Recurrent inhibition in the cat distal forelimb.

The pattern of recurrent pathways from motor axon collaterals to motoneurones has been investigated in the spinal cord of anaesthetized cats with intracellular techniques. Only recurrent inhibitory postsynaptic potentials (RIPSPs) were observed so far. They were very pronounced between the mechanical synergists acting at the elbow. The triceps muscles received substantial effects also from the extensor carpi ulnaris (ECU) and from palmar-located medialis (M) and ulnaris (Ul) muscles. The effects were much smaller in the motor nuclei acting on the wrist (ECU, EC radialis (ECR]. The motor nuclei to the muscles acting on the phalanges (extensor digitorum communis and lateralis, extensor indicis proprius, and abductor pollicis longus (EDC, EDL, EIP, APL] neither received nor emitted any recurrent effects, which may indicate a lack of the recurrent system in these distal motor nuclei. A comparison of the recurrent pattern with the distribution of monosynaptic excitation from large muscles spindle afferents (Ia) shows that there is a partial overlap of both systems only at the elbow joint.