Interactions from both inhibitory and excitatory interneurons are necessary components of cortical processing that contribute to the vast amount of motor actions executed by humans daily. As transcranial magnetic stimulation (TMS) over primary motor cortex is capable of activating corticospinal neurons trans-synaptically, studies over the past 30 years have provided how subtle changes in stimulation parameters (i.e., current direction, pulse width, and paired-pulse) can elucidate evidence for two distinct neuronal networks that can be probed with this technique. This article provides a brief review of some fundamental studies demonstrating how these networks have separable excitatory inputs to corticospinal neurons. Furthermore, the findings of recent investigations will be discussed in detail, illustrating how each network’s sensitivity to different brain states (i.e., rest, movement preparation, and motor learning) is dissociable. Understanding the physiological characteristics of each network can help to explain why interindividual responses to TMS exist, while also providing insights into the role of these networks in various human motor behaviors.

来自抑制性和兴奋性中间神经元的相互作用是皮层处理的必要组成部分，有助于人类每天执行的大量运动动作。由于初级运动皮层上的经颅磁刺激 (TMS) 能够跨突触激活皮质脊髓神经元，因此过去 30 年的研究提供了刺激参数（即电流方向、脉冲宽度和成对脉冲）的细微变化如何可以阐明可以用这种技术探测的两个不同神经元网络的证据。本文简要回顾了一些基础研究，展示了这些网络如何对皮质脊髓神经元产生可分离的兴奋性输入。此外，将详细讨论最近的调查结果，说明每个网络对不同大脑状态（即休息、运动准备和运动学习）的敏感性是如何分离的。了解每个网络的生理特征有助于解释为什么存在个体间对 TMS 的反应，同时还可以深入了解这些网络在各种人类运动行为中的作用。