Control of locomotor cycle durations

In intact animals and humans, increases in locomotor speed are usually associated with decreases in step cycle duration. Most data indicate that the locomotor central pattern generator (CPG) shortens cycle duration mainly by shortening the durations of extensor rather than flexor phases of the step cycle. Here we report that in fictive locomotion elicited by electrical stimulation of the midbrain locomotor region (MLR) in the cat, spontaneous variations in cycle duration were due more to changes in flexor rather than extensor phase durations in 22 of 31 experiments. The locomotor CPG is therefore not inherently extensor- or flexor-biased. We coined the term "dominant" to designate the phase (flexion or extension) showing the larger variation. In a simple half-center oscillator model, experimental phase duration plots were fitted well by adjusting two parameters that corresponded to background drive ("bias") and sensitivity ("gain") of the oscillator's timing elements. By analogy we argue that variations in background drive to the neural timing elements of the CPG could produce larger variations in phase duration in the half-center receiving the lower background drive, i.e., background drive may determine which half-center is dominant. The fact that data from normal cats were also fitted well by the model indicates that sensory input and central drive combine to determine locomotor phase durations. We conclude that there is a considerable flexibility in the control of phase durations in MLR-induced fictive locomotion. We posit that this may be explained by changes in background excitation of neural timing elements in the locomotor CPG.