Responses of red nucleus neurons to antidromic and synaptic activation

An account is given of the responses of 432 red nucleus (RN) neurons with axons projecting down the spinal cord. Almost half were in an initial series of 18 experiments on anesthetized cats, and the remainder were in a second series of 12 experiments on decerebrate unanesthetized cats. The differences between the two series were of little significance. All recording was from single neurons using extracellular glass microelectrodes that were inserted throught the right superior colliculus and directed to the right red nucleus at a standard orientation. Identification of RN neurons was both by location, checked by subsequent histology, and by antidromic invasion from the spinal cord. The spinal stimulating electrodes were placed in proximity to the left rubrospinal tract at C2 and L2 segmental levels. Axonal conduction velocities were calculated from the latency differential between the L2 and C2 antidromic responses and were usually in the range 60-130 m/s, 97% of all neurons located in the red nucleus had axons projecting to the C2 level, and 37% projected to the L2 level. The responses of 229 RN neurons were observed with stimulation applied to the contralateral (left) interpositus nucleus. In 10 (5%) there were antidromic responses to both interpositus and C2 stimulation, a finding in good agreement with the anatomical description of rare axon collaterals from rubrospinal fibers to the interpositus nucleus. In 209 (91%), there was a clear monosynaptic excitation. The impulse generation was at a latency usually of 1.0-1.8 ms, which a modal value of 1.4 ms. The afferent inputs to RN neurons were provided by stimulation either of predominantly cutaneous nerves in all four limbs or of cutaneous mechanoreceptors of the contralateral forelimb and hindlimb...