| Abstract: | The course, caudal extent and termination of the opossum rubro-spinal tract were determined by employing the Nauta-Gygax and Fink-Heimer techniques on the spinal cords of animals with lesions either within the red nucleus or within the descending fibers emanating from that nucleus. Prior to placement of the lesion, the area at the tip of the electrode was stimulated and the concomitant movements of skeletal muscles were palpated. The rubro-spinal tract traversed the entire length of the spinal cord and was located within the dorsolateral portion of the lateral funiculus. The tract was partially separated from the surface throughout cervical and thoracic levels by fibers of the dorsal spino-cerebellar tract. More caudally the rubro-spinal tract coursed immediately beneath the cord surface. At cervical levels, rubro-spinal fibers ended extensively within the lateral portion of laminae V and VI (Rexed, '52) and minimally within the medial portions of the same laminae. Fascicles also ended within lamina VII. At thoracic levels, lamina VI disappeared and rubro-spinal fibers terminated within the lateral and, to a lesser extent, the medial portions of laminae V and VII. Lamina VI was present at lumbar and rostral sacral levels and the majority of rubro-spinal fibers ended within the lateral part of that lamina. However, a few fibers terminated within laminae V and VII at such levels. At caudal sacral and coccygeal levels lamina VI disappeared and rubro-spinal bundles ended within laminae V and VII. Stimulation of the caudal red nucleus initiated contraction of the flexor musculature of the contralateral forelimb, and to a lesser degree, the contralateral hindlimb. Contraction of the contralateral paraxial musculature was also noted at cervical and thoracic levels during such experiments. Stimulation of the rubro-spinal tract and adjacent tegmentum immediately caudal to the red nucleus resulted in similar movements, but they were essentially limited to the side of the stimulation. |
