PATHWAY FOR TASTE RECEPTORS
Receptors for taste sensation
are the type III cells of taste buds. Each taste bud is innervated by about 50
sensory
nerve fibers and each nerve fiber supplies at least five taste buds through its
terminals.
FIRST ORDER NEURON
First order neurons of taste
pathway are in the nuclei of three different cranial nerves, situated in
medulla
oblongata. Dendrites of
the neurons are distributed to the taste buds. After arising
from taste buds, the fibers reach the cranial nerve nuclei by running
along the following nerves :
1. Chorda
tympani fibers of facial
nerve, which run from anterior two third of tongue
2. Glossopharyngeal nerve fibers, which run from posterior
one third of the tongue
3. Vagal fibers, which run from taste buds in other regions.
Axons from
first order neurons in the nuclei of these nerves run together in medulla
oblongata and terminate in the nucleus of tractus
solitarius.
3. Sour
4. Bitter
5. Umami.
Man can perceive more than
100 different tastes. Other taste sensations are just the combination of two
or more primary taste
sensations.
Combination of Taste
Sensation with Other Sensations
Sometimes, taste sensation
combines with other sensations to give rise to a different sensation. For
example, combination of taste, smell and touch senses, gives rise to sensation of flavor. Combination of taste with pain
gives rise to sensation of ginger.
DISCRIMINATION OF DIFFERENT TASTE
SENSATIONS
Earlier, it was believed that
different areas of tongue were specialized for
different taste sensation. Now it
is clear that all areas of
tongue give response to all types of taste sensations. Usually, in low
concentration of taste substance, each taste bud gives response to one primary
taste stimulus. However, in high concentration, the taste buds give response to
more than one type of taste stimuli.
It is also
clear now that each afferent nerve fiber from the taste buds carry
impulses of one taste sensation.
TASTE SENSATIONS AND CHEMICAL
CONSTITUTIONS
Substances causing sour or
salt tastes are mostly electrolytes. Bitter and sweet tastes are caused by
electrolytes or
non-electrolytes.
SWEET TASTE
Sweet taste is produced
mainly by organic substances like monosaccharides, polysaccharides, glycerol,
alcohol, aldehydes, ketones
and chloroform. Inorganic substances, which produce sweet taste are lead and beryllium.
SALT TASTE
Salt taste is produced by
chlorides of sodium, potassium and ammonium, nitrates of sodium and
potassium. Some sulfates,
bromides and iodides also produce salt taste.
SOUR TASTE
Sour taste is produced
because of hydrogen ions in acids and acid salts.
BITTER TASTE
Bitter taste is produced
by organic substances like quinine, strychnine, morphine, glucosides, picric
acid
and bile salts and inorganic
substances like salts of calcium, magnesium and ammonium. Bitterness of the salts
is mainly due to cations.
UMAMI
Umami is the recently
recognized taste sensation. Umami is a Japanese word, meaning ‘delicious’.
Receptors of this taste sensation
respond to glutamate, particularly monosodium
glutamate (MSG), which is a common ingredient in Asian food. However, excess MSG
consumption is proved to produce Chinese
restaurant syndrome in
some people taking Chinese food regularly. Common symptoms are headache,
flushing,
sweating, perioral numbness, chest pain. In severe conditions, airway
swelling and obstruction
and cardiac arrhythmia
occur.
Threshold for Taste
Sensations
Sweet taste Sugar : 1 in
200 dilution
Salt taste Sodium chloride
: 1 in 400 dilution
Sour taste Hydrochloric
acid : 1 in 15,000 dilution
Bitter taste Quinine : 1
in 2,000,000 dilution.
Bitter taste has very low
threshold and sweet taste has a high threshold. Threshold for umani is not
known.
TASTE TRANSDUCTION
Taste transduction is the
process by which taste receptor converts chemical energy into action potentials
in the
taste nerve fiber. Receptors of taste sensation are chemoreceptors,
which are stimulated by substances dissolved
in mouth by saliva. The dissolved substances act on
microvilli of taste receptors exposed in the taste pore. It causes the development of receptor potential in the receptor cells. This in turn, is responsible for the generation of action potential in the sensory neurons.
Taste Receptor
Generally, taste receptor
is a G-protein coupled receptor (GPCR). It is also called G protein
gustducin However, several other receptors are also involved in taste
sensation. Transduction mechanism is different in each
taste receptor cells.
SWEET RECEPTOR
Receptor for sweet taste
is GPCR. The sweet substances bind to receptor and cause depolarization
via cyclic AMP.
SALT RECEPTOR
Receptor for salt taste is
called epithelial sodium channel (ENaC). It acts
like ENaC receptors in other
parts of the body. When sodium
enters, this receptor releases glutamate, which causes depolarization.
SOUR RECEPTOR
Sour sensation also has
the same ENaC receptor. The proton (hydrogen) enters the receptor and causes
depolarization. It is believed
that besides ENaC, other receptors such as hyperpolarization-activated
cyclic nucleotide-gated cation channel (HCN) also are involved
in sour sensation.
BITTER RECEPTOR
Bitter receptor is a GPCR.
In bitter receptor, the sour substances activate phospholipase C through G
proteins. It causes production
of inositol triphosphate (IP3), which initiates depolarization
by releasing
calcium ions.
UMAMI RECEPTOR
Umami receptor is called metabotropic
glutamate receptor (mGluR4). Glutamate causes depolarization
of
this receptor. Exact
mechanism of depolarization is not
clearly understood.
Activation of umami taste receptor is intensified by the
presence of guanosine monophosphate
(GMP) and inosine monophosphate (IMP).
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