Neurobiology of the Gustatory-Salivary Reflex

Abstract

All neural information resulting from chemical stimulation of taste buds in the oral cavity, pharynx and larynx travels via the facial (VII), glossopharyngeal (IX) and vagus (X) nerves to terminate in the nucleus of the solitary tract (NST) in the brainstem. The NST is responsible for initial processing and distribution of chemosensory information. At higher relays in the central nervous system the processes of detection, discrimination and affective responses occur resulting in the sensation we call taste and the behavioral reactions to that sensation. In addition, the NST connects to efferent motor systems involved in oral facial motor reflexes and systems controlling the initiation and flow of saliva. Thus, the NST plays a pivotal role in the neural processing of chemosensory information derived from stimulation of taste buds. Beginning in 1961 (Pfaffmann et al., 1961) a large number of investigators in different laboratories have examined the NST using anatomical, and neurophysiological techniques. The topographical projections of the VII, IX and Xth nerves conveying sensory information to the NST have been determined using different methods in several species (Torvik, 1956; Norgren, 1981; Whitehead and Frank, 1983; Hamilton and Norgren, 1984). The morphology of the NST has been studied and the neuronal architecture defined (Whitehead, 1988). Neurons in the NST have been described as belonging to three major anatomical types—multipolar, elongate and ovoid (Whitehead, 1988; Lasiter and Kachele, 1988; King and Bradley, 1994; Mistretta and Labyak, 1994), and using immunocytochemistry the presence of GABA and other neuropeptides has been described (Lasiter and Kachele, 1988; Barry et al., 1993).