Smell helps us to distinguish pleasant from unpleasant or dangerous substances at a distance; taste involves direct contact But both are chemical senses that depend on foreign molecules touching sensory structures called receptor cells. In some ways, smell and taste may be the most primitive senses. Sometimes for example in the discernment of some of the more subtle flavors both work together more closely than most of us realize.
Smell is detected by sensory cells in two patches of olfactory epithelium, one in the roof of each nasal cavity, just below the cranium. Between them, the two patches occupy approximately one square inch (5 square centimeters), yet scientists calculate that they include millions of rodlike bodies projecting from the buried sensory cells. Each rod ends in filaments, or “hairs,” the total area of which may exceed that of the skin surface.
These sensitive hairs, in common with the entire surface of each nasal cavity, are moistened by the mucous membrane that lines the nose. This membrane is kept warm by a rich supply of blood vessels and so warms and moistens air breathed in through the nostrils. Moisture plays a key role in the sense of smell, for chemical receptors can only detect substances that can be dissolved.
Probably only two per cent of breathed-in air passes close to the cell receptors. Yet these are so sensitive that a single molecule of some substances is enough to excite one receptor ending for instance, the human olfactory system can sense less than one hundred millionth of a gram of musk. Furthermore, some people can distinguish as many as 10,000 different odors.
Exactly how smell works remains debatable. But the stereochemical theory of olfaction suggests that most odors are combinations of a few primary odors produced by distinctively shaped chemical molecules that fit into matching olfactory sites, like keys into locks. This suggests there may be different basic odors, in rather the same way as there are different basic tastes.
From the olfactory receptors, nerve signals travel to the two olfactory bulbs projecting from the brain, then on by complex routes to a diffuse olfactory region associated with the limbic system in the brain.
Compared with our sense of smell, our sense of taste seems poorly developed. Most tastesensitive cells occur on the upper part of the tongue. A very few are found on the palate, lingual tonsils, and epiglottis. Groups of these receptors form each taste bud, and there can be many such buds on one papilla, or small projection on the tongue’s upper surface, which feels rough because of the scores of papillae found there. Babies possess tens of thousands of taste buds, but numbers decrease with age: an adult normally has about 9,000.
Four types of taste buds, found in the papillae, enable us to distinguish between sweet, sour, salty, and bitter tastes. But the receptor cells that make up our taste buds do not have structural or functional differences that correspond to these tastes. The idea of the four categories of taste seems to be something that is learned. Taste categories may be only characteristics of taste; they tell us little about how the taste sense functions.
While olfactory signals pass through the olfactory lobes, taste signals travel through cranial nerves straight to the brain stem, then on to the brain’s higher centers.
The taste of food
Taste and smell combine to help give many foods their flavors. This is apparent in the fact that flavors are difficult or impossible to distinguish when the mucous membranes are inflamed during an infection, such as a cold. The taste of food also depends upon its temperature: taste receptors are most highly sensitive to foods at temperatures of 85—105° F. (30— 40° C).
In time, nerve cells adapt to prolonged exposure to certain tastes or odors so that these stimuli are no longer noticed. Furthermore, adaptation to one taste may also alter sensitivity to others. Thus, coffee tastes unusually bitter if you drink it after eating ice cream, while salt-adaption heightens sensitivity to bitter, sweet, and sour substances.