ABOUT TASTE

An overview of taste

The human sense of taste allows us to decide if food is beneficial (contains nutrients and should be consumed), or if it is dangerous and should be rejected (contains toxic substances).

Salty
Pleasant in small quantities

Sour
Pleasant in small quantities

Umami
Japanese for savory

Sweet
Signals the presence of carbohydrates

Bitter
Almost universally unpleasant

When we eat, digestive enzymes in saliva begin to dissolve food into base chemicals that are washed over the papillae, which are interpreted as tastes by the taste buds.

Lingual papillae

Lingual papillae are the structures on the tongue responsible for its characteristic rough texture, and increase the area of contact with food.

Taste buds are found in Foliate Papillae, Circumvallate Papillae, and Fungiform Papillae. Taste buds are not found in Filiform Papillae, which cover most of the front two thirds of the tongue’s surface.

Taste buds

Taste buds are composed of 50-150 polarized neuroepithelial cells. Adults have roughly 5,000 taste buds in the oral cavity.

Taste receptor cells have a life span of about two weeks, and are constantly being replaced. Mature cells only expresses one type of taste receptor.

Taste receptors

The human sense of taste is composed of five receptor types. Salty and Sour receptors are ion channels, while Umamai, Sweet and Bitter receptors are typically dimeric GPCRs (G protein-coupled receptors):

Salty

Sodium salts deploarize taste cells directly via Na+ influx through the epithelial sodium channel (ENaC).

Sour

Acids, in the form of protons (H+), also permeate ENaC, activate H+-activated cation (X+) channels and inhibit apical K+ channels.

Umami

The amino acid L-glutamate activates the truncated forms of G protein-coupled receptors metabotropic glutamate receptor 1 (mGluR1) and metabotropic glutamate receptor 4 (mGluR4), and the heterodimeric receptor complex taste receptor type 1 TAS1R1/TAS1R3. Other amino acids, such as arginine, activate ionotropic glutamate receptors.

Sweet

Artificial sweeteners activate both ionotrophic receptors linked to cation channels, and the heterodimeric receptor complex taste receptor type 1 TAS1R2/TAS1R3. Natural sugars apparently activate the heterodimeric receptor complex taste receptor type 1 TAS1R2/TAS1R3, as well as the homodimeric receptor complex taste receptor type 1 TAS1R3/TAS1R3.

Bitter

Compounds such as PTC, denatonium, and PROP activate heterodimeric receptor complex taste receptor type 2 TAS2Rx/TAS2Ry, where TAS2Rx and TAS2Ry are each one of 25 different human type 2 bitter taste receptors. Other bitter compounds have been shown to inhibit apical K+ channels.

Upon encountering tastants, taste receptors transmit signal intracellularly via second messenger molecules, resulting in cell depolarization and action potential, which is transmitted through taste nerve fibers to the brain. Once taste signals reach the brain, several digestive neural pathways are activated, including salivation and secretory activity in the stomach.