Traditionally, the red nucleus of the cat is divided into two parts: a large-celled, magnocellular, division (RNm) and a small-celled, parvicellular, division (RNp). The RNm projects to the spinal cord and receives input from the cerebellar interpositus nucleus. The RNp projects to the inferior olive and receives input from the cerebellar dentate nucleus. In this report, we reexamine the connections of the red nucleus using the bidirectional tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP). Our findings demonstrate that the cat RNp has a large caudal and lateral region that projects to contralateral spinal cord and not to the inferior olive. The spinally projecting region of RNp receives input from the dentate nucleus and a lateral segment of anterior interpositus. Cervical projections from the red nucleus show a topography with the rostral portion of RNp favoring upper segments and the caudal portion of RNm favoring lower segments. The results show that dentate output can influence spinal activity without passing through the cerebral cortex. For the control of movements such as reaching and grasping, we suggest that RNp and dentate focus on the control of proximal limb musculature, whereas RNm and the anterior interpositus focus on the control of distal limb musculature. We also suggest that other species are likely to have a small-celled area of red nucleus projecting to the spinal cord.
