Monosynaptic rubrospinal projections to distal forelimb motoneurons in the cat

In the present electrophysiological experiments, we examined whether or not there are short-latency excitatory postsynaptic potentials (EPSPs) suggesting monosynaptic rubro- and corticomotoneuronal connections in the cervical spinal segments in cats. Under pentobarbital anesthesia and immobilization with gallamine triethiodide, intracellular recordings were made from forelimb motoneurons (Mns) following stimulation of the red nucleus (RN) and the cerebral peduncle (CP). Mns comprising the musculocutaneous nerve (Musc. Mns), the radial nerve (Rad. Mns), the ulnar nerve (Uln. Mns), and the median nerve (Med. Mns) were identified by their antidromic activation. Single-pulse stimulation of RN produced EPSPs with a mean amplitude of 0.35 mV in the majority of forelimb Mns (88 of 122, 72%). Half of the forelimb Mns (23 of 46) of the C8-T1 segments in which RN-EPSPs were detected by single shocks exhibited segmental latencies of RN-EPSPs shorter than 1.0 ms, while only 3 of 43 (7%) forelimb Mns in the C6-7 segments showed such latencies. These results suggest that rubromotoneuronal connections are, at least in part, monosynaptic for forelimb Mns that innervate in particular the hand and digit muscles. When paired-pulse stimuli (interpulse interval of 3 ms) were applied to RN, the second EPSPs were potentiated moderately (mean amount of temporal facilitation 24%) for EPSPs with short segmental latencies (< 1.0 ms). By contrast, single shocks to CP rarely produced EPSPs in forelimb Mns (9 of 106, 8%). There was no obvious evidence to suggest monosynaptic connections between corticospinal fibers and the forelimb Mns. The present results suggest the presence of preferential direct rubrospinal projection to Mns innervating distal forelimb muscles in the cat.