This paper provides a concise physiological overview of the human sense of taste, explaining how taste receptor cells within taste buds detect dissolved molecules called tastants and transmit signals to the brain. It covers the five recognized taste sensations — salty, sour, sweet, bitter, and umami — and describes the distinct mechanisms by which each is triggered at the cellular level. The paper also examines evidence that taste perception ultimately occurs in the brain rather than on the tongue, discusses the roles of hormones such as leptin and aldosterone in modulating taste, and briefly addresses how taste sensitivity changes across the human lifespan.
Physiologically speaking, taste is the ability to respond to dissolved molecules and ions — called tastants — through taste receptor cells clustered in taste buds. "Each taste bud has a pore that opens out to the surface of the tongue, enabling molecules and ions taken into the mouth to reach the receptor cells inside" (Kimball 2009). Every taste bud can contain upwards of 50–100 taste cells. These cells have transmembrane protein receptors on their apical surfaces, "which admit the ions that give rise to the sensations of salty and sour" and "bind to the molecules that give rise to the sensations of salty, sour, sweet, bitter, and umami" (Kimball 2009).
Contrary to the popular image found in many older textbooks, each taste bud contains all five identified taste sensations. However, it is true that every individual taste cell appears to be restricted to expressing only a single type of receptor, with the exception of bitter receptors.
"The majority of taste buds on the tongue are located within papillae, the tiny projections that give the tongue its velvety appearance. During chewing, chemicals from food called tastants enter the taste pores of the taste buds, where they interact with molecules on fingerlike processes called microvilli on the surfaces of specialized taste cells. The interactions trigger electrochemical changes in the taste cells that cause them to transmit signals that ultimately reach the brain. The impulses are interpreted, together with smell and other sensory input, as flavors" (Smith & Margolskee 2001, pp. 1–2). The sensory experience of eating is therefore not derived solely from the strict sense of taste; the sensation of flavor ultimately resides in the brain.
One experiment that demonstrated the brain's central role in taste involved mice that had been genetically altered to express sweet responses in the taste cells that normally respond to bitter flavors. These mice responded to bitter substances as though they were sweet (Kimball 2009). "Unlike normal mice, the altered mice did not prefer sweet foods or avoid bitter substances: they did not avidly drink highly sweetened water and instead drank solutions of very bitter compounds as readily as they did plain water. The researchers also showed that key nerves had a reduced electrical response to sweet and bitter tastants but could still respond to salts and acidic compounds" (Smith & Margolskee 2001, p. 2). It is neurons — not taste molecules or receptors — that ultimately "produce" the experience of taste.
Although tastes are not restricted to different regions of the tongue, there are meaningful differences in the mechanisms underlying each major taste sensation. "The chemicals that produce salty and sour tastes act directly through ion channels, whereas those responsible for sweet and bitter tastes bind to surface receptors that trigger a cascade of signals to the cells' interiors, ultimately resulting in the opening and closing of ion channels" (Smith & Margolskee 2001, p. 2).
At least one of the receptors for salty substances "is an ion channel that allows sodium ions (Na+) to enter directly into the cell" (Kimball 2009). Sour ion channels are activated by the release of acids into the taste cell (Kimball 2009). Sweet substances bind to G-protein-coupled receptors in the cell. "Humans have genes encoding 25 different bitter receptors," although every individual taste cell appears to respond to certain bitter-tasting molecules in preference to others (Kimball 2009). Finally, umami — the response to glutamic acid found in foods such as those containing monosodium glutamate (MSG) — is considered by some researchers to be a fifth distinct taste, and like sweet and bitter, its response is linked to G-protein-coupled receptors.
"Leptin, aldosterone, and taste regulation"
"How taste sensitivity changes from infancy to adulthood"
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