Hindlimb stepping movements in complete spinal rats induced by epidural spinal cord stimulation

The locomotor ability of the spinal cord of adult rats deprived of brain control was tested by epidural spinal cord stimulation. The studies were performed on six rats that had a complete spinal cord transection (T7-T9) and epidural electrode implantations 2-3 weeks before testing was initiated. The stimulating epidural electrodes were implanted at the T12-L6 spinal segments. Epidural electrical stimulation of the dorsal surface of the spinal cord at frequencies between 1 and 50 Hz and intensities between 1 and 10 V without any pharmacological facilitation was used. Stimulation at each of the lumbar spinal cord segments elicited some rhythmic activity in the hindlimbs. However, stimulation at most segmental levels usually evoked activity in only one leg and was maintained for short periods of time (< 10s). Bilateral hindlimb locomotor activity was evoked most often with epidural stimulation at 40-50 Hz applied at the L2 segment. A necessary condition for initiation of locomotor activity was providing a specific amount (at least 5%) of body weight support. Therefore, the rat spinal cord isolated from brain control is capable of producing bilateral stepping patterns induced most readily by epidural stimulation applied at the L2 spinal segment. Furthermore, the induced stepping patterns were dependent on sensory feedback associated with weight bearing.