Projections from the pontine nuclei proper and reticular tegmental nucleus onto the cerebellar cortex in the cat. An autoradiographic study.

After injections of 0.5 microliter of tritiated leucine and/or proline into various parts of the pontine nuclei proper or the pontine tegmental reticular nucleus (N.r.t.) of 34 cats, labeled terminals of pontocerebellar fibers were found in the cerebellar cortex. Fibers from the pontine nuclei and N.r.t. terminate as mossy fibers in the granular layer of the cerebellum, and no evidence is obtained of labeled fibers in the molecular layer. The pontocerebellar projection is, in general, bilateral with a contralateral preponderance, and a complex organization has been shown to exist in the cat. Clear evidence of divergence of this projection from a small pontine area has been demonstrated. Thus, the dorsolateral nucleus has a heavy projection to lobule VII, besides modest projections to lobules VI, VIII, and IX, crus I and II, paraflocculus, and paramedian lobule. On the other hand, a particular cerebellar region receives afferent fibers from several pontine regions, confirming previous HRP studies. For example, lobule VII receives heavy projections from parts of the dorsolateral, peduncular, and paramedian nuclei, less heavy projections from the lateral part of the lateral nucleus, and some from other parts of the pontine nuclei. This is a convergent feature of the pontocerebellar projections. In addition, small adjoining areas within a pontine subdivision have different patterns of cerebellar projections, shwing preferential sites of terminations. This suggests some degree of localization within the pontine nuclei. The cerebellar projection from the N.r.t. shows an essentially similar organization as the projection from the pontine nuclei proper, an apparent difference being only that the former is more extensive in the fields of termination than the latter. Some evidence for a parasagittal termination of pontocerebellar projections to the paramedian lobule has been found in this study. However, this is not as clear-cut as such patterns in the cerebellar projections from the spinal cord, cuneate nucleus, and lateral reticular nucleus shown recently in rat and cat.