Sensory Physiology
225
acid transport protein fi rst identifi ed in the lingual papillae of
rodents may soon be added to the list. Research has shown that
blocking these transporters inhibits the preference for the taste
of foods with high lipid content, and reduces the production
of fat-digesting enzymes by the gastrointestinal system. If con-
fi rmed in humans, this fatty acid transporter could become the
sixth member of the taste receptor family, and might explain
our tendency to over-indulge on high-calorie, high-fat foods.
Each group of tastes has a distinct signal transduction
mechanism. Salt taste is detected by a simple mechanism in which
ingested sodium enters channels in the receptor cell membrane,
depolarizing the cell and stimulating the production of action
potentials in the associated sensory neuron. Sour taste is stim-
ulated by foods with high acid content, such as lemons, which
contain citric acid. Hydrogen ions block potassium channels in
the sour receptors, and the loss of the hyperpolarizing potas-
sium leak current depolarizes the receptor cell. Sweet receptors
have integral membrane proteins that bind natural sugars like
glucose, as well as artifi cial sweetener molecules like saccharin
and aspartame. Binding of sugars to these receptors activates a
G-protein-coupled second-messenger pathway (Chapter 5) that
ultimately blocks potassium channels and thus generates a depo-
larizing receptor potential. Bitter fl
avor is associated with many
poisonous substances, especially plant alkaloids like strychnine
and arsenic. There is an obvious evolutionary advantage in
recognizing a wide variety of poisonous substances, and thus
there are many varieties of bitter receptors. All of those types,
however, generate receptor potentials via G-protein-mediated
second-messenger pathways and ultimately evoke the negative
sensation of bitter fl avor. Umami receptor cells also depolarize
via a G-protein-coupled receptor mechanism.
Each
afferent
neuron
synapses
w
ith
more
than
one
receptor cell, and the taste system is organized into inde-
pendent coded pathways into the central nervous system.
Figure 7–44
Taste receptors. (a) Top view of the tongue showing several types of lingual papillae. (b) and (c) Cross section of one type of papilla with taste
buds. (d) Pores in the sides of papillae open into taste buds, which are composed of support cells, taste receptor cells, and basal cells.
Vallate
papillae
Taste bu
Connective tissue
Gustatory
(taste) cell
Supporting
cell
Basal cell
Sensory
fiber
Epithelium
of tongue
Taste hair
Taste pore
Taste buds
Papillae
Lingual papilla
Taste pore
Taste bud
100
m
(a)
(c)
(d)
(b)
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