Increase in flexor but not extensor corticospinal motor outputs following ischemic nerve

27 The human motor cortex is capable of rapid and long-lasting reorganization. This 28 reorganization is evident globally, in the form of shifts in body part representations, and at 29 the level of individual muscles in the form of changes in corticospinal excitability. 30 Representational shifts provide an overview of how various body parts reorganize relative to 31 each other but do not tell us whether all muscles in a given body part reorganize in the same 32 manner and to the same extent. Transcranial magnetic stimulation (TMS) provides 33 information about individual muscles and can therefore inform us about the uniformity of 34 plastic changes within a body part. Here, we used TMS to investigate changes in 35 corticospinal excitability of forearm flexors and extensors after inflation of a tourniquet 36 around the wrist. Motor evoked potential (MEP) amplitudes and input/output (I/O) curves 37 were obtained from wrist flexors and extensors simultaneously before and during block. TMS 38 was delivered to the optimal site for eliciting MEPs in the flexors in Experiment 1, the 39 extensors in Experiment 2, and both the flexors and extensors in Experiment 3. In all 40 experiments flexor MEP amplitude increased during block while extensor MEP amplitude 41 showed no systematic change, and the slope of flexor but not extensor I/O curves increased. 42 Flexor H-reflex amplitude normalised to the maximal M-wave showed negligible changes 43 during block, suggesting that the increase in corticospinal excitability in the flexors cannot be 44 completely explained by increased excitability at the level of the spinal cord. These findings 45 show that forearm flexors and extensors differ in their potential for plastic changes, and 46 highlight the importance of investigating how experimentally-induced plasticity affects 47 anatomically close, but functionally distinct, muscle groups. It also suggests that 48 rehabilitation interventions that aim to alter cortical organization should consider the 49 differential sensitivity of various muscle groups to plasticity processes. 50