Horseradish peroxidase labeling of the central pathways in the medulla of the ampullary nerves in the chicken, Gallus gallus.

The purpose of the present study was to place horseradish peroxidase on the distal processes of the three ampullary nerves in 6-8-week-old chickens so that we could identify the ganglion cells associated with each nerve and trace the specific central pathways taken by each nerve in the brainstem. We are especially interested in the pathways of the colossal vestibular fibers, which may play a role in a fast reflex pathway as suggested by their large caliber and electrotonic mode of transmission in the tangential vestibular nucleus. The cells of origin of each ampullary nerve occupy discrete portions of the vestibular ganglion. Those vestibular ganglion cells giving rise to the posterior ampullary nerve (PAN) occupy the posterior portion of Scarpa's ganglion; the ganglion cells producing the anterior (AAN) and lateral (LAN) ampullary nerves occupy the anterior ganglion, within the dorsal and ventral portions, respectively. Centrally the vestibular fibers occupy discrete portions of the tangential vestibular nucleus before bifurcating into ascending and descending tracts. The tangential nucleus receives afferents from the colossal fibers, which form spoon endings, and also from the fine vestibular fibers, which form small terminals. The ascending fibers of the posterior ampullary nerve are associated with the nucleus piriformis; the ascending fibers of the anterior and lateral ampullary nerves occupy discrete cell groups of the vestibulo-cerebellar nucleus. All three ampullary nerves have descending branches that course through the retrotangential nucleus into the descending vestibular nucleus (DVN). Within the descending vestibular nucleus, the descending fibers of the posterior ampullary nerve run dorsally and centrally, whereas fibers of the anterior ampullary nerve course ventromedially, and the lateral ampullary nerve fibers take a ventrolateral course until all three fiber bundles converge in the posterior tip of the descending vestibular nucleus. The ascending and descending fibers of each ampullary nerve form collaterals that pass to the ventrolateral and dorsomedial parts of the medial vestibular nucleus. These collaterals are derived exclusively from the fine and medium diameter vestibular fibers. Some of these ascending fibers form a distinctive tract that courses posteriorly within medial regions of the dorsomedial part of the medial vestibular nucleus. The colossal vestibular fibers, which are found within all three ampullary nerves, conform to the ampullary pathways as described, excluding the innervation of the medial vestibular nucleus.