Sensory impulses for taste arise from several sites in the oropharynx and are transmitted to the medulla via several cranial nerves (V, VII, IX, and X). The main pathway arises on the anterior two-thirds of the tongue; these taste fibers first run in the lingual nerve (a major branch of the mandibular segment of the trigeminal [V] cranial nerve). After coursing within the lingual nerve for a short distance, the taste fibers diverge to enter the chorda tympani (a branch of the facial [VII] nerve); thence they pass through the pars intermedia and geniculate ganglion of the seventh nerve to the rostral part of the nucleus of the tractus solitarius in the posterolateral medulla, where all taste afferents converge (see the following text and Fig. 44-3).
From the posterior one-third of the tongue, soft palate, and palatal arches, the sensory taste fibers are conveyed via the glossopharyngeal (IX) nerve and ganglion nodosum to the nucleus of the tractus solitarius. Taste fibers from the extreme dorsal part of the tongue and the few that arise from taste buds on the pharynx and larynx run in the vagus (X) nerve. The gustatory nucleus is situated in the rostral and lateral parts of the nucleus tractus solitarius, which receive the special afferent (taste) fibers from the facial and glossopharyngeal nerves. Probably both sides of the tongue are represented in this nucleus.
Fibers from the palatal taste buds pass through the pterygopalatine ganglion and adjacent to greater superficial petrosal nerve fibers, joining the facial nerve at the level of the geniculate ganglion, and proceed to the nucleus of the tractus solitarius (see Fig. 44-3). Possibly, some taste fibers from the tongue may also reach the brainstem via the mandibular division of the trigeminal nerve. The presence of this alternative pathway probably accounts for reported instances of unilateral taste loss that have followed section of the root of the trigeminal nerve and instances in which no loss of taste has occurred with section of the chorda tympani.
The second sensory neuron for taste has been difficult to identify. Neurons from the gustatory segment of the nucleus solitarius project to adjacent nuclei (e.g., dorsal motor nucleus of the vagus, ambiguus, salivatorius superior and inferior, trigeminal, and facial nerves), which serve viscerovisceral and viscerosomatic reflex functions, but those concerned with the conscious recognition of taste are currently considered to form an ascending pathway to a pontine parabrachial nucleus. From the latter, two ascending pathways have been traced (in animals). One is the solitariothalamic lemniscus to the ventroposteromedial nucleus of the thalamus. A second passes to the ventral parts of the forebrain, to parts of the hypothalamus (which probably influences autonomic function), and to other basal forebrain limbic areas in or near the uncus of the temporal lobe. Other ascending fibers lie near the medial lemniscus and are both crossed and uncrossed. Experiments in animals indicate that taste impulses from the thalamus project to the tongue–face area of the postrolandic sensory cortex. This is probably the end station of gustatory projections in humans as well, insofar as gustatory hallucinations have been produced by electrical stimulation of the parietal and/or rolandic opercula (Hausser-Hauw and Bancaud). Penfield and Faulk evoked distinct taste sensations by stimulating the anterior insula.
Clinical Manifestations of Disorders of Taste
Testing of Taste Sensation