Brainstem and spinal projections of the deep cerebellar nuclei in the monkey, with observations on the brainstem projections of the dorsal column nuclei.

Abstract The brainstem and spinal projections of fibers arising in each of the deep cerebellar nuclei were compared. The brainstem terminations of fibers arising in the dentate and interposed nuclei are similar. Both cerebellofugal systems follow similar intracerebellar trajectories, leave the cerebellum via the brachium conjunctivum, decussate and distribute terminal ramifications contralaterally in the red nucleus, the nucleus reticularis tegmenti pontis and the inferior olivary complex. Although the zones innervated by the dentate and interposed nuclei are similar within the nucleus reticularis tegmenti pontis, these two deep cerebellar nuclei project differentially to the red nucleus, and to the inferior olivary complex. The dentate nucleus projects only to the parvocellular red nucleus and the principal olivary nucleus, whereas the interposed nucleus projects only to the magnocellular red nucleus and the two accessory olivary nuclei. No terminations were detected in the oculomotor nuclei following injections into the dentate nucleus. The fastigial projections to the brainstem are extensive, bilateral, and independent of the dentatofugal and interpositofugal systems. The majority of the fastigio-brainstem fibers decussate within the cerebellum, then leave the cerebellum via the uncinate fasciculus. Contralaterally, the fastigio-brainstem projections terminate in the nucleus reticularis tegmenti pontis, Deiter's nucleus, portions of the descending vestibular nucleus, the pontine nuclei, the pontine raphe, and parts of the reticular formation from the midbrain caudally. The fastigial projection to the nucleus reticularis tegmenti pontis terminates in a zone clearly independent of the dentate and interpositus projections. The ipsilateral fastigio-brainstem terminations are less extensvie than the contralateral. They occur in Deiter's nucleus, in portions of the descending vestibular nucleus, and at the rostralmost end of the mesencephalic reticular formation near the pretectal area. The terminations of fibers projecting from the dorsal column nuclei to the inferior olivary complex occur in zones which are coextensive with cerebellar termination zones. Both the fibers of the dorsal column nuclei and of the anterior interposed nucleus terminate in the dorsal accessory olive. Other than the dorsal accessory olive, the only other brainstem projection from the dorsal column nuclei is to the external nucleus of the inferior colliculus. Projections to the spinal cord arise in the interposed and fastigial nuclei. The interpositospinal projection leaves the cerebellum via the ipsilateral brachium conjunctivum, decussates, travels in the ventral funiculus of the spinal cord, and terminates on interneurons in the intermediate zone. By contrast, the fastigiospinal projection decussates within the cerebellum, leaves the cerebellum via the contralateral uncinate fasciculus, travels in the lateral funiculus, and terminates around motoneurons in the ventral horn. No spinal projections were detected from the dentate nucleus.