The four primary taste sensations are sweet, sour, salty, and bitter. Some investigators recognize other taste sensations, including alkaline, metallic, and most recently umami, which seems to involve responsiveness to the amino acid glutamate (see box below). Each of the many flavors we experience daily is believed to result from one of the primary sensations or from some combination of two or more of them. The way we experience flavors may also reflect the concentration of chemicals as well as the sensations of smell, texture (touch), and temperature. Furthermore, chemicals in some foods—chili peppers and ginger, for instance—may stimulate pain receptors that cause a burning sensation.
Current evidence suggests that all taste cells are responsive to all taste sensations, although for a given taste cell, one taste sensation is likely to predominate. Due to the distribution of taste cells, responsiveness to particular sensations may vary from one region of the tongue to another. For example, responsiveness to a sweet stimulus may be greatest at the tip of the tongue, responsiveness to sour greatest at the margins of the tongue, responsiveness to bitter at the back of the tongue, and the responsiveness to salt quite widely distributed. In practice, perhaps due to the fact that all taste receptor cells are sensitive to all of these stimuli to some degree, there is a wide range of individual variations in these responses. More importantly, it may be the pattern of these responses from differentially sensitive receptor cells that provides the brain with the information necessary to create what we call taste.
Sweet receptors are usually stimulated by carbohydrates, but a few inorganic substances, including some salts of lead and beryllium, also elicit sweet sensations.
Acids stimulate sour receptors. The intensity of a sour sensation is roughly proportional to the concentration of the hydrogen ions in the substance being tasted. Ionized inorganic salts mainly stimulate salt receptors. The quality of the sensation that each salt produces depends upon the kind of positively charged ion, such as Na+ from table salt, that it releases into solution. A variety of chemicals stimulates bitter receptors, including many organic compounds. Inorganic salts of magnesium and calcium produce bitter sensations, too. Extreme sensitivity to bitter tastes is inherited—this is why diet colas taste just like regular cola to some people, but are very bitter to others.
One group of bitter compounds of particular interest are the alkaloids, which include a number of poisons such as strychnine, nicotine, and morphine. The fact that we spit out bitter substances may be a protective mechanism to avoid poisonous alkaloids in foods.
Taste receptors, like olfactory receptors, rapidly undergo sensory adaptation. One can avoid the resulting loss of taste by moving bits of food over the surface of the tongue to stimulate different receptors at different moments.
Although taste cells are very close to the surface of the tongue and are therefore exposed to environmental wear and tear, the sense of taste is not as likely to diminish with age as is the sense of smell. This is because taste cells are modified epithelial cells and divide continually. A taste cell functions for only about three days before it is replaced.
The sense of taste also reflects what happens to food as it is chewed. Most foods are chemically complex, and so they stimulate different receptors. In an experiment to track the actual act of tasting, chemists collected samples of air from participants' nostrils as they bit into juicy red tomatoes. An analytical technique called mass spectrometry revealed that chewing activates a sequence of chemical reactions in the tomato as its tissues are torn, releasing first aromatic hydrocarbons, then after a 30-second delay, products of fatty acid breakdown, and, finally, several alcohols. This gradual release of stimulating molecules is why we experience a series of flavors as we savor a food.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.