The course of spinocerebellar fibers in the rat spinal cord was investigated by injecting horseradish peroxidase into the cerebellar anterior vermis after complete transection of the left inferior and right superior cerebellar peduncles. By this procedure, fibers passing via the inferior cerebellar peduncles (icp-fibers) were labeled retrogradely on the right side of the spinal cord, whereas fibers passing via the superior cerebellar peduncles (scp-fibers) were labeled on the left side. Crossed icp-fibers were located diffusely in the anterior and lateral funiculi in the sacral to lower lumbar segments. They gradually migrated laterally and dorsally in these funiculi and received many uncrossed icp-fibers moving laterally in the lateral funiculus from the gray substance in the upper lumbar to lower thoracic segments. These mixed fibers shifted more dorsally and laterally in the anterior and lateral funiculi to aggregate in the narrow peripheral zone of the lateral funiculus in the upper thoracic and lower cervical segments, and received many crossed fibers in the upper cervical segments. There were more icp-fibers than scp-fibers through the spinal cord. However, the extent of scp-fibers in the anterior and lateral funiculi was essentially the same as that for icp-fibers, except that a few scp-fibers were found in the dorsolateral marginal zone of the lateral funiculus. It has been generally accepted that the dorsal spinocerebellar tract ascends in the dorsal half of the lateral funiculus and enters the cerebellum via the inferior cerebellar peduncle, whereas the ventral spinocerebellar tract ascends in the ventral half of it and takes the superior cerebellar peduncle route. The results of this study suggest that it is necessary to revise this concept.

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