Peripheral and central distribution of major branches of the facial taste nerve in the carp.

The major pathways of the peripheral facial taste system in the carp, Cyprinus carpio, are the maxillary (Max), mandibular (Mand), palatine (Pal) and recurrent nerve rami. The peripheral distribution of the sensory fibers of these branches (B) was determined by means of electrophysiological techniques. Max.B., Mand.B. and Pal.B., each of which arises from the gasserian-geniculate ganglionic complexes, were found to innervate respectively, the upper lip and the adjacent skin, the internal and external surface of the lower lip region, and the upper lip and the anterior palate, ipsilaterally. The recurrent nerve sends fibers mainly via dorsal and ventral branches of the posterior lateral line nerve (NPLL), and a pectoral branch of the occipito-spinal nerve. The dorsal and ventral branches of NPLL innervate respectively, the dorsal fin and the adjacent body surface, and the remainder of the body surface. The pectoral branch supplies the pectoral fin. The central connections of the above branches were also examined by using the techniques of transganglionic tracing with horseradish peroxidase (HRP). HRP was applied to each of the branches, and its penetration of the brainstem was carefully followed. Labeled fibers were observed only in the ipsilateral region of the brainstem. When Max.B or Mand.B. was treated with HRP, labeled fibers were observed in the facial sensory root and in the descending trigeminal root. When Pal.B. was treated, however, they were traced only to the facial sensory root; thus indicating that the former two branches are trigeminofacial complexes and the latter is a pure facial nerve. Labeled fibers for NPLL were found in the facial sensory root as well as in bundles projecting to the lateral line areas. The facial fibers of Max.B. and Mand.B. innervate respectively in the dorsal-intermediate portion of the rostral half of the facial lobe, and in the ventral portion of the caudal half of the lobe. Those of Pal.B. however, cover a large area of the lobe anteroposteriorly except for the dorsal and ventral portions. The recurrent fibers of NPLL and the pectoral B. end in the dorsal-medial portion of the caudal half of the lobe. Thus the results of this study show that there is a topographical relation between the receptive field of the 6 peripheral nerve branches and their locus of representation in the facial lobe. Similarly, that the gustatory system through Pal.B. is represented on the facial lobe in a disproportionately large area compared to that of the other 5 branches.