Red nucleus projections to distinct motor neuron pools in the rat spinal cord.

Despite being one of the more extensively investigated descending pathways of the rat spinal cord, the termination pattern and postsynaptic targets of the rubrospinal tract (RST) still present some unresolved issues. In addition to locomotor functions, the RST is implicated in the control of limb movements such as reaching and grasping. Although a strong RST projection onto interneurons of intermediate Rexed's laminae V and VI have been described through the entire length of the rat spinal cord, the existence of direct rubro-motoneuronal connections have not been demonstrated. In the present study, anterograde tracing of the rat RST with biotinylated dextran amine (BDA) was combined with injections of cholera toxin beta-subunit (CTbeta) into selected groups of forelimb muscles to analyze in detail the rubral projections to the forelimb areas of the cervical spinal cord. The double-staining procedure suggested a direct projection from the RST to specific populations of motoneurons. Three populations of forelimb muscles were distinguished, i.e., paw, "distal" muscles; forearm, "intermediate" muscles; and upper arm, "proximal" muscles. A somatotopic distribution of the corresponding motor neuron pools was present in the spinal cord segments C4-Th1. Rubrospinal axons were seen in close apposition to the distal and intermediate muscle motoneurons, but were consistently absent in the most ventrally situated motor column projecting to proximal muscles. Microstimulation of the red nucleus resulted in electromyographic responses with shorter latency in the distal forelimb muscles than in proximal muscles. These experiments support a specific, preferential role of the RST in distal forelimb muscle control.