The cerebellofugal projections in the brachium conjunctivum of the rat. I. The contralateral ascending pathway,

Abstract

The cerebellofugal projections in the contralateral ascending pathway of the brachium conjunctivum (B.C.) in the rat have been investigated in 23 animals using the Fink‐Heimer technique to demonstrate the axonal degeneration resulting from complete B.C. lesions (7), partial B.C. lesions (14) and control lesions dorsal to the B.C. (2). All of the 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. The contralateral ascending pathway ascends rostrally from the decussation of the B.C. through the ventromedial midbrain tegmentum to the diencephalon. In the midbrain, cerebellofugal fibers terminate heavily throughout the red nucleus including the nucleus minimus, while others pass dorsolaterally and dorsomedially from the ascending tract to terminate in adjacent midbrain nuclei. The dorsolaterally directed fibers terminate in the midbrain reticular formation, the stratum griseum profundum of the superior colliculus, the anterior pretectal nucleus and the nucleus of the posterior commissure; the dorsomedially directed fibers terminate in the principal oculomotor nucleus, the nucleus of Dark‐schewitsch, interstitial nucleus of Cajal and the central gray matter. A considerable number of the cerebellofugal fibers proceed more rostrally within the prerubral field and enter the thalamus by two routes. Most follow a direct subthalamic course within field H of Forel and after contributing fibers to the zona incerta, enter the caudal pole of the ventromedial nucleus (Vm) of the thalamus to terminate throughout Vm and the ventrolateral complex (Vl). Others enter via the internal medullary lamina and terminate throughout the parafascicular and central lateral nuclei. A small number of cerebellothalamic fibers form a commissural projection to Vl on the opposite side. The finding that the cerebellothalamic projections to the ventral nucleus are distributed throughout Vm and Vl establishes the Vm‐Vl complex as the homologue in the rat of the ventral anterior and ventral lateral nuclei in the primate thalamus.