The cerebellopontine system in the rat. I. Autoradiographic studies

This study utilized light microscopic autoradiographic procedures to describe the projections from the three major subdivisions of the deep cere‐ bellar nuclei (DCN) to the basilar pontine nuclei (BPN). Although the vast majority of cerebellopontine axons reached the BPN via the descending limb of the brachium conjunctivum (BC) after crossing the midline within the midbrain, a relatively small number of ipsilaterally projecting fibers was also observed. Fascicles of cerebellopontine axons left the main bundle of de‐scending limb fibers throughout much of the rostrocaudal length of the BPN and passed around and through the medial lemniscus and cerebral peduncle to enter the pontine gray. The lateral cerebellar nucleus gave rise to the largest number of cerebellopontine fibers, whose terminal fields exhibited both diffuse and patchlike labeling patterns within each of the major subdi‐ visions of the BPN including medial, ventral, lateral, and dorsal areas. Pro‐ jections from the interpositus complex exclusive of its posterior division were fewer and less widely distributed than those from the lateral nucleus. Interpositopontine fibers terminated primarily in the caudal one‐half of the BPN in medial, ventral, and lateral regions and overlapped somewhat with projections from the lateral cerebellar nucleus. Pontine projections ema‐ nating from the medial cerebellar nucleus were the fewest and most re‐ stricted in distribution relative to the other two cerebellar efferent systems. Such fibers formed a patchlike network of terminal fields which extended throughout much of the rostrocaudal length of the BPN in medial and dorso‐ medial regions. A relatively small but considerable number of ipsilateral cer‐ ebellopontine fibers terminated in pontine regions, which often mirrored the typical contralateral projection fields. Although it proved difficult to deter‐ mine the precise origin of the ipsilateral fiber systems, it appeared that each of the three major DCN subdivisions made some contribution. Also it was apparent that considerable overlap existed between cerebellopontine projec‐ tion zones and those of other pontine afferents including sensorimotor, vi‐ sual, and auditory cortices, the superior colliculus, and the mammillary nu‐ clei of the hypothalamus. Moreover, cerebellopontine terminal fields were congruent in some instances with discrete clusters of BPN neurons which serve as the source of pontocerebellar fiber systems, reaching portions of the lateral cerebellar hemispheres, posterior vermis, and the paraflocculus.