PROJECTIONS OF THE INDIVIDUAL VESTIBULAR END-ORGANS IN THE BRAIN-STEMOF THE SQUIRREL-MONKEY

The central nervous system (CNS) projections of primary afferent neuro ns from individual vestibular receptors were studied using horseradish peroxidase (HRP) or biocytin labeling in 14 ears from 7 adult squirre l monkeys using the technique developed in the chinchilla (Lee et al., 1989,1992). The specificity of labeling was verified by examining the location of the labeled fibers and cell bodies in the vestibular nerv e and Scarpa's ganglion, Labeled fibers and cells were restricted to n erves and areas belonging to groups of cells in either the superior or the inferior ganglion of the vestibular nerve. In the vestibular nerv e root, labeled primary afferent fibers also exhibited a receptor-depe ndent segregation at the entrance to the medulla. Fibers from the HSC and the SSC were found rostrally and those from the PSC and the SAC we re found in the caudal area, The UTR fibers were situated intermediate between these two groups of fibers. (A bundle of fibers, probably ves tibular efferents, was identified immediately rostrally and ventromedi ally to the UTR fibers.) The primary afferent fibers bifurcated into s econdary ascending and descending fibers at the lateral border of the vestibular nuclei, forming a longitudinal rostrocaudal vestibular trac t, The secondary fibers from individual end-organs occupied specific l ocations in the tract: the UTR fibers were dorsal to the SSC and the H SC fibers, PSC fibers were found most medially, and the SAC fibers occ upied the lateralmost area. The secondary UTR fibers overlapped consid erably with those of the SSC and the HSC. The orderly receptor-depende nt segregation of fibers was more prominent in the descending tracts t han in the ascending tracts. In the vestibular nuclei complex the loca tion of the tertiary branches of various end-organs exhibited consider able overlap within the major vestibular nuclei (SN, superior nucleus; LN, lateral nucleus; MN, medial nucleus; DN, descending nucleus). The re were still differences, however, in the projection pattern. Fibers from the SAC ran primarily in the lateral area, fibers from the SSC an d the UTR were found ventromedially to the SAC fibers, and the HSC pro jected slightly medially to the fibers from the SSC. The PSC fibers pr ojected most medially, The UTR and SAC sent numerous fibers to the cer ebellum. Fibers from the semicircular canals projected through the ros trodorsal region of the SN and presumably also projected to the cerebe llum. The precise termination of fibers was evaluated by studying the location of labeled boutons, which were identified in all major vestib ular nuclei. Labeled boutons from all the receptors were in the rostra l and central areas of the SN, and in the MN mainly in the rostral two -thirds. In the LN, boutons from all the receptors were in the rostrov entral part, most of which were from the UTR and SAG. No labeled bouto ns were in the caudodorsal part of this nucleus. Labeled boutons in th e DN primarily surrounded the descending tract fibers and were particu larly prominent medially, In specimens in which superior vestibular ne rve receptor organs were scratched vestibular efferent fibers were als o labeled, These fibers traveled in the most ventral part of the vesti bular nerve root and projected in the ventral aspect of the LN to labe led soma in the ipsilateral and contralateral brain stem. Specificity the in projection patterns of efferent fibers from different end-organ s could not be ascertained.