A comparative study of the neurons of origin of the spinocerebellar afferents in the rat, cat and squirrel monkey based on the retrograde transport of horseradish peroxidase.

The cell bodies of the neurons of the spinocerebellar pathways were examined using large injections of horseradish peroxidose into the cerebellum. Sections of each spinal segment were examined with both the DAB and the de Olmos O-dianisidine techniques. Results common to all three species were found. In Clarke's nucleus, the central cervical nucleus, and the spinal border cells there were many heavily labeled cells. Clarke's nucleus was found to project primarily ipsilaterally; the spinal border cells primarily contralaterally; and the central cervical nucleus bilaterally. In addition to these aggregates of spinocerebellar neurons there were numerous labeled neurons scattered throughout the spinal grey. Labeled neurons were found in all portions of the spinal grey except the substantia gelatinosa and lateral cervical nucleus and occurred in all spinal segments. They varied in morphology from large multipolar neurons, found predominantly in the ventral horn to small globular and fusiform neurons that were most abundant in the dorsal horn. These cells were found to project both ipsilaterally and contralaterally. Results common to only two of the species examined were also found. In the dquirrel monkey and the cat, but not the rat, the marginal layer of the dorsal horn in all segments of the spinal contained numerous labeled neurons. These marginal neurons were especially numerous in the squirrel monkey, where as many as 13 to 16 labeled neurons per section of the dorsal horn were found. In the rat and the squirrel monkey but not the cat, some intensely labeled large multipolar neurons were found in the sacral and caudal segments. These are the cells of Stilling's nucleus, a column of cells similar in position and orientation to that of Clarke's column but different in its projections and details of cytoarchitecture. Thus we have not only confirmed that Clarke's nucleus, the central cervical nucleus, and the spinal border cells project to the cerebellum but we have also found several new sources of spinocerebellar afferents.