Modelling study of spinal generators structure; the role of alpha motoneurons, Renshaw cells and Ia interneurons in locomotion.

The properties of a model system composed of nets of motoneurons, Ia interneurons and Renshaw cells corresponding to the set of muscles driving one joint were analyzed from the point of view of their participation in locomotion control, i.e., generation of signals driving individual muscles. The model was constructed of neuron-like analog elements. The relations between the structure of the system, its control by tonic and phasic inputs and the properties of the generated signals were examined. The pattern of activity of motoneuron-like elements was considered to reflect the physiological pattern of muscle activity during locomotion. The network modelled in this way was an adjustable system, i.e., the parameters of the generated pattern could be varied by both tonic and phasic signals. A phasic signal acting on a background of moderate tonic signals seemed to enable a broader adjustment of the parameters of the generated pattern than tonic signals only. The best correspondence between generated and physiological patterns was obtained when the Renshaw cells were additionally and selectively inhibited by tonic signals.