Chemical senses

In the last decade, studies using approaches from molecular biology have substantially advanced our understanding of the early events in olfaction and taste. The many odorants that we can recognize may well interact with many distinct receptor proteins. Of the four taste qualities that we recognize, studies on salty and sour suggest that these tastes involve ion channels in the membrane of receptor cells while sweets and bitters bind to receptor proteins. Some volatiles (pheromones) play special roles in reproductive behavior via the vomeronasal organ (VNO) and the accessory olfactory system. Initial belief that humans lack a VNO has been questioned recently, thus raising the fascinating possibility of human pheromones. The roles that taste and smell play in the world of the newborn are very different. Acceptance of sweet and rejection of bitter appear to be hard-wired while the affect associated with odors depends much more on experience. Genetic variation may produce total losses (Kallman's syndrome produces anosmia and familial dysautonomia produces ageusia) or losses specific to certain stimuli. The best known of the specific anosmias is that for androstenone, which has no smell to some, a urinous smell to others, and a smell like sandalwood to still others. Analogous to the specific anosmias, some individuals are unable to taste PROP while others, supertasters, perceive PROP to be exceedingly bitter. Clinical studies reveal pathologies responsible for total or partial losses. The olfactory system, dependent on one cranial nerve, is more vulnerable than taste, and total anosmia is a relatively common clinical problem. Three cranial nerves carry taste and two of those nerves inhibit one another such that damage to one disinhibits the other and preserves over-all taste function. Total ageusia is very rare. Throughout these studies we see that taste and olfaction have different properties and often different functions (e.g. odor and reproduction). Yet taste and smell can also be integrated to determine what does or does not enter the body. In Adrian's words, "we are dealing with the sense organs which signal the quality of the air we breathe and that of the food and drink we propose to swallow."