Figure 1.

Schematic representation of the major gustatory input pathways from the periphery (lower left-hand side) to the rostral nucleus of the solitary tract (rNST) and their associated local hindbrain circuits (right side) and ascending forebrain projections in the rodent model [see 61]. For simplicity, descending projections from forebrain taste structures to hindbrain nuclei are not shown. Percentages in light blue boxes indicate approximate proportion of total oral taste buds found in each oral region with remaining taste buds scattered in other areas of the oropharyngeal epithelium. The gustatory afferent fibers of the VIIth, IXth, and Xth cranial nerves terminate in a rough orotopic fashion with significant overlap in the rNST. The caudal NST (cNST) receives sensory input from the viscera through the vagus nerve (X). The gustatory functions associated with the depicted circuits and structures remain largely speculative. Nerve transection studies indicate that input from the gustatory branches of the facial nerve (VII), but not the IXth cranial nerve, are necessary for stimulus identification (red text and symbols) [see 62,63]. Ingestive motivation (blue text and symbols) appears to depend on input from the VIIth and IXth cranial nerves. There is evidence supporting the necessity of the chorda tympani branch of facial nerve (CT-VII) in the maintenance of cephalic phase insulin responses [56], and it is possible that other nerve branches are critical for other taste-evoked physiological reflexes. Recent findings have revealed at least two classes of neurons in the geniculate ganglion. One class synapses on rNST neurons that project to the parabrachial nucleus; the other class synapses on rNST neurons that project to the reticular formation [64]. This segregated projection pattern is consistent with the hypothesis that different peripheral afferent taste fibers can contribute differentially to various gustatory functions. The taste buds of the laryngeal epithelium are innervated by the superior laryngeal branch of the vagus (SLN-X); based on their response properties and location, the laryngeal taste buds are thought to help protect the airways [e.g., 68]. The efferent limb of such protective reflexes involve several of the meduallary structures shown. Neurons in the rNST project to the parabrachial nucleus and also contribute to local medullary circuits that contribute both to salivation [e.g., 66] and taste-evoked oromotor reflexes [e.g., 65]. The medullary reticular formation also receives forebrain projections (not shown) that play a role in voluntary ingestion (ingestive motivation). The central circuitry subserving other taste-triggered physiological reflexes, including insulin release, remains to be fully described (dashed line). Finally, although there is substantial evidence supporting the role of the ventral forebrain gustatory pathway in ingestive motivation, the hypothesized role of the thalamocortical gustatory pathway in stimulus identification [67] awaits explicit behavioral tests. GSP-VII: greater superficial petrosal branch of the facial nerve; LT-IX: lingual-tonsillar branch of the glossopharyngeal nerve