Forelimb and hindlimb stepping by the anesthetized rat elicited by electrical stimulation of the pons and medulla.

This study determined the lower brainstem sites at which electrical stimulation elicits stepping movements of the forelimbs and hindlimbs. Rats (N = 45), anesthetized with nembutal, were fixed in a stereotaxic apparatus so that their limbs contacted a moving treadmill belt. Electrical stimulation (100 microA, 10-sec trains, 0.5-msec cathodal pulses, 50-Hz pulse frequency) was applied every 200 micron through 173 movable electrodes. Well coordinated quadrupedal stepping was elicited by stimulation at dorsal posterior mesencephalic sites including the inferior collicular commissure, the central gray, the nucleus cuneiformis and lateral aspects of the pedunculopontine tegmental nucleus. Caudal and ventral to this general region, sites supporting quadrupedal stepping appeared mainly in or near the spinal trigeminal nucleus. Stepping with only the forelimb and hindlimb contralateral to the stimulation site was associated with the corticospinal tract, the lateral pontis oralis, the lateral pontis caudalis and the ventral reticular nucleus of the medulla. Bilateral forelimb stepping was associated with the trigeminal system and the gigantocellular reticular nucleus. At the level of the rostral medulla, systems involved in bilateral forelimb stepping and contralateral hindlimb stepping appear to be located medially. Systems concerned with bilateral hindlimb stepping appear to be located laterally.