Chapter 15
The Special Senses
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15
receptors detect substances present in minute amounts. Te
other receptors are less sensitive. ±aste receptors adapt rapidly,
with partial adaptation in 3–5 seconds and complete adaptation
in 1–5 minutes.
Taste Transduction
Te mechanisms of taste transduction are
only now beginning to become clear. Tree different mecha-
nisms underlie how we taste.
Salty taste is due to Na
1
influx through Na
1
channels, which
directly depolarizes gustatory epithelial cells.
Sour is mediated by H
1
, which acts intracellularly to open
channels that allow other cations to enter.
Bitter, sweet, and umami responses share a common mecha-
nism, but each occurs in a different cell. Each taste’s unique
set of receptors is coupled to a common G protein called
gustducin
. Activation leads to the release of Ca
2
1
from intra-
cellular stores, which causes cation channels in the plasma
membrane to open, thereby depolarizing the cell and releas-
ing the neurotransmitter A±P.
The Gustatory Pathway
Afferent fibers carrying taste information from the tongue are
found primarily in two cranial nerve pairs. A branch of the
facial
nerve
(VII), the
chorda tympani
, transmits impulses from taste
receptors in the anterior two-thirds of the tongue
(Figure 15.23)
.
Te lingual branch of the
glossopharyngeal nerve
(IX) services
the posterior third and the pharynx just behind. ±aste impulses
from the few taste buds in the epiglottis and the lower pharynx
are conducted primarily by the
vagus nerve
(X).
Tese afferent fibers synapse in the
solitary nucleus
of
the medulla, and from there impulses stream to the thalamus
and ultimately to the
gustatory cortex
in the insula. Fibers also
project to the hypothalamus and limbic system structures, re-
gions that determine our appreciation of what we are tasting.
An important role of taste is to trigger reflexes involved in
digestion. As taste impulses pass through the solitary nucleus,
they initiate reflexes (via synapses with parasympathetic nuclei)
that increase secretion of saliva into the mouth and of gastric
juice into the stomach. Saliva contains mucus that moistens
food and digestive enzymes that begin digesting starch. Acidic
foods are particularly strong stimulants of the salivary reflex.
On the other hand, when we eat revolting or foul-tasting sub-
stances, the taste may initiate protective reactions such as gag-
ging or reflexive vomiting.
Influence of Other Sensations on Taste
±aste is 80% smell. When nasal congestion (or just pinching
your nostrils) blocks access to your olfactory receptors, food
tastes bland. Without smell, our morning coffee would lack its
richness and simply taste bitter.
Te mouth also contains thermoreceptors, mechanorecep-
tors, and nociceptors, and the temperature and texture of foods
can enhance or detract from their taste. “Hot” foods such as
chili peppers actually bring about their pleasurable effects by
exciting pain receptors in the mouth.
Basic Taste Sensations
Normally, our taste sensations are complicated mixtures of
qualities. However, when taste is tested with pure chemical
compounds, all taste sensations can be grouped into one of five
basic modalities: sweet, sour, salty, bitter, and umami.
Sweet
taste is elicited by many organic substances including
sugars, saccharin, alcohols, some amino acids, and some lead
salts (such as those found in lead paint).
Sour
taste is produced by acids, specifically their hydrogen
ions (H
1
) in solution.
Salty
taste is produced by metal ions (inorganic salts); table
salt (sodium chloride) tastes the “saltiest.”
Bitter
taste is elicited by alkaloids (such as quinine, nicotine,
caffeine, morphine, and strychnine) as well as a number of
nonalkaloid substances, such as aspirin.
Umami
(u-mam
9
e; “delicious”), a subtle taste discovered by
the Japanese, is elicited by the amino acids glutamate and as-
partate, which appear to be responsible for the “beef taste” of
steak, the characteristic tang of aging cheese, and the flavor
of the food additive monosodium glutamate.
In addition, there is growing evidence for our ability to taste
long-chain fatty acids from lipids. Tis possible sixth modality
may help explain our liking for fatty foods.
Keep in mind that many substances produce a mixture of
these basic taste sensations, and taste buds generally respond to
all five. However, it appears that a single taste cell has receptors
for only one taste modality.
Although taste maps that assign specific tastes to various
areas of the tongue are common, researchers have known for
years that these maps are dubious. All areas that contain taste
buds can detect all modalities of taste.
±aste likes and dislikes have homeostatic value. Umami
guides the intake of proteins, and a liking for sugar and salt
helps satisfy the body’s need for carbohydrates and minerals (as
well as some amino acids). Many sour, naturally acidic foods
(such as oranges, lemons, and tomatoes) are rich sources of vi-
tamin C, an essential vitamin. On the other hand, intensely sour
tastes warn us of spoilage. Likewise, many natural poisons and
spoiled foods are bitter. Consequently, our dislike for sourness
and bitterness is protective.
Physiology of Taste
For a chemical to be tasted it must dissolve in saliva, diffuse into
a taste pore, and contact the gustatory hairs.
Activation of Taste Receptors
Gustatory epithelial cells con-
tain neurotransmitters. When a food chemical, or
tastant
, binds
to receptors in the gustatory epithelial cell membrane, it induces
a graded depolarizing potential that causes neurotransmitter re-
lease. Binding of the neurotransmitter to the associated sensory
dendrites triggers generator potentials that elicit action poten-
tials in these fibers.
Te different gustatory epithelial cells have different thresh-
olds for activation. In line with their protective nature, the bitter
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