Sensation Of Taste Physiology Notes

Taste Buds

The sense organs for taste or gustatory sensation are the taste buds. The taste buds are ovoid bodies with a diameter of 50-70 p. In adults, about 10,000 taste buds are present. The number is more in children, in old age, many taste buds degenerate and, the sensitivity of taste becomes less.

1. Situation Of Taste Buds: Most of the taste buds are present on the papillae of the tongue. Taste buds are also situated in the mucosa of the epiglottis, palate, pharynx, and the proximal part of the esophagus. Three types of papillae are located on the tongue:

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1. Filiform papillae
2. Fungiform papillae
3. Circumvallate papillae.

1. Filiform Papillae: Filiform papillae are small and conical-shaped papillae situated over the dorsum of the tongue. The papillae contain less number of taste buds (only a few).

2. Fungiform Papillae: Fungiform papillae are round in shape and are situated over the anterior surface of the tongue near the tip. Numerous fungiform papillae are present. The number of taste buds In each is moderate (up to 10).

3. Circumvallate Papillae: Circumvailate papillae are large structures present on the posterior part of the tongue and are many in number. These papillae are arranged in the shape of a ‘V’. Each papilla contains many taste buds (up to 100).

2. Structure Of Taste Bud:

The taste bud is a bundle of taste receptor cells, with supporting cells embedded in the epithelial covering of the papillae. Each taste bud contains about 40 cells, which are the modified epithelial cells. The cells of taste buds are divided into four groups:

1. Type 1 cells or sustentacular cells
2. Type 2 cells
3. Type 3 cells
4. Type 4 cells or basal cells.

• Type 1 cells and type 4 cells are supporting cells. Type 3 cells are the taste receptor cells. The function of type 2 cells is unknown. Type 1, 2, and 3 cells have projections called microvilli. The microvilli project into an opening in the epithelium covering the tongue. The opening is called the taste pore.
• The neck of each cell is attached to the neck of the other. All the cells of the taste bud are surrounded by epithelial cells. There are tight junctions between epithelial cells and the neck portion of the type 1. 2 and 3 cells, so that only the tip of these cells is exposed to fluid in the oral cavity.
• The cells of taste buds undergo a constant cycle of growth, apoptosis, and regeneration.

Pathway For Taste

1. Receptors: Receptors for taste sensation are the type 3 cells of taste buds. Each taste bud is innervated by about 5C sensory nerve fibers and each nerve fiber supplies at least 5 taste buds through its terminals.

2. First-Order Neuron: First-order neurons of the taste pathway are in the nuclei of three different cranial nerves. The dendrites of the neurons are distributed to the taste buds. After arising from taste buds, the fibers run along the following nerves:

• Chorda tympani fibers of the facial nerve, which run from the anterior two-thirds of the tongue
• Glossopharyngeal nerve fibers, which run from the posterior one-third of the tongue
• Vagal fibers, which run from taste buds in other regions.

Axons of the first-order neurons run together in the medulla oblongata and terminate in the nucleus of tractus solitarius.

3. Second-Order Neuron: Second-order neurons are in the nucleus of tractus solitarius. Axons of the second-order neurons, run through the medial lemniscus and terminate in the posteroventral nucleus of the thalamus.

4. Third Order Neuron: The third-order neurons are in the posteroventral nucleus of the thalamus. The axons from the third-order neurons project into the parietal lobe of the cerebral cortex.

5. Taste Center: The center for taste sensation is in the opercular insular cortex, i.e. in the lower part of the postcentral gyrus, which receives cutaneous sensations from the face. Thus, the taste fibers do not have an independent cortical projection.

Primary Taste Sensations

The primary or fundamental taste sensations are divided into four types:

1. Sweet
2. Salt
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 sensations. Sometimes, the taste sensation is combined with other sensations like pain (ginger) or temperature (flavor).

Discrimination Of Different Taste Sensations

• Earlier, it was believed that different areas of the tongue were specialized for different taste sensations. Now it is clear that all areas of the tongue give a response to all types of taste sensations. Usually, in low concentrations of taste substances, each taste bud gives a response to one primary taste stimulus.
• However, in high concentrations, 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 carries impulses of one taste sensation.

Taste Sensations And Chemical Constitutions

The substances causing sour or salt tastes are mostly electrolytes. Bitter and sweet tastes are caused by electrolytes or nonelectrolytes.

• Sweet Taste: Sweet taste is produced mainly by organic substances like monosaccharides, polysaccharides, glycerol, alcohol, aldehydes, ketones, and chloroform. The 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 a 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. The bitterness of the salts is mainly due to cations.
• Umami: Umami is a 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 proven to produce Chinese restaurant syndrome in some people taking Chinese food regularly. The common symptoms are headache, flushing, sweating, perioral numbness, and 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 a very low threshold and sweet taste has a high threshold. The threshold for umani is not known.

Taste Transduction

• Taste transduction is the process in which the taste rece¬ptor converts chemical energy into action potentials in the taste nerve fiber. Receptors of taste sensation are chemoreceptors, which are stimulated by substances dissolved in the mouth by saliva. The dissolved substances act on the 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, the 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. The transduction mechanism is different in each taste receptor cell.
    • Sweet Receptor: A receptor for sweet taste is a GPCR. The sweet substances bind to the receptor and cause depolarization via cyclic AMP.
    • Salt Receptor: The receptor for salt taste is called the epithelial sodium channel (ENaC). It acts like the ENaC receptors in other parts of the body. When sodium enters, this receptor releases glutamate which causes depolarization.
    • Sour Receptor: Source 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 receptors, the sour substances activate phospholipase C through G proteins. It causes the production of inositol triphosphate (IP3) which causes depolarization by releasing calcium ions.
    • Umami Receptor: Umami receptor is called the metabotropic glutamate receptor (mGluR4). Glutamate causes depolarization of this receptor. The exact mechanism of depolarization is not clearly understood. Activation of umami taste receptors is intensified by the presence of guanosine mono¬phosphate (GMP) and inosine monophosphate (IMP).

Applied Physiology-Abnormalities Of Taste Sensation

1. Ageusia:

• Loss of taste sensation is called ageusia. The taste buds in the anterior two-thirds of the tongue are innervated by the chorda tympani branch of the facial nerve. The chorda tympani nerve receives taste fibers from the tongue via the lingual branch of the mandibular division of the trigeminal nerve.
• So, the lesion in the facial nerve, chorda tympani, or mandibular division of the trigeminal nerve causes loss of taste sensation in the anterior two-thirds of the tongue. The lesion in the glosso-pharyngeal nerve leads to loss of taste in the posterior one-third of the tongue.
• Temporary loss of taste sensation occurs due to the drugs like captopril and penicillamine, which contain the sulfhydryl group of substances.

2. Hypogeusia: The decrease in the taste sensation is known as hypogeusia. It is due to an increase in the threshold for different taste sensations. However, the taste sensation is not completely lost.

3. Taste Blindness: Taste blindness is a rare genetic disorder in which the ability to recognize substances by taste is lost.

4. Dysgeusia: The disturbance in the taste sensation is called dysgeusia. It is found in the temporal lobe syndrome, when the anterior region of the temporal lobe in this condition, the paroxysmal ^nations of taste and smell occur which are usually unpleasant.