Taste the Sense of Taste:

Taste

The sense of taste: An overview

Physiologically speaking, taste is simply the ability to respond to dissolved molecules and ions called "tastants" through the use of 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 contains upwards of 50 -- 100 taste cells. The 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 in many old textbooks, each taste bud contains all five indentified taste sensations. However, it is true that every single taste cell does seem 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 not derived solely from what is strictly the sense of taste and the sensation of taste -- like all sensations -- ultimately resides in the brain.

One experiment that proved that taste is located in the brain involved mice that had been genetically altered to express sweet responses in the taste cells that normally were sensitive to bitter flavors. The mice responded 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). Neurons, not taste molecules are receptors are what 'produce' taste.

Although tastes are not relegated to different regions on the tongue, it is important to note that there are slight differences between the major taste sensations. "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 bucket brigade of signals to the cells' interiors that ultimately results 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). The hormone aldosterone is thought to increase the number of salt receptors to better enable humans to maintain a normal sodium level in the body. Sour ion channels are liberated by the release of acids into the taste cell (Kimball 2009).

Sweet substances bind to G-protein-coupled receptors in the cell and the hormone leptin inhibits sweet cells by opening their K+ channel (Kimball 2009). Leptin thus could be a way for the body to signal 'enough' for a sweet sensation, and leptin abnormalities have been linked to some manifestations of obesity. "Humans have genes encoding 25 different bitter receptors," although every single taste cell seems to respond to certain bitter-tasting molecules in preference to other bitter-tasting molecules (Kimball 2009). Finally, some researchers have called umami, the response to glutamic acid -- like processed foods monosodium glutamate (MSG) to be a 'fifth' taste. Like sweet and bitter flavors, this response is linked to G-protein-coupled receptors.