23.9: Taste: an action potential serves up a taste sensation to the brain.

Look at your tongue in the mirror. It is covered with bumps. Embedded within these bumps are taste buds—more than 10,000 in all. Within each taste bud are 60–80 sensory receptor cells, called chemoreceptors (which are modified epithelial cells that synapse with sensory neurons), that are stimulated when particular chemicals in food dissolve in saliva and bind to proteins on the receptor cell surface. Much as a lock will open only when the proper key is inserted, particular taste receptors allow only specific food molecules, with exactly the right shape, to bind (FIGURE 23-18).

Taste chemoreceptors fall into five groups, depending on which type of molecule chemically stimulates them: sweet, salty, sour, bitter, or a recently discovered, but difficult to describe, savory taste called umami. These different types of chemoreceptors occur all across the tongue (although the different groups tend to be concentrated in different regions of the tongue). A rich variety of tastes is possible because most foods stimulate unique combinations of the different taste receptors. Foods also release molecules into the air in the mouth, which stimulate smell receptors within the nasal cavity.

Not all animals use their mouth to taste things. Some insects have chemoreceptors on their legs, and they “taste” things just by touching them. Other animals have taste receptors on their antennae or tentacles. Regardless of where the food meets the taste receptors, the process is the same: chemical binding triggers an action potential in accompanying sensory neurons that delivers a taste sensation to the brain.

Because of the way that humans sense taste, it’s possible to fool your brain. Your brain never actually “knows” the true identity of the food on your tongue. Rather, it senses a particular taste based solely on the combination of receptor cells that is stimulated. If a molecule that is not sugar has a chemical structure closely resembling sugar, it can stimulate the same taste-bud receptors and be perceived by the brain as sugar (FIGURE 23-19). Many non-nutritive sugar-substitute molecules, such as saccharin and sucralose, do exactly this. They have three-dimensional arrangements of atoms that are similar to sugar molecules such as sucrose, glucose, or fructose, but have structures that cannot be broken down by any enzymes in the human body. Consequently, when we consume them, we sense the sweet taste of sugar but don’t actually derive any energy from the molecules. Instead, they pass through our digestive tract unaltered.

Aspartame also resembles sugar closely enough to stimulate sugar receptors in taste buds. Molecularly, however, it is quite different: it is made from two amino acids. Unlike the other artificial sweeteners, it can be broken down and releases energy (and hence contains calories), but it is so efficient at stimulating sweet-taste receptors that it can generate a strong sugary taste even when used in tiny amounts.