Structure of Taste receptors and Mechanism of Gustation

The sensation of taste is called gustation or gustatory response.
Various chemoreceptors present in the tongue help in getting the sense of taste.
The sense of taste (gustation) and smell (olfaction) are clearly interrelated. That’s why, a person whose nasal passages are blocked by a cold cannot get the proper taste of food effectively.

Location and Structure of Taste receptors:

The surface of our tongue is covered with many small protuberances or outgrowths called papillae, which give a tongue its bumpy appearance.
They are most numerous on the dorsal or upper surface of the tongue and are also found on the palate (roof of the mouth), throat and posterior surface of the epiglottis.
There are three main types of papillae;
1. Fungiform papillae:
  - They are mushroom-like papillae which are scattered singly.
  - They are found especially near the tip of the tongue.
  - Each fungiform papilla contains 1 to 8 taste buds.
2. Circumvallate papillae:
  - These papillae have a wall around themselves.
  - 10 to 12 such papillae form two rows parallel to the V-shaped sulcus terminalis (a groove separating the anterior two thirds of the tongue from the posterior third) near the posterior third of the tongue.
  - Each circumvallate papilla contains 90 to 250 taste buds.
3. Filiform papillae:
  - They are the pointed structures near the anterior two-thirds of the tongue.
  - Filiform papillae do not necessarily contain taste buds.

What are taste buds?

Taste buds are the receptor organs which are located within the crevices of most papillae and are responsible for the sense of taste.
There are about 10,000 taste buds in our tongue.
Taste buds are barrel-shaped clusters of chemoreceptor (gustatory) cells along with sustentacular (supporting) cells.
Gustatory and sustentacular cells are arranged like alternating segments of an orange.
Each taste bud contains about 25 receptors.
The more numerous supporting cells act as reserve cells which replenish the receptor cells when they die.
Mature receptor cells have a life of only about 10 days, and they usually can be replaced from reserve cells in about 10 hours.
Babies don’t have well developed taste buds and the frequency of replacement of taste receptors decreases as we get older and this explains why our sense of taste may diminish with age.
Extending from the free end of each receptor cell are short taste hairs (microvilli).
These microvilli project through the tiny outer opening of the taste bud, called the taste pore, into the surface epithelium of the oral cavity.
It is thought that gustatory sensations are initiated on the taste hairs, but before a substance can be tasted it must be in solution.
Saliva containing ions or dissolved molecules of the substance to be tasted enters the taste pore and interacts with the receptor sites on the taste hairs.

Basic taste sensations:

Although all gustatory cells are structurally identical, each cell has many different types of receptor sites.
Because the proportion of different types varies from cell to cell, each taste cell can respond to a variety of stimuli.
There are four generally recognized basic taste sensations which are; sweet, sour, salty and bitter.
Recently, there has been a fifth sensation identified called umami which means “essence of deliciousness” in Japanese. Its taste is often described as the meaty, savory deliciousness that deepens flavor.
We can taste many subtle flavors because of combinations of the four basic sensations, complemented by an overlay of odors.
Taste perception is also enhanced by information about the texture, temperature, spiciness and odor of food.

Areas of response to basic taste sensations;

The areas of response to the four basic tastes are located on specific parts of the tongue.
- Sweet: In the front or anterior part of the tongue
- Sour: On the dorsal lateral sides of the tongue
- Salty: On the dorsal lateral sides of the tongue
- Bitter: At the back or posterior part of the tongue
The taste buds for umami taste are distributed all over the tongue.
Salt and sweet are perceived most acutely on the tongue but bitter and sour are perceived more acutely on the palate.

Mechanism of Gustation:

The exact mechanisms that stimulate a neuroepithelial taste cell are not known, although many theories have been proposed.
Different types of potential gustatory stimuli may cause proteins on the surface of the receptor plasma membrane to change the permeability of the membrane.
This change in permeability leads to the opening and closing of ion channels to sodium ions.
When sodium ions enter the cell, they depolarize it and generate a receptor potential that stimulates the nerve endings in taste neurons.
Variations in the intensity of tastes are produced by differences in the firing frequencies of nerve impulses.

Neural pathways of Gustation:

Taste impulses are conveyed from the anterior two thirds of the tongue to the brain by a branch of the facial nerve.
Impulses from the posterior third of the tongue are carried to the brain by the Glossopharyngeal (IX) nerve and from the palate and pharynx by the Vagus (X) nerve.
The taste fibers of all three cranial nerves terminate in the nucleus solitarius in the medulla oblongata.
From there, axons project to the thalamus and then to the ‘taste center’ in the parietal lobe of the cerebral cortex.

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