Nucleus interpositus projection to spinal interneurons in monkeys

Abstract

Direct spinal projections from the deep cerebellar nuclei have recently received considerable attention in catsS, 10, and some other species1,3, 9. These reports have focused primarily on the fastigiospinal neurons, which are thought to influence upper cervical motoneurons monosynaptically 10. Retrograde transport studies z have indicated that in monkeys, the nucleus interpositus also projects to the spinal cord. During the course of more detailed studies of the output connections of the monkey deep cerebellar nuclei, we have identified the course and mode of termination of interpositospinal axons. Their terminations are dorsally situated, on the somata and proximal dendrites of interneurons in the intermediate zone. Injections of [3H]leucine and [aH]proline were made into various foci within the deep cerebellar nuclei of cynomolgus and rhesus monkeys s. After survival periods of 5-8 days, serial sections of the brains and spinal cords were prepared for autoradiography 4. In experiments in which the injections were confined to the nucleus interpositus, all labeled efferent fibers leave the cerebellum via the superior cerebellar peduncle and decussate at mesencephalic levels. Although many of these labeled fibers, after decussation, either terminate in the reticulotegmental nucleus, the magnocellular red nucleus, or proceed rostrally towards the diencephalon, a substantial number project caudally. In the ventrolateral tegmentum there is a diffuse array of labeled fibers, most of them descending to terminate in the inferior olivary complex. However, clusters of concentrated label, indicative of a few large diameter axons are segregated dorsomedially and proceed caudally along the lateral surface of the pyramidal decussation into the medial part of the ventral funiculus of the spinal cord (Fig. 1). Up to 20 labeled axons can be identified in this position in each experiment. Labeled axons can be traced into the central gray from the caudal medulla to the third cervical segment (they were not traced further). Grain concentrations indicative of labeling of terminal