Central Gustatory Pathways

Taste receptors synapse with dendritic elements of the primary afferent neurons of the gustatory pathway, causing the neurons to fire in response to appropriate stimuli above threshold levels. These afferent fibers represent the peripheral projection of the primary sensory neurons of the gustatory pathway, and they arise from cell bodies within three cranial nerve ganglia (Fig. 7). Afferent fibers within the facial nerve (cranial nerve VII) arise from the geniculate ganglia and innervate the anterior two-thirds of the tongue. Fibers from the glossopharyngeal nerve (cranial nerve IX) arise from the inferior glossopharyngeal ganglia and innervate the posterior one-third of the tongue. Fibers from the vagus nerve (cranial nerve X) arise from the inferior vagal ganglia and innervate the smattering of taste buds found in throat regions, including the glottis, epiglottis, and pharynx. Central projections from these three cranial nerve ganglia enter the brain stem along the lateral aspect of the medulla and converge to make synaptic contact with cells in the rostral or gustatory division of the solitary nucleus of the medulla. Ascending fibers from the solitary nucleus make a synaptic connection in the ventral posterior medial nucleus of the thalamus. Thalamic neurons then project to the primary gustatory cortex in the insular and orbitofrontal regions.

Compared to other special sensory systems, relatively little is known about central processing of taste information. It has been suggested that taste is not encoded along "labeled lines,'' with specific neurons designated for each basic taste. Apparently, there is only a crude spatial map of basic tastes, and the discrimination of the many thousands of recognizable tastes is encoded as a population response, with each individual taste neuron having rather broadly tuned response characteristics. Each taste bud is composed of taste receptors, with similar response characteristics reflecting a preference for one or perhaps two basic tastes. Furthermore, taste buds in different regions of the tongue show different preferences (see Fig. 5). Along the lateral edge of the tongue there are three definable areas: (1) the most tongue

FIGURE 7 Central projections of taste buds. Fibers from taste buds enter the ventral surface of the medulla via three pairs of cranial neurons and synapse in the rostral or gustatory division of the solitary nucleus. Secondary neurons carrying gustatory information project to the ventral posterior medial nucleus of the thalamus, and the information is then relayed to the cerebral cortex for conscious appreciation. (A) Lateral view of the cerebral cortex with the lateral sulcus opened to expose the insula; the gustatory cortex is shown in blue. (B) Coronal section taken in the plane indicated by the dashed line in (A). (C) Oral cavity and dorsal view of the brain stem showing the three cranial nerves that carry information from the taste buds. Note that the information is processed ipsilaterally.

anterior, which is responsive to saltiness; (2) the middle area, which overlaps area 1 and is most responsive to sourness; and (3) the posterior area, which responds to bitterness. Taste buds near the tip of the tongue are most responsive to sweet tastes; however, these preferences are best expressed at threshold concentrations of the stimulant. At higher concentrations multiple stimulants are effective in eliciting a response from a given taste bud. Primary and secondary neurons in the gustatory pathway receive input from many individual taste buds, and they do not show a strong spatial map of taste-specific regions of the tongue.

Suggested Readings

Buck LB, Axel R. A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 1991; 5:175-187. Kauer JS. Coding in the olfactory system, in: Fingerand TE, Silver WL, Eds., Neurobiology of taste and smell. New York: Wiley, 1987, pp. 205-231.

McLaughlin S, Margolskee RF. The sense of taste. Am Sci 1994; 82:538-545.

Reed RR. Signaling pathways in odor detection. Neuron 1992; 8:205-209.

Reed RR. How does the nose know? Cell 1990; 60:l-2. Roper SD. The cell biology of the vertebrate taste receptors. Annu Rev Neurosci 1989; 12:329-353.

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