A two compartment model of the stepping generator: analysis of the roles of a stage-setter and a rhythm generator.

Recent studies on locomotion of the mesencephalic cat demonstrated that activation of the spinal stepping generator and the postural control system are dependent phenomena (Mori et al., 1978, 1980). This has motivated the construction of a new model of the stepping generator to account for interactions with the postural control system. The present model consists of two main compartments, the rhythm generator and the stage-setter. The rhythm generator generates rhythmic bursting discharges of extensor and flexor alpha motoneurons. The function of the stage-setter is to set and reset the excitability of extensor alpha motoneuron to a number of desired levels. This study analyzes interactions in this model between rhythm generating and postural control system. By adding a concept of "stage-setting" to the rhythm generator model, we succeed in simulating a variety of locomotor patterns observed in the mesencephalic cat, including "stepping automatism" (Mori et al., 1979).