Morphology of single primary vestibular afferents originating from the horizontal semicircular canal in the cat

The central projections of physiologically characterized vestibular nerve fibers originating from the horizontal semicircular canal were studied in the vestibular nuclei of adult cats after intracellular staining with horseradish peroxidase (HRP). First, primary nerve fibers were physiologically classified as regular or irregular types on the basis of the regularity of the spontaneous discharge pattern. Then, these two types of fibers were morphologically analyzed and compared following HRP intraaxonal injection. The two types of axons showed a basically similar trajectory in the four major vestibular nuclei. They bifurcated into an ascending and a descending branch in the ventrolateral part of the lateral vestibular nucleus (LVN). The ascending branch extended rostrally and gave off one or two collaterals in the superior vestibular nucleus (SVN), although some of the ascending branches further ran rostrally into the cerebellum. The collaterals, while running medially, gave rise to fine terminal branches with en passant boutons in the SVN, and further coursing caudally, they entered the rostral part of the medial vestibular nucleus (MVN). The descending branch, while running caudally in the lateral part of the LVN and the inferior vestibular nucleus (IVN), gave off several thick collaterals to the MVN and extensive terminals were present in the IVN and MVN. In each primary axon, about one‐third of the total terminal boutons were distributed in each of the SVN, the MVN, and the IVN. In contrast to this similarity of the overall axonal trajectory within the vestibular nuclei, both types of axons exhibited several marked differences in diameter and in the mode of terminal arborization. In almost every part of the ramification, the irregular‐type fibers were thicker than the regular‐type fibers. In the regular‐type axons, many small terminal boutons (mean size, 2.4 × 1.4 μm, N = 2,739) were located in close proximity (100–150 μm) to the parent collateral. In the irregular‐type axons, slightly larger terminal boutons (mean size, 3.0 × 1.7 μm, N = 1,287), were spread more widely (200–300 μm) around their collaterals. These clear morphological differences between the regular‐type and the irregular‐type terminal axons were consistently observed in any vestibular nucleus.