A caudal spinal source of cervical and forelimb inhibition during sleep

This study sought to determine whether caudal spinal neurons with axons ascending the spinal cord contribute to inhibitory phenomena during sleep. Spinal cords were transected at the eighth thoracic segment, usually under Halothane anesthesia, in seven cats with implanted recording electrodes. After transection, release of forelimb extensor tone (Schiff-Sherrington phenomenon) appeared during wakefulness and was evident when killed several weeks later. A mildly opisthotonic posture with increased tone in forelimb extensors was assumed and maintained as the cats passed from drowsiness into synchronized sleep. They never lay in a normal curled-up posture but with arousal could overcome the extensor posture. This effect largely vanished after 48 hr. Thus ascending fibers play a role in inhibiting antigravity tone in synchronized sleep. Release of epaxial and limb extensor motor neurons during paradoxical sleep could be detected for as long as 1 week after transection. Proximal limb and neck movements occurred more frequently than normally, and the head could even be held off the floor. However, collapses of neck and forelimb extensor tone accompanied rapid eye movements. These results suggest that ascending fibers influence tonic inhibitory mechanisms operating on motor neurons during paradoxical sleep, but not the phasic inhibition associated with rapid eye movements.