Neuromuscular patterns of stereotypic hindlimb behaviors in the first two postnatal months. III. Scratching and the paw-shake response in kittens.

Neuromuscular patterns of scratching and the paw-shake response were studied in normal kittens from birth to postnatal day 60. Onset of both behaviors coincided with the development of secure weight-bearing posture and occurred on postnatal day 21 for scratching and postnatal day 26 for paw shaking. At onset, cycle periods for scratching (5-6 Hz) and paw shaking (8-10 Hz) were similar to that for adult cats, and EMG patterns were adult-like. The scratch cycle consisted of reciprocal flexor and extensor bursts of equal duration, while the shake cycle consisted of coactive knee extensor and ankle flexor bursts alternately active with ankle extensor bursts. The lack of scratching and paw shaking during the first 3 postnatal weeks and the adult-like EMG patterns at onset are consistent with the hypothesis that pattern-generating circuits within lumbosacral segments are available early in development but inhibited by the rostral neuraxis until postural control is sufficient to accommodate the response. To eliminate rostral inputs, including descending input critical for postural control, kittens were spinalized at the T12 level, and onset of paw shaking was accelerated. In kittens spinalized at birth, paw-shake onset occurred on postnatal day 14, while in kittens spinalized on postnatal day 14, onset occurred 48 h after spinalization. In all spinal kittens, however, knee extensor activity was disrupted and not normal by postnatal day 60. Mature neuromuscular patterns for scratching and paw shaking are available at onset of the behavior during normal development. Spinalization hastens the onset of paw shaking but the normal neuromuscular synergy is disrupted as well as the temporal structure of the multi-cycle response. Disruptions following spinalization may be due to altered development of spinal pattern generators or aberrant feedback from atypical hindlimb motions due to a retardation of hindlimb growth and an alteration of muscle contractile properties in spinal kittens.