Coordination of neck and hindlimb extension during the initiation of locomotion elicited by hypothalamic stimulation

We tested the hypothesis that during the initiation of stepping elicited by hypothalamic stimulation, hindlimb extension was coordinated with head extension in the sagittal plane. Chronic stimulation electrodes (monopolar stainless-steel, 125 microm diameter) were implanted bilaterally into the perifornical hypothalamus of anaesthetized rats (N = 15) under stereotaxic control. Under freely moving and awake conditions, 18 sites which reliably elicited forward locomotion at a latency of approximately 3 s were tested in a videotaping session. The locomotor stimulation was a constant current train of 5 s duration composed of biphasic pulses at 40-50 Hz. The videotape records were digitized at a sampling rate of 6 Hz for seven points on the rat: Nose, pinnae, midpoint of inter-pinnae line, right forepaw, right hindpaw and base of tail. A characteristic pattern of coordinated movements preceded, by approximately 0.5 s, the execution of the first locomotor step. The pattern included a movement of the pelvis in the anterior or superior direction that was produced by hindlimb extension and an extension of the neck forward along the sagittal plane. There was considerable flexibility in this pattern, but it was invariant to the extent that it occurred at a variety of latencies and after several types of head movements. Associated with the coordinated extensions of the neck and hindlimbs was a lowering of the head angle which had a more variable time course. These data indicate that there is significant coupling between the systems that produce hindlimb extension and control head position when the rat prepares to step.