Fetal transplants rescue axial muscle representations in M1 cortex of neonatally transected rats that develop weight support

Intraspinal trans plants of fetal spinal tissue partly alleviate motor defi cits caused by spinal cord injury. How transplants modify body representati on and muscle recruitment by motor cortex is currently largely unknown. We compared electromyographic responses from motor cortex stimulation in norma l adult rats, adult rats that received complete spinal cord transection at the T-8-T-10 segmental level as neonates (TX rats), and similarly transecte d rats receiving transplants of embryonic spinal cord (TP rats). Rats were also compared among treatments for level of weight support and motor perfor mance. Sixty percent of TP rats showed unassisted weight-supported locomoti on as adults, whereas similar to 30% of TX rats with no intervention showed unassisted weight-supported locomotion. In the weight-supporting animals w e found that the transplants enabled motor responses to be evoked by micros timulation of areas of motor cortex that normally represent the lumbar axia l muscles in rats. These same regions were silent in all TX rats with trans ections but no transplants, even those exhibiting locomotion with weight su pport. In weight-supporting TX rats low axial muscles could be recruited fr om 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 repr esentation in cortex. The data demonstrate that spinal transplants allow th e 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 indirectl y rather than use explicit hindlimb control during weight-supported locomot ion.