Organization of the Vagus in Elasmobranchs: Its Bearing on a Primitive Gnathostome Condition

Abstract

The vagus nerve of the clearnose skate, Raja eglanteria, on the basis of its central and peripheral patterns and in light of the embryonic origin of its innervation fields, is viewed as a collector of separate elements. The peripheral elements include a series of branchial nerves to a segmented pharynx, an intestinal nerve to an unsegmented gut, a nerve or nerves to the heart, and an accessory nerve to a cucullaris muscle. The central elements include a sensory column, a dorsal motor column, and a ventral motor column. The dorsal motor column and sensory column are segmented in register with the branchial and intestinal nerves. Motoneurons that supply the branchial muscles of somitic origin are only located in the rostral segmented portion of the dorsal motor column. Preganglionic parasympathetics to the enteric plexus, presumably derived from circumpharyngeal crest, form the caudal portion of the dorsal motor column and are probably also present in the rostral segmented portion. Cardiac preganglionic parasympathetics to a visceral field of cardiac crest origin occur in the rostral portion of the ventral motor column as well as in the dorsal motor column. Accessory motoneurons that supply the cucullaris, likely a part of the general body musculature, are unrelated to other vagal motoneurons and form a separate nucleus (caudal ventral motor nucleus) located at spinal levels. The central and peripheral vagal nerve patterns of elasmobranchs suggest a highly conserved, ancestral gnathostome condition.