Twitches of the limbs during REM sleep in adult mammals result from de
scending motor activation from the brainstem. In contrast, many sponta
neous movements in embryos appear similar to REM-related twitches and
result from the local firing of spinal motor neurons. To determine whi
ch mechanism produces twitches in neonates, we analyzed twitching in 5
- and 8-day-old rat pups that had spinal cords transected in the lower
thoracic region. This transection separated motor units controlling f
orelimb movements from motor units controlling hindlimb movements. Spi
nal transection did not significantly affect the amount of forelimb tw
itching. In contrast, the amount of hindlimb twitching in transected p
ups was reduced by only 35%-50%. Given that hindlimb twitching was not
eliminated by spinal transection, it is concluded that there are 2 in
dependent mechanisms producing twitches at these ages.