Discipline of Physiology, Adelaide Medical School, The University of Adelaide, Adelaide, Australia.
Neuromodulation. 2021 Jul;24(5):813-828. doi: 10.1111/ner.13314. Epub 2020 Dec 9.
The corticospinal volley produced by application of transcranial magnetic stimulation (TMS) over primary motor cortex consists of a number of waves generated by trans-synaptic input from interneuronal circuits. These indirect (I)-waves mediate the sensitivity of TMS to cortical plasticity and intracortical excitability and can be assessed by altering the direction of cortical current induced by TMS. While this methodological approach has been conventionally viewed as preferentially recruiting early or late I-wave inputs from a given populations of neurons, growing evidence suggests recruitment of different neuronal populations, and this would strongly influence interpretation and application of these measures. The aim of this review is therefore to consider the physiological, functional, and clinical evidence for the independence of the neuronal circuits activated by different current directions.
To provide the relevant context, we begin with an overview of TMS methodology, focusing on the different techniques used to quantify I-waves. We then comprehensively review the literature that has used variations in coil orientation to investigate the I-wave circuits, grouping studies based on the neurophysiological, functional, and clinical relevance of their outcomes.
Review of the existing literature reveals significant evidence supporting the idea that varying current direction can recruit different neuronal populations having unique functionally and clinically relevant characteristics.
Further research providing greater characterization of the I-wave circuits activated with different current directions is required. This will facilitate the development of interventions that are able to modulate specific intracortical circuits, which will be an important application of TMS.
经颅磁刺激（TMS）作用于初级运动皮层产生的皮质脊髓束反射由一系列通过中间神经元回路的突触传入产生的波组成。这些间接（I）波介导 TMS 对皮质可塑性和皮质兴奋性的敏感性，可通过改变 TMS 诱导的皮质电流的方向来评估。虽然这种方法传统上被认为优先招募来自特定神经元群体的早期或晚期 I 波输入，但越来越多的证据表明招募了不同的神经元群体，这将强烈影响这些测量的解释和应用。因此，本综述的目的是考虑不同电流方向激活的神经元回路的独立性的生理学、功能和临床证据。
为提供相关背景，我们首先概述 TMS 方法，重点介绍用于量化 I 波的不同技术。然后，我们全面回顾了使用线圈方向变化来研究 I 波回路的文献，根据研究结果的神经生理学、功能和临床相关性对研究进行分组。
对现有文献的回顾表明，有大量证据支持这样一种观点，即改变电流方向可以招募具有独特功能和临床相关特征的不同神经元群体。
需要进一步研究以更好地表征不同电流方向激活的 I 波回路。这将有助于开发能够调节特定皮质内回路的干预措施，这将是 TMS 的一个重要应用。
