Central projections of the octavolateralis nerves of the clearnose skate, Raja eglanteria.

Abstract

The central projections of first-order lateral line and octavus nerve afferents of the clearnose skate, Raja eglanteria, were determined by nerve degeneration and horseradish peroxidase techniques. The octavolateralis area of the medulla, which receives these afferents, is organized into dorsal, intermediate, and ventral longitudinal columns of cells and neuropil. Fibers that innervate the electroreceptive sense organs enter the dorsal longitudinal column via the dorsal root of the anterior lateral line nerve and terminate within the dorsal nucleus. Mechanoreceptive fibers from neuromasts of the head and trunk are carried by the ventral root of the anterior lateral line nerve and posterior lateral line nerve, respectively. Both nerves enter the intermediate longitudinal column and terminate throughout the rostrocaudal extent of the intermediate nucleus. Fibers of the ventral root of the anterior lateral line nerve are confined to the medial portion of the intermediate nucleus and posterior lateral line nerve fibers to the lateral portion. In addition, ascending mechanoreceptive fibers from both head and trunk neuromasts project to the vestibulolateral lobe of the cerebellum. Octavus nerve afferents enter the medulla and terminate primarily within the four octaval nuclei that comprise the ventral longitudinal column. Rostrocaudally, these nuclei are the anterior, magnocellular, descending, and posterior octaval nuclei. A few ascending axons continue beyond the anterior octaval nucleus and course to the vestibulolateral lobe of the cerebellum. Some descending axons emanate from the descending octaval nucleus and course to the reticular formation and intermediate nucleus. Therefore, electroreceptive lateral line, mechanoreceptive lateral line, and octavus nerve afferents project ipsilaterally and terminate predominantly within separate medullary nuclei. The significance of octavus nerve projections to the intermediate nucleus and overlap of mechanoreceptive and octavus afferents within the vestibulolateral lobe of the cerebellum cannot be determined until it is known which fibers of the inner ear sense organs project to these areas. Retrograde transport of horseradish peroxidase results in the labeling of large multipolar cells, both ipsilaterally and contralaterally, within a column of gray that lies dorsolateral to the reticular formation. These cells are interpreted as the cell of origin of the efferent components of the anterior and posterior lateral line nerves.