Plasticity of the human motor system following muscle reconstruction: a magnetic stimulation and functional magnetic resonance imaging study.

OBJECTIVE: Although motor system plasticity in response to neuromuscular injury has been documented, few studies have examined recovered and functioning muscles in the human. We examined brain changes in a group of patients who had a muscle transfer.
METHODS: Transcranial magnetic stimulation (TMS) was used to study a unique group of 9 patients who had upper extremity motor function restored using microneurovascular transfer of the gracilis muscle. The findings from the reconstructed muscle were compared to the homologous muscle of the intact arm. One patient was also studied with functional magnetic resonance imaging (fMRI).
RESULTS: TMS showed that the motor threshold and short interval intracortical inhibition was reduced on the transplanted side while at rest but not during muscle activation. The difference in motor threshold decreased with the time since surgery. TMS mapping showed no significant difference in the location and size of the representation of the reconstructed muscle in the motor cortex compared to the intact side. In one patient with reconstructed biceps muscle innervated by the intercostal nerves, both TMS mapping and fMRI showed that the upper limb area rather than the trunk area of the motor cortex controlled the reconstructed muscle.
CONCLUSIONS: Plasticity occurs in cortical areas projecting to functionally relevant muscles. Changes in the neuronal level are not necessarily accompanied by changes in motor representation. Brain reorganization may involve multiple processes mediated by different mechanisms and continues to evolve long after the initial injury. SIGNIFICANCE: Central nervous system plasticity following neuromuscular injury may have functional relevance.