Fetal Transplants Rescue Axial Muscle Representations in M1 Cortex of Neonatally Transected Rats That Develop Weight Support

Fetal transplants rescue axial muscle representations in M1 cortex of neonatally transected rats that develop weight support. J. Neurophysiol. 80: 3021-3030, 1998. Intraspinal transplants of fetal spinal tissue partly alleviate motor deficits caused by spinal cord injury. How transplants modify body representation and muscle recruitment by motor cortex is currently largely unknown. We compared electromyographic responses from motor cortex stimulation in normal adult rats, adult rats that received complete spinal cord transection at the T8-T10 segmental level as neonates (TX rats), and similarly transected rats receiving transplants of embryonic spinal cord (TP rats). Rats were also compared among treatments for level of weight support and motor performance. Sixty percent of TP rats showed unassisted weight-supported locomotion as adults, whereas approximately 30% of TX rats with no intervention showed unassisted weight-supported locomotion. In the weight-supporting animals we found that the transplants enabled motor responses to be evoked by microstimulation of areas of motor cortex that normally represent the lumbar axial muscles in rats. These same regions were silent in all TX rats with transections but no transplants, even those exhibiting locomotion with weight support. In weight-supporting TX rats low axial muscles could be recruited from the rostral cortical axial representation, which normally represents the neck and upper trunk. No operated animal, even those with well-integrated transplants and good weight-supported locomotion, had a hindlimb motor representation in cortex. The data demonstrate that spinal transplants allow the development of some functional interactions between areas of motor cortex and spinal cord that are not available to the rat lacking the intervention. The data also suggest that operated rats that achieve weight support may primarily use the axial muscles to steer the pelvis and hindlimbs indirectly rather than use explicit hindlimb control during weight-supported locomotion.