Anatomy of the gustatory system in the hamster: central projections of the chorda tympani and the lingual nerve.

The sensory modalities of taste and touch, for the anterior tongue, are relegated to separate cranial nerves. The lingual branch of the trigeminal nerve mediates touch: the chorda tympani branch of the facial nerve mediates taste. The chorda tympani also contains efferent axons which originate in the superior salivatory nucleus. The central projections of these two nerves have been visualized in the hamster by anterograde labelling with horseradish peroxidase (HRP). Afferent fibers of the chorda tympani distribute to all rostral-caudal levels of the solitary nucleus. They synapse heavily in the dorsal half of the nucleus at its rostral extreme; synaptic endings are sparser and located laterally in caudal regions. These taste afferents travel caudally in the solitary tract and reach different levels by a series of collateral branches which extend medially in the the solitary nucleus, where they exhibit preterminal and terminal swellings. Taste afferent axons range in diameter from 0.2 micrometer to 1.5 micrometers. The thickest axons project exclusively to the rostral and intermediate subdivisions of the solitary nucleus; the find ones may distribute predominantly to the caudal subdivision. Afferent fibers of the lingual nerve terminate heavily in the dorsal one-third of the spinal nucleus of the trigeminal nerve and also as a dense patch in the lateral solitary nucleus at the midpoint between its rostral and caudal poles. This latter projection overlaps that of the chorda tympani. Thus the two sensory nerves which subserve taste and touch from coincident peripheral fields on the tongue converge centrally on the intermediate subdivision of the solitary nucleus. Efferent neurons of the superior salivatory nucleus were labelled retrogradely following application of HRP to the chorda tympani. These cells are located ipsilaterally in the medullary reticular formation ventral to the rostral pole of the solitary nucleus; their dendrites are oriented dorsoventrally. The efferent axons course dorsally, form a genu lateral to the facial somatomotor genu, and course ventrolaterally through the spinal nucleus of the trigeminal nerve to exit the brain ventral to the entering facial afferents.