Pathway of taste receptors

 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).