The sensory receptors for taste (taste buds) are distributed over the surface of the tongue and, in smaller numbers, over the soft palate, pharynx, larynx, and esophagus. Mainly they are located in the epithelium along the lateral surfaces of the circumvallate and foliate papillae and to a lesser extent on the surface of the fungiform papillae. The taste buds are round or oval structures, each composed of up to 200 vertically oriented receptor cells arranged like the staves of a barrel. The superficial portion of the bud is marked by a small opening, the taste pore or pit, which opens onto the mucosal surface. The tips of the sensory cells project through the pore as a number of filiform microvilli (“taste hairs”). Fine, unmyelinated sensory fibers penetrate the base of the taste bud and synapse directly with the sensory taste cells, which have no axons.
The taste receptors are activated by chemical substances in solution and transmit their activity along the sensory nerves to the brainstem. There are four primary and readily tested taste sensations that have been long known: salty, sweet, bitter, and sour; recently a fifth, umami, signifying a savory taste—the taste of glutamate, aspartate, and certain ribonucleotides—has been added. The full range of taste sensations is much broader, consisting of combinations of these elementary gustatory sensations. Older notions of a “tongue map,” which implied the existence of specific areas subserving one or another taste, are incorrect. Any one taste receptor is capable of responding to a number of sapid substances but each is preferentially sensitive to one substance. In other words, the receptors are only relatively specific. The sensitivity of these receptors is remarkable: as little as 0.05 mg/dL of quinine sulfate will arouse a bitter taste when applied to the base of the tongue.
A G-protein transduction system (gustducin), similar to the one for olfaction, has been found to be operative in signaling taste sensations in the tongue receptors. A discussion of this system can be found in the commentary by Brand.
The receptor cells of the taste buds have a brief life cycle (about 10 days), being replaced constantly by mitotic division of adjacent basal epithelial cells. The number of taste buds, not large to begin with (approximately 10,000), is gradually reduced with age; also, changes occur in the taste cell membranes, with impaired function of ion channels and receptors (Mistretta). Gustatory (and olfactory) acuity diminishes with age (everything begins to taste and smell the same). According to Schiffman, taste thresholds for salt, sweeteners, and amino acids are 2 to 2.5 times higher in the elderly than in the young. The reduction in the acuity of taste and smell with aging may lead to a distortion of food habits (e.g., excessive use of salt and other condiments) and contribute to the anorexia and weight loss of elderly persons.
Richter has explored the biologic role of taste in normal nutrition. Animals made deficient in sodium, calcium, certain vitamins, proteins, etc., will automatically select the correct foods, on the basis of their taste, to compensate for their deficiency. Interesting genetic polymorphisms in the receptor for sweet substances in rats have been found to underlie differences in the proclivity to ingest sweet substances, and a similar system has been proposed in humans (Chaudhari and Kinnamon).
Neural Innervation of the Tongue