Central projections of the vagus nerve in Chelon labrosus Risso (Teleostei, O. Perciformes)

Abstract

The primary projections of the vagus nerve and its nuclei in a perciform teleost, Chelon labrosus, are described after application of hoseradish peroxidase (HRP) labeling and Nissl and silver staining methods. The vagus nerve has three peripheral roots, rostral, medial and caudal, which are formed from viscerosensory and visceromotor rootlets. In addition to viscerosensory and visceromotor rootlets, the caudal root has a cutaneous root and a 'descending' root. Dorsally in the caudal root, a fiber bundle enters the medulla from the retroocular and opercular skin region; this bundle joins the descending trigeminal root and courses with it rostrally and caudally. Fibers from another rootlet, which enters dorsolaterally, course caudally towards the spinal cord. This rootlet has not been described in other fishes. The vagal viscerosensory nucleus, where most of the viscerosensory fibers terminate, forms a column in which two types of neurons can be distinguished. The most caudal part of the nucleus is continuous with the commissural nucleus, into which vagal nerve projections also enter. No vagal projections were found in the area postrema. In the visceromotor column, two types of neurons are seen lateroventral to the IV ventricle.