Brainstem projection of the vestibular nerve in the guinea pig: An HRP (horseradish peroxidase) and WGA‐HRP (wheat germ agglutinin‐HRP) study

Abstract

We explored the course and termination of primary vestibular afferent fibers within the brainstem of the guinea pig by means of anterograde axonal transport of horseradish peroxidase (HRP) and wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Primary vestibular afferent fibers distribute within the entire vestibular nuclear complex, with the exception of the dorsal part of the lateral vestibular nucleus. The superior vestibular nucleus is characterized by the concentration of terminals within its central part. Although terminal labeling is weaker within the periphery, no area completely lacks primary input. The lateral vestibular nucleus can be divided into a ventral and a dorsal part; within the ventral part small and giant cells receive primary afferent fibers, whereas no significant terminal labeling occurs in the dorsal part. The medial vestibular nucleus shows the most uniform labeling, although the lateral part of its rostral third has a few more terminals than the medial half. Primary projection to the descending vestibular nucleus is widespread, although in its rostrodorsal part it is less impressive. Of the small cell groups commonly associated with the vestibular nuclear complex, only group y receives abundant primary input. Whereas group z completely lacks labeled fibers as well as terminals, single primary axons can be observed passing groups x and f. However, no terminals can be found within the borders of these two cell groups. Scanty projections can be detected within the prepositus hypoglossi nucleus, as well as within the external cuneate nucleus, the cochlear nucleus, the abducent nucleus, and parts of the reticular formation.