Neural network model for muscle force control based on the size principle and inhibition by Renshaw cells

A neural network model consisting single motor cortex output cell, actual number of a motoneuron, Renshaw cells and muscle units was constructed. The relation between discharge frequency of α motoneuron and that of cortex output cells of the model was investigated. Two types of neural network model were constructed. One consists of 100 or more motor units (model A). The other consists of the same number of motor units and Renshaw cells (model B). For the model A, the discharge frequency of α motoneuron increased almost linearly with increasing that of motor cortex output cells. All of α motoneuron showed this properties. For the model B, the discharge frequency of α motoneuron did not increase so much as that for model A. The shape of the discharge frequency curve, α motoneuron versus cortex output cell, of the model B was similar to that of α motoneuron firing rate versus force curve observed in monkey or human skeletal muscle. However, while small-sized α motoneuron fired, middleand large sized α motoneuron did not fire. It seems that Renshaw cells in model B had too strong inhibition for α motoneuron. These results suggest that inhibition of Renshaw cells play an important role in recruitment of motor units