Taste (gustation) and smell (olfaction) are two senses that work together in humans to determine the flavor and palatability of foods and beverages and to signal the presence of dangerous gases and toxic foodstuffs. In many animals, these senses play a critical role in locating food, detecting predators, and recognizing appropriate mating partners.
Despite their close association, taste and smell are anatomically and functionally distinct. In the case of taste, chemicals that evoke sweet, sour, bitter, and salty sensations stimulate taste-bud receptors located in the throat and on the tongue and palate. This stimulation triggers nerve cells to send signals to the brain stem located in the base of the brain. Odors register in the brain when airborne chemicals stimulate receptors located on the olfactory epithelium, a small patch of tissue positioned high in the nose. These receptor cells send signals into the brain through a thin section of the skull located between the eyes, termed the cribriform plate. Signals are conducted from an overlying olfactory bulb to higher brain centers associated with arousal, emotional phenomena, and cognition.
The olfactory system is vitally important in determining food flavors. During chewing and swallowing, odor-laden air is forced from the rear of the oral cavity to the olfactory receptors, evoking many flavor sensations that people usually associate with taste but that are almost completely dependent on the sense of smell. If the nose is held while food is being swallowed, the free flow of air to the olfactory receptors is prevented, resulting in a decrease or elimination of the perception of the food's taste.
Numerous factors influence the ability to smell. For instance, women, on average, have a better sense of smell than men. Older persons typically develop anosmia, or loss of smell function. Various diseases, head injuries, and exposure to airborne toxins, including cigarette smoke, may cause a lessened smell function or a distortion of smell function, such as perceiving a bad smell while smelling pleasantly scented flowers. Recent studies suggest that alterations in the ability to smell may be an early first sign of such disorders as Alzheimer disease and parkinsonism.
Fewer factors influence taste perception. Cigarette smoking tends to cause hypogeusia, or a lessened taste function, by dulling the ability to taste bitter-tasting chemicals. Some medications produce dysgeusia, or distortion of taste function, by inducing unpleasant or persistent taste sensations.
The invention of practical chemosensory tests has resulted in the ability to make simple and accurate measurements of taste and smell in a manner similar to the evaluation of hearing or vision. For this reason, the testing of these senses is now routinely performed in a number of major medical centers.