This diagram depicts intracortical neuronal populations within the primary motor cortices that experience excitability changes after stroke. Black neurons reflect inhibitory influences and white neurons represent excitatory populations. A) Diagram of M1s interactions in the healthy brain that result in modulation of excitability in pyramidal tract (PT) neurons as tested by TMS. B) Changes in activity of these networks after stroke. Ipsilesional short interval intracortical inhibition (SICI) within M1 is decreased compared to the contralesional M1. At movement onset, interhemispheric inhibition at 10ms inter-stimulus intervals (IHI₁₀) from contralesional to ipsilesional M1 is greater in the stroke brain than in the healthy brain. Whether this change in IHI₁₀ is due to an increase in the transcallosal glutamatergic elements or ipsilesional inhibitory networks, and how IHI₁₀ interacts with SICI in the stroke brain have yet to be elucidated.