The cerebellofugal projections in the brachium conjunctivum of the rat. II. The ipsilateral and contralateral descending pathways.

The cerebellofugal projections in the ipsilateral and contralateral descending pathways of the brachium conjunctivum (B.C.) in the rat have been investigated in 22 animals using the Fink-Heimer technique to demonstrate the axonal degeneration resulting from complete B.C. lesions (6), partial B.C. lesions (14) and control lesions dorsal to the B.C. (2). The incidental degeneration resulting from the concomitant involvement of the structures surrounding the B.C. is accounted for in terms of known fiber pathways and from the results in the control experiments. This study confirms Ramón y Cajal's ('03) original observation that cerebellofugal fibers in the B.C. project caudally throughout the length of the hindbrain via both ipsilateral and contralateral descending pathways. The fibers forming the ipsilateral descending pathway proceed ventrally from the B.C. at the level of the trigeminal motor nucleus, turn caudally and terminate within nucleus reticularis parvocellularis (Rpa). In particular, fibers within this pathway terminate densely in two cytoarchitecturally distinct Rpa subnuclei--nucleus "k" (Meessen and Olszewski, '49) and a caudal linear subnucleus--which project to the cerebellum (Faull, '77). The contralateral descending pathway (B.C.de) roceeds caudally from the decussation of the B.C. within the ventromedial region of the magnocellular nuclei of the reticular formation of the pons and medulla. Cerebellofugal fibers of the B.C.de terminate in a distinctive pattern within precerebellar brainstem nuclei: densely throughout the middle third of nucleus reticularis tegmenti pontis; in a longitudinal zone of each of the three pontine gray subnuclei; in the principal and dorsal accessory nuclei of the inferior olive; and sparsely within nucleus reticularis paramedianus. Fibers in the B.C.de also terminate within the magnocellular nuclei of the reticular formation, principally the nuclei reticularis pontis oralis and caudalis.