Neuromuscular patterns of stereotypic hindlimb behaviors in the first two postnatal months. II. Stepping in spinal kittens.

From birth to postnatal day 60, neuromuscular patterns for airstepping and treadmill stepping were assessed in kittens spinalized (T12) at birth (Day-1) or at the end of the second postnatal week (Day-14). Within 72 h after spinalization, all kittens displayed stepping motions, but exteroceptive facilitation (e.g. tail pinch) was required to initiate and sustain both behaviors. In Day-14 spinal kittens, the hindlimbs spontaneously and alternately airstepped, but in Day-1 spinal kittens exteroceptive stimulation was usually necessary to evoke airstepping, and the hindlimbs stepped synchronously. Kittens in both groups developed atypical neuromuscular patterns; flexor bursts were nearly twice as long in duration as extensor bursts. Development of bipedal treadmill stepping was similar for Day-1 and Day-14 spinal kittens, but differed from that for normal kittens. Tested at the same belt speeds, stepping was more easily elicited in spinal kittens, bouts of repetitive stepping were longer, and cycle periods were shorter than in normal kittens until postnatal week 6. Spinal kittens, however, seldom exhibited adequate weight support during hindlimbs stepping, and the neuromuscular patterns associated with bipedal stepping were atypical. For spinal kittens, the relationship between the extensor burst duration and the cycle period was reduced substantially, and flexor activity was initiated earlier in the step cycle and was longer in duration than that for normal kittens. These atypical intralimb synergies may have been the consequence of altered lumbosacral circuits produced by the spinal transection. It is also possible that these spinal circuits, lacking rostral input, were particularly susceptible to abnormal motion-dependent feedback resulting from reduced hindlimb weight support.