Central course and terminal arborizations of single primary vestibular afferent fibers from the horizontal canal in the cat.

Physiologically identified single primary vestibular afferents originating from the horizontal canal in the cat were intraaxonally stained with horseradish peroxidase. All afferents examined gave off one collateral to the interstitial nucleus of Cajal located in the vestibular root bundle in the brainstem. Then these fibers bifurcated into ascending and descending branches in the lateral vestibular nucleus. The ascending branch distributed terminals mainly in the middle part of the superior vestibular nucleus. The descending branch issued several collaterals in the ventrolateral regions of the lateral and inferior vestibular nuclei. These collaterals ran horizontally and reached the medial vestibular nucleus. Terminals were observed mainly in the boundaries between the medial nucleus and the lateral or inferior nucleus. Two types of primary vestibular afferents may be distinguished. Type A fibers had terminal arborizations with plenty of boutons en passage and boutons terminaux, while type B fibers those with a few boutons. The axonal diameter of the main trunk was smaller in type A fibers than in type B fibers.