Distribution of primary vestibular fibers in the brainstem and cerebellum of the monkey.

Attempts were made to determine the central projections of ganglion cells innervating individual semicircular ducts in the monkey by implanting or injecting tritiated amino acids (leucine and/or proline), or horseradish peroxidase (HRP), selectively into a single ampulla. Central transport via the vestibular ganglion in animals receiving isotope implants or injections fell into three categories: (1) transport from ganglion cells innervating all receptive elements of the labyrinth, (2) transport from ganglion cells innervating the three semicircular ducts, and (3) transport from cells of the inferior vestibular ganglion innervating the posterior semicircular duct. Transneuronal transport of isotope was observed in secondary vestibular fibers in animals where proline was used and survival exceeded 12 days. Transneuronal labeling of secondary auditory fibers was independent on the [3H]amino acid used, and occurred with survivals of 10 or more days. HRP implanted into the ampulla of the lateral semicircular duct in several animals produced retrograde transport to efferent vestibular and cochlear neurons, but did not result in transganglionic labeling of primary vestibular or auditory fibers. Primary vestibular fibers terminate throughout the superior (SVN) and medial vestibular nuclei (MVN). Within SVN, terminals are most pronounced in its central large-celled portion, but extend into peripheral parts of the nucleus, except for a small medial area near its junction with the oral pole of MVN. Primary projections to MVN are homogenously distributed throughout the nucleus excepting a small circular area of sparse terminals along its ventral margin. Primary vestibular afferents terminate mainly in rostral and caudal portions of the inferior vestibular nucleus (IVN), but do not reach cell group 'f'. Projections to the lateral vestibular nucleus (LVN) are restricted to its ventral part. Primary projections to the accessory vestibular nuclei reach the interstitial nucleus of the vestibular nerve (NIVN) and cell group 'y'. Fibers project beyond the vestibular nuclei (VN) to terminate ipsilaterally in the accessory cuneate nucleus (ACN), the subtrigeminal lateral reticular nucleus (SLRN), and well-defined portions of the reticular formation (RF). Projections to SVN and MVN are derived primarily from ganglion cells innervating the semicircular ducts, while projections to caudal IVN, cell group 'y' and ACN are related mainly to macular portions of the vestibular ganglion. NIVN receives both macular and duct afferents.(ABSTRACT TRUNCATED AT 400 WORDS)