The motor nuclei and primary projections of the facial nerve in the monitor lizard Varanus exanthematicus.

Abstract

The location of the motor nuclei and the projection of the primary afferent fibers of the facial nerve of the reptile Varanus exanthematicus were studied by means of the HRP method and the anterograde degeneration technique. The motor nuclei are located ventrolaterally in the rhombencephalon and are constituted by a medial cell group consisting of large, polygonal cells and a lateral cell group consisting of medium-sized, spindle-shaped, and multipolar cells. From HRP applications to the various branches of the facial nerve it could be concluded that the medial cell group represents the branchiomotor nucleus and the lateral cell group the superior salivatory nucleus. The efferent axons from the motor nuclei course dorsomedially toward the fourth ventricle, where they form a genu, and exit from the brainstem in the ventral fiber bundle of the facial nerve. The primary afferent fibers enter the brainstem in the dorsal bundle of the facial nerve. This bundle courses medially, enters the solitary tract, and diverges into rostrally and caudally running fibers. Part of the caudally directed fibers leave the solitary tract and course laterally toward the descending trigeminal tract. Some fibers enter the nucleus of this tract. There was no noticeable terminal degeneration in the solitary tract or in the descending trigeminal tract or its nucleus.