Gustatory pathway

First-order neurons are pseudounipolar cells in cranial nerve sensory ganglia: facial nerve — geniculate ganglion (chorda tympani from the anterior two-thirds of the tongue; greater petrosal nerve from the soft palate); glossopharyngeal nerve — inferior (petrosal) ganglion (posterior third of the tongue including circumvallate papillae); vagus nerve — inferior (nodose) ganglion (epiglottis, pharynx and larynx via the internal laryngeal nerve). Note that the chorda tympani travels with the lingual nerve (V3) in the infratemporal fossa — so lingual nerve injury causes loss of both taste and general sensation on the anterior tongue.

Their central processes enter the medulla, descend in the solitary tract and synapse in the rostral (gustatory) part of the nucleus of the solitary tract (NTS) — VII most rostrally, then IX, then X. Second-order neurons project ipsilaterally in the central tegmental tract to the ventral posteromedial nucleus (VPM; its parvocellular part) of the thalamus. Third-order neurons project through the posterior limb of the internal capsule to the primary gustatory cortex in the anterior insula and frontal operculum (near the face area of the somatosensory cortex). From there information reaches the orbitofrontal cortex (secondary taste cortex), where it combines with smell, texture and temperature to produce flavour and is weighted by hunger and reward. In primates the ascending taste pathway is largely ipsilateral.

The NTS also sends taste information to brainstem reflex centres — the salivatory nuclei, dorsal motor nucleus of the vagus and nucleus ambiguus — for salivation, gastric secretion, swallowing and gagging, and to the hypothalamus and amygdala (taste-aversion learning, appetite).