Ultrastructural evidence for direct monosynaptic rubrospinal connections to motoneurons in Macaca mulatta

The magnocellularis division of the red nucleus of the Macaca mulatta, a midbrain structure involved in processing motor information, is known by light microscopic analysis to project, via the rubrospinal tract, to the contralateral intermediate horn of the spinal cord. Physiological studies, however, provide additional evidence for direct monosynaptic connections to motoneurons subserving distal musculature. This electron microscopic study demonstrates, by analyzing the anterograde transport of 5% wheatgerm agglutinin-horseradish peroxidase injected into the red nucleus, the presence of labeled terminals synapsing upon somata and proximal dendrites of motoneurons in the lateral portion of the ventral horn of the cervical enlargement of the spinal cord. We conclude that this anatomical evidence confirms the presence of direct monosynaptic connections to spinal motoneurons in the primate.