An electron microscopic study of rubrospinal projections to the lumbar spinal cord of the opossum

The rubrospinal system is a major suprasegmental input to the important interneuronal pool at the base of the lumbar dorsal horn in the North American opossum. After appropriate lesions, rubral axons and their synaptic terminals were found in electron micrographs of lamina IV, V and VI as well as within the dorsal extreme of lamina VII. Degenerating terminals contact small diameter dendrites in the lateral terminal zone and large dendritic profiles in the medial terminal zone. Correlating these data with the dendritic arborizations of interneurons in Golgi preparations and with existing physiologic studies, it appears that interneurons in the intermediate and medial aspects of lamina V, VI and VII receive rubral input on both their proximal and distal dendrites.