Plastic Changes in Lumbar Locomotor Networks after a Partial Spinal Cord Injury in Cats

After an incomplete spinal cord injury (SCI), we know that plastic reorganization occurs in supraspinal structures with residual descending tracts. However, our knowledge about spinal plasticity is rather limited. Our recent studies point to changes within the spinal cord below the lesion. After a lateral left hemisection (T10), cats recovered stepping with both hindlimbs within 3 weeks. After a complete section (T13) in these cats, bilateral stepping was seen on the next day, a skill usually acquired after several weeks of treadmill training. This indicates that durable plastic changes occurred below the lesion. However, because sensory feedback entrains the stepping rhythm, it is difficult to reveal central pattern generator (CPG) adaptation. Here, we investigated whether lumbar segments of cats with a chronic hemisection were able to generate fictive locomotion-that is, without phasic sensory feedback as monitored by five muscle nerves in each hindlimb. With a chronic left hemisection, the number of muscle nerves displaying locomotor bursts was larger on the left than on the right. In addition, transmission of cutaneous reflexes was relatively facilitated on the left. Later during the acute experiment, a complete spinalization (T13) was performed and clonidine was injected to induce rhythmic activities. There were still more muscle nerves displaying locomotor bursts on the left. The results demonstrate that spinal networks were indeed modified after a hemisection with a clear asymmetry between left and right in the capacity to generate locomotion. Plastic changes in CPG and reflex transmission below the lesion are thus involved in the stepping recovery after an incomplete SCI.