Monosynaptic excitation of motoneurons innervating forelimb muscles following stimulation of the red nucleus in cats.

We examined the rubrospinal projection to forelimb motoneurons (Mns) in cats, in comparison with the corticospinal projection. Under pentobarbital anesthesia, intracellular recordings were made from forelimb Mns following stimulation of the red nucleus (RN) and the cerebral peduncle (CP). Single pulse stimulation of the RN produced excitatory postsynaptic potentials (EPSPs) in the majority of forelimb Mns (58/80, 72%). Segmental latencies of RN-EPSPs were shorter than 1.2 ms in 14 out of 22 Mns in which RN-EPSPs were detected in the C8-T1 segments, and in 3 of 36 Mns in the C6-C7 segments. The results suggest monosynaptic rubro-motoneuronal connections in a substantial portion of forelimb Mns particularly in the C8-T1 segments. There was no evidence suggesting monosynaptic connections between cerebral cortex and forelimb Mns.