|Figure 35-18 Pheromone-producing glands of an ant.
Chemoreception is the oldest and
most universal sense in the animal
kingdom. It probably guides behavior
of animals more than any other sense.
Unicellular forms use contact chemical
to locate food and
adequately oxygenated water and to
avoid harmful substances. These
receptors elicit an orientation behavior,
called chemotaxis, toward or
away from the chemical source. Most
metazoans have specialized distance
, which are often
developed to a remarkable degree of
sensitivity. Distance chemoreception,
usually called smell or olfaction,
guides feeding behavior, location and
selection of sexual mates, territorial
and trail marking, and alarm reactions
of numerous animals.
|Figure 35-19 Taste receptors. A,
Surface of human tongue
of maximum sensitivity to the four
taste sensations. B, Position of taste buds
on a taste papilla. C, Structure of a taste bud.
Social insects and many other animals,
including mammals, produce
species-specific compounds, called
pheromones, that constitute a highly
developed chemical language. Pheromones
are a diverse group of organic
compounds that an animal releases to
affect the physiology or behavior of
another individual of the same species.
Ants, for example, are walking batteries
of glands (Figure 35-18) that
produce numerous chemical signals.
These include releaser pheromones,
such as alarm and trail pheromones,
and primer pheromones, which alter
endocrine and reproductive systems of
different castes in the colony. Insects
bear a variety of chemoreceptors on
the surface of the body for sensing
specific pheromones, as well as other,
In all vertebrates and in insects,
the senses of taste
clearly distinguishable. Although there
are similarities between taste and smell
receptors, in general taste is more
restricted in response and is less sensitive
than smell. Central nervous system
centers for taste and smell are located
in different parts of the brain.
In vertebrates, taste receptors are
found in the mouth cavity and especially
on the tongue (Figure 35-19),
provide a means for judging
foods before they are swallowed. A taste bud
consists of a cluster of
receptor cells surrounded by supporting
cells; it is provided with a small
external pore through which the slender
tips of the sensory cells project.
Chemicals being tasted apparently
combine with specific receptor sites on
microvilli of the receptor cells.
Although the mechanisms are different
for each basic taste sensation, receptor
cells are depolarized by the specific
chemical to which the cell is sensitive
and action potentials are generated.
These impulses are transmitted across
chemical synapses and travel
along sensory neurons to specific
brains regions. Because receptor cells
are subject to wear and tear by abrasive
foods, taste buds have a short life
(5 to 10 days in mammals) and are
continually being replaced.
The four basic taste sensations
possessed by humans—sour, salty, bitter,
and sweet—are each attributable to
a different kind of taste bud. The tastes
for salty and sweet are found mainly
on the tip, bitter at the base, and sour
along the sides of the tongue. Of these,
the bitter taste is by far the most sensitive,
because it provides early warning
against potentially dangerous substances,
many of which are bitter.
|Figure 35-20 Human olfactory epithelium.
A, The epithelium is
a patch of tissue positioned
in the roof of the
nasal cavity. B, It is
composed of supporting
cells, basal cells
, and olfactory receptor cells
cilia protruding from their free ends.
Smell is more complex than taste,
and until very recently odor research
has lagged behind other areas of sensory
physiology. Although the olfactory
sense is a primal sense for many animals,
used for identification of food,
sexual mates, and predators, olfaction is
most highly developed in mammals.
Even humans, although a species not
celebrated for detecting smells, can discriminate
perhaps 20,000 different
odors. A human nose can detect 1/25 of
one-millionth of 1 mg of mercaptan, the
odoriferous substance of skunks. Even
so, our olfactory abilities compare
poorly with those of other mammals
that rely on olfaction for survival. A dog
explores new surroundings with its
nose much as we do with our eyes. A
dog’s nose is justifiably renowned; with
some odorous sources a dog’s nose is at
least a million times more sensitive than
ours. Dogs are assisted in their proficiency
by having a nose located close
to the ground where odors from passing
creatures tend to linger.
Olfactory endings are located in a
special epithelium covered by a thin
film of mucus, positioned deep in the
nasal cavity (Figure 35-20). Within the
epithelium lie millions of olfactory
neurons, each with several hairlike
cilia protruding from the free end.
Odor molecules entering the nose bind
to receptor proteins located in the cilia;
this binding generates an electrical signal
that travels along axons to the
olfactory bulb of the brain. From here
odor information is sent to the olfactory
cortex where odors are analyzed.
Odor information is then projected to
higher brain centers where they affect
emotions, thoughts, and behavior.
Recently, using techniques of gene
cloning and molecular hybridization), researchers discovered a large
family of genes that appears to code
for odor reception in mammals (including
humans). Each of the 500 to
1000 genes discovered encodes a separate
type of odor receptor. Since mammals
can detect at least 20,000 different
odors, each receptor must respond to
several odor molecules, and each odor
molecule must bind with several types
of receptors, each of which responds
to a part of the molecule’s structure.
Brain mapping techniques have shown
that each olfactory neuron projects to a
characteristic location on the olfactory
bulb, providing a two-dimensional
map that identifies which receptors
have been activited in the nose. In
addition, olfactory neurons expressing
the same odor receptor gene converge
to a fixed olfactory bulb region, which
might provide an explanation for the
extremely high sensitivity of smell.
Projected to the brain, odor information
is recognized as a unique scent.
Because flavor of food depends on
odors reaching the olfactory epithelium
through the throat passage, taste
and smell are easily confused. All
“tastes” other than the four basic ones
(sweet, sour, bitter, salty) result from
flavor molecules reaching the olfactory
epithelium in this manner. Food loses
its appeal during a common cold
because a stuffy nose blocks odors rising
from the mouth.