The neural control of cyclic motor activity in the fetal rat (Rattus norvegicus).

The spontaneous behavior of rat fetuses (in a saline bath with fetal-placental-uterine connections intact) was observed directly for 30 minutes on Day 20 of gestation. Rearleg and nonrearleg movements from fetuses with a mid-thoracic spinal cord transection or sham operation were analyzed for cyclic organization. Oscillations in rearleg activity occurred at the same frequency in fetuses with spinal cord transections (0.74 cycle/min) and sham-operated fetuses (0.72 cycle/min). However, oscillations in nonrearleg activity were much slower in the fetuses with spinal cord transections (0.30 vs. 0.77 cycle/min). Other characteristics of the cyclic patterns in motor activity were unaffected. The findings demonstrate 1) the caudal half of the spinal cord can generate cyclic output in the absence of descending input from the brain, 2) there is no single timing center, and 3) rostral sources are slower.