Further observations on the cerebellar projections from the pontine nuclei and the nucleus reticularis tegmenti pontis in the rhesus monkey.

The projections from the pontine nuclei and the necleus reticularis tegmenti pontis (N.r.t.) onto the flocculus, uvula, and the paramedian lobule were studied with retrograde transport of horseradish peroxidase n the rhesus monkey. The main findings are as follows: There is a conspicuous tendency for labeled cells to occur in numerous discrete clusters in the pontine nuclei after injections of these parts of the cerebellum. There appears to be very limited overlap between pontine cell groups projecting to the flocculus, the uvula, and the paramedian lobule, respectively. The flocculus appears to receive a substantial projection from the pontine nuclei. The projection is almost totally crossed (3% ipsilateral), and arises mainly laterally in the rostral half of the pons but in addition from a minor group dorsomedially. The flocculus receives a bilateral projection (slight contralateral preponderance) from medial and dorsomedial parts of the NRT. The number of labeled cells in the NRT was 13% of the number in the pontine nuclei. the uvula is amply supplied from the pontine nuclei. The projection takes origin throughout the rostrocaudal extent of the pons, from one medial and one dorsolateral region. Labeled cells are found in greatest number dorsolaterally in the rostral half of the pons. In the caudal N.r.t., one medial and one lateral cell group were labeled after injection of the uvula. The number of labeled cells in the N.r.t. was only 4% of the number in the pontine nuclei. Findings with regard to the paramedian lobule confirm and extend earlier observations in the monkey (Brodal, '79, '80). The present results are discussed in relation to HRP studies of the pontocerebellar projection in lower animals. Several possible species differences are noted--for example, with regard to projections to the flocculus. There is some evidence that the pontocerebellar projection is more precisely organized in the monkey than in lower animals.