Abstract
Using intracellular staining with horseradish peroxidase (HRP), Sato et al. (1989) demonstrated the course and termination in the vestibular nuclei of electrophysiologically identified single axons originating from the three semicircular canals. All semicircular canal-activated afferents terminated on all vestibular nuclei. The morphology of physiologically identified otolith-activated afferents (Imagawa et al. 1995, 1998) and 2nd order vestibular neurons (Meng et al. 2002) was investigated by similar methodology with HRP. The ascending branches of otolith axons give off collaterals with boutons in the caudal part of the superior nucleus. The descending branches coursed caudally through the lateral part of the descending nucleus, and gave off collaterals with boutons in the lateral, descending and medial vestibular nuclei. In summary, our data now complete the catalogue of the morphology of individual primary neurons innervating vestibular end organs, i.e. the three canals and the two otolith orngas. As a whole, canal and otolith afferents basically project to the same vestibular nuclei, although there are noticeable quantitative differences. Otolith projections to areas outside the vestibu-lar nuclei where ipsilateral abducens nucleus and reticular formation are present are quite different (Uchino et al. 1994, 1996).
The morphology of vestibular neurons was studied by similar methodology with HRP (See Wilson and Melvill Jones 1979). The otolith-activated 2nd order vestibular neurons were pyramidal, elongated and ovoidal in shape. Most of the labeled cells were medium to large. There was no apparent correlation between morphology and the different types of otolith- and canal-activated vestibular neurons. Thus, it seems likely that the functional type of vestibular neurons cannot be presumed on the basis of their morphology alone.
