The gustatory system in mammals

Taste buds occur in five distinct populations in the mammalian oral cavity. The chemical sensitivity of these receptors varies from one population to another and among species as well. Taste buds degenerate when denervated and reappear when gustatory axons reinvade an area. Branches of three cranial nerves-VII, IX, and X-convey gustatory information to the medulla, where they terminate in the rostral two thirds of the nucleus of the solitary tract. Individual gustatory afferent fibers normally respond to several classes of sapid chemicals; they are broadly tuned. The tuning, however, is not random. For a given neuron, the best stimulus predicts the order of effectiveness of other stimulus qualities. Taste neurons in the first and second central relays, in the medualla and pons, respectively, are even more broadly tuned than those on the periphery but maintain a similar orderliness in the effectiveness of different sapid qualities. Much less is known about the response properties of taste neurons in more rostral areas. Even the basic anatomy of the gustatory system in the forebrain remains in question, because its organization differs in rodents and primates.