Morphometric and experimental studies of the red nucleus in the albino rat.

Cytoarchitectural observations showed that the red nucleus of the albino rat consists of three distinct neuronal populations. Neurons with coarse Nissl bodies occupy the caudal end of the red nucleus and extend in diminishing number to the rostral tip. Neurons with finely granular Nissl material are the predominant cell type at the rostral tip of the red nucleus and interdigitate with the coarse neurons except at the caudal end of the nucleus. Coarse neurons, in contrast to fine neurons, are multiangular in contour and tend to be larger, although the two populations overlap in size. A population of interneurons, almost entirely smaller than the other cell types and less numerous, is ubiquitous within the red nucleus. Injections of horseradish peroxidase (HRP) at different levels of the spinal cord established that the coarse neurons on the contralateral side are the source of the rubrospinal tract and are topographically organized. The dorsal-medial part of the red nucleus emits axons which project to the cervical cord and the ventral-ventrolateral part of the nucleus to the lumbar cord. The thoracic cord receives projections from rubral neurons at intermediate positions. Further, coarse neurons from the entire rostrocaudal axis of the red nucleus contribute fibers to the rubrospinal tract.