DUAL MECHANISMS OF TWITCHING DURING SLEEP IN NEONATAL RATS

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.