Supraspinal influence on the development of motor behavior in the fetal lamb

To examine the involvement of supraspinal inputs in the maturation of motor activity patterns in the developing fetal lamb, we recorded spontaneous electromyographic activity from spinally innervated muscles at approximately 45, 65, and 95 days gestation (G45, G65, and G95; term = 147 days). At G45, fetal activity occurred in synchronized activity-inactivity cycles of approximately 2 min duration, with the activity phase lasting 22.2 +/- 4.8 s and the inactivity phase lasting 95.4 +/- 13.3 s (mean +/- standard error of the mean, n = 5). At G65 and G95, the organization of activity was clearly different from that at G45 in that it was no longer cyclic, nor was the discharge of different muscles synchronized. By contrast, after spinal cord transection at G62, synchronised cyclic activity occurred in muscles innervated by segmental levels below the transection, both at G65 and G95. At G65 the duration of the activity phase of the cycle was 53.5 +/- 6.0 s, while the inactivity phase lasted 171.6 +/- 22.1 s; these durations did not alter between G65 and G95. Since spinal cord transection leads to the motor behavior of the G65 fetus reverting to the cyclic pattern characteristic of the G45 fetus, we conclude that supraspinal inputs begin to modulate the output of the spinal pattern generators at some stage between G45 and G65. The observation that spinally transected fetuses generate identical behavior at G65 and G95, both in terms of its cyclic character and the duration of cycles, suggests that spinal circuits undergo little autonomous development over this period; that is, the altered behavior observed in the developing intact fetus reflects the influence of supraspinal inputs on the motor circuits of the spinal cord.