On the interaction between spinal locomotor generators in quadrupeds.

(1) The spinal locomotor generator of Miller and Scott may be regarded as a circuit of 4 neural 'elements' two of which are composite (i.e. they comprise two neurones). An analysis of all 4-element circuits with inhibitory interconnections reveals only one suitable arrangement. In that sense Miller and Scott's model is unique. (2) A set of simple but non-linear differential equations is proposed to describe the behaviour of 4 interacting populations of elements. The resulting system exhibits stable oscillatory behaviour under suitable conditions. (3) A set of 4 such oscillators, suitably coupled, is proposed as a model for the spinal control of corresponding joint muscles in each of 4 limbs. (4) The resulting system exhibits 4 types of oscillatory behaviour which correspond to the Trot, Pace, Gallop and Bound gaits of quadruped locomotion. (5) Mechanisms are suggested to account for transitions between gaits.