Smell and Taste — The Chemical Senses
Olfaction (smell) and gustation (taste) are senses that respond to minute traces of chemical molecules that foods and many other substances emit. These two senses are closely linked. If you've ever had a bad cold that made it difficult to smell, you may have noticed that your sense of taste was suppressed as well. In humans, there are perhaps 10 million olfactory receptors in the nasal cavities that respond to specific odorants or combinations of them when they enter the nasal tracts from the surrounding air. The number of receptors stimulated determines the strength of the smell. The particular combination stimulated determines the quality of the smell, which of course can range from pleasant and enticing to thoroughly obnoxious.
Signals from the olfactory receptors fan out to a number of areas of the brain, and compared to the other senses, they take a much shorter route. They also go to an area deep in the cortex called the amygdala, which is intimately involved with emotional responses — you therefore very quickly feel pleasure or disgust in response to an odor. All of this is consistent with the view that smell is the oldest and most basic sense in terms of evolution, and it is likely that your ancient ancestors had a much keener sense of smell than you do — as do dogs, cats, and many other animals that rely more on smell than on vision or hearing.
In gustation, chemical molecules from substances you eat enter the saliva and stimulate taste buds concealed in small bumps on the tongue and also fan out to areas of the brain. You have about 10,000 taste buds, which are maximally responsive to molecules that give rise to “sweet,” “salty,” “sour,” or “bitter” tastes. But the delicate experiences of flavor that you're capable of depend on a complex interplay between taste and smell, as anyone who's ever had a stuffy nose is aware of. When you have a cold, for example, you can still sense the four basic tastes, but that's about it.
Many animals have special glands that emit chemical molecules called pheromones to signal their readiness to mate and arouse sexual interest in potential partners. Humans also emit pheromones — your underarms, for example, contain glands that are especially active in secreting them. Some theorists have proposed that pheromones also stimulate sexual interest, but so far the jury is still out.
All of your senses tend to decline as you age, and smell and taste are no exception. Older people may remark that foods prepared the same way as when they were younger don't taste as good, and they may find themselves using more sugar, salt, or other spices to compensate.