Observations on the secondary vestibulocerebellar projections in the macaque monkey.

The distribution of retrogradely labeled cells in the nuclei of the vestibular nuclear complex following injections of horseradish peroxidase in various parts of the cerebellar cortex (except the nodulus and paraflocculus) has been mapped in the macacus rhesus monkey. In the main the findings correspond to those made in other mammalian species (cf. Table 1). The flocculus receives afferents bilaterally from the superior, medial and descending vestibular nucleus, group y, the interstitial nucleus of the vestibular nerve and also from the abducent nucleus. The projection to the posterior vermis (lobules VIII and IX), especially to lobule IX, is more abundant than that to lobules VI-VII. The projection to the anterior lobe vermis appears to be modest. Evidence for projections to the cerebellar hemispheres was not obtained. Whether the lateral vestibular nucleus projects to the cerebellum in the macaque is uncertain. The regular occurrence of weakly labeled cells among heavily labeled ones suggests that many of the cerebellar projecting cells may have axonal branches passing to other destinations. The findings lend support to the notion that there are precise topical relations within the entire secondary vestibulocerebellar projection. For example, in the medial nucleus the sites of origin of fibers to the flocculus and uvula are different. Surprisingly, many cells in group z were found to project to the uvula and - to a lesser extent - to lobule VIII. The group z may, therefore, not be a pure relay nucleus in a spinothalamic pathway, as generally assumed. The rather marked cerebellar projection of the abducent nucleus, especially to the flocculus, is of interest for the analysis of cerebellar control of eye movements in the macaque.