Distribution of recurrent inhibition in the cat forelimb.

This chapter reviews experiments on the distribution of recurrent pathways from motor axon collaterals to alpha motoneurones in the brachial enlargement of the cat. In anaesthetized cats intracellular recording from identified forelimb motoneurones was used to reveal the pattern of recurrent inhibition or excitation following stimulation of muscle nerves. The recurrent connections of the motor nuclei acting on the elbow follow the tight mechanical agonism of the muscles involved. Extensive bidirectional recurrent inhibitory connections were found between motor nuclei innervating elbow and wrist muscles. It is suggested that one group of these connections supports the organization of limb extension, the other group organization of limb flexion. The supinator and the pronator teres motornuclei have identical recurrent connections. Co-convergence from elbow flexor and extensor motornuclei in one and the same motoneurone is frequent. It is suggested that this pattern may serve the stabilization of the radio-ulnar plane. Neither homonymous nor heteronymous recurrent actions were observed in the radial motornuclei acting on the digits, which suggests a lack of recurrent axon collaterals in these nuclei. These results draw attention to the fact that recurrent inhibition is not evenly distributed between all limb motor nuclei.