 10
days. If the sensory nerve is cut, the taste buds it innervates de-
generate and eventually disappear.
In humans, the taste buds are located in the mucosa of the
epiglottis, palate, and pharynx and in the walls of
papillae
of
the tongue (Figure 14–6). The
fungiform papillae
are
rounded structures most numerous near the tip of the tongue;
the
circumvallate papillae
are prominent structures arranged
in a V on the back of the tongue; the
foliate papillae
are on