Locomotor plasticity after spinal injury in the chick

Functional recovery after spinal cord injury likely depends, in part, on the reorganization of undamaged spinal circuitry. Segmental afferent input from the limbs remains largely intact after spinal injury and may provide an important source of activation and regulation of the spinal circuits that have lost descending input as a result of the injury. This purpose of this study was to investigate the contribution of cutaneous afferent inputs to the recovery of motor function after spinal injury in the chick. After lateral thoracic spinal hemisection, the motion of the ipsilateral limb was impaired during both walking and swimming. By 2 weeks postoperatively, limb motion recovered to preoperative values for walking but not for swimming. It was hypothesized that phasic afferent inputs experienced during walking, but not swimming, contributed to recovery of limb motion during walking. When a source of phasic cutaneous input was provided during swim training sessions, limb motion gradually improved to preoperative values. After 2 weeks of training, this improved motion was retained even after the source of cutaneous stimulation was removed. The proposed mechanism is an experience-dependent strengthening of the circuits activated during the improved limb motion, leading to a permanent change in limb action during swimming. Thus, the afferent inputs experienced during movement repetition are important during the acquisition of learned movements after spinal injury. These results are discussed in terms of behavioral, physiological, and anatomical evidence for spinal plasticity in other species. It is concluded that the spinal cord has significant plastic capabilities, and efforts should be directed toward maximizing the contribution of this plasticity to functional recovery after spinal cord injury.