Patterns of projections from the pontine nuclei and the nucleus reticularis tegmenti pontis to the posterior vermis in the rhesus monkey: a study using retrograde tracers.

In an attempt to estimate the relative importance of the various afferent systems impinging on the oculomotor regions of the posterior cerebellar vermis of rhesus monkeys in quantitative terms, we made small injections of the retrograde tracers fast blue, fluorogold, and cholera toxin into different parts of a region of the posterior vermis, spanning lobuli VI through VIII. We found that the vast majority of cells retrogradely labeled by injections of lobulus VII and its vicinity lay in the pontine nuclei (PN), the nucleus reticularis tegmenti pontis (NRTP), and subnuclei a and b of the medial accessory olive. The remaining retrogradely labeled cells were distributed among a number of other brainstem nuclei or regions including the paramedian pontine reticular formation (PPRF). A quantitative analysis showed that the projection from the NRTP to the posterior vermis was focused on lobulus VII. While the projection from the PN as a whole demonstrated a preference for the more caudal parts of the posterior vermis, a closer look at the different regions of the PN revealed that cells located in the dorsal parts of the PN showed the same preference for lobulus VII as cells in the NRTP. The dorsal PN are a major gateway for cortical input to the cerebellum, related to visual processing and visually guided eye movements. Conversely, the NRTP, likewise involved in visually guided eye movements, is much more dependent on subcortical afferents. The observed convergence of input derived from the dorsal PN and the NRTP in oculomotor lobulus VII therefore suggests that a major function of this part of the vermis might be the integration of cortical and subcortical signals important for visually guided eye movements.