On the function of recurrent inhibition in the spinal cord.

Recurrent inhibition of alpha-motoneurons, via motor axon collaterals and Renshaw cells, obviously reduces the response (output) from a motor nucleus to a given synaptic input. It is proposed that the supraspinal covergence on Renshaw cells allows recurrent inhibition to serve as a variable gain regulator at motoneuronal level. This would allow for an optimal resolution in the force control during weak as well as strong contractions. Renshaw cells are not only inhibiting alpha-motoneurons but also gamma-motoneurons and IA inhibitory interneurons. It is argued that this distribution is meaningful since all these receptive neurons act together as a functional unit, forming an "output stage" of the motor system.