Division of Neurology, Krembil Neuroscience Centre and Toronto Western Research Institute, University Health Network, University of Toronto, Toronto, Ontario, Canada.
J Neurophysiol. 2011 Feb;105(2):749-56. doi: 10.1152/jn.00640.2010. Epub 2010 Dec 8.
Transcranial magnetic stimulation (TMS) to the primary motor cortex (M1) produces a series of corticospinal descending waves, with a direct (D) wave followed by several indirect (I) waves. TMS inducing posterior-anterior (PA) current in the brain predominantly recruits the early I1-wave, whereas anterior-posterior (AP) directed current preferentially recruits the late I3-wave. However, it is not known whether I-waves elicited by different current directions are mediated by the same neuronal populations. We studied the neuronal mechanisms mediating I-waves by examining the influence of short-latency afferent inhibition (SAI) on various I-waves. SAI was tested with electrical median nerve stimulation at the wrist followed by TMS to the contralateral M1 at different current directions. Surface electromyograms and single motor units were recorded from the first dorsal interosseous muscle. SAI was weaker for the AP compared with that for the PA current direction. With increasing median nerve stimulation intensities, SAI increased for the PA direction but showed a U-shaped relationship for the AP direction. SAI produced more inhibition of late I-waves generated by PA than those generated by AP current direction. We conclude that late I-waves generated by PA and AP current directions are mediated by different neuronal mechanisms.
经颅磁刺激（TMS）作用于初级运动皮层（M1）会产生一系列皮质脊髓下行波，直接（D）波后跟着几个间接（I）波。TMS 在大脑中诱导后前（PA）电流主要募集早期 I1 波，而前向后（AP）方向的电流则优先募集晚期 I3 波。然而，目前尚不清楚不同电流方向引发的 I 波是否由相同的神经元群体介导。我们通过研究短潜伏期传入抑制（SAI）对各种 I 波的影响来研究介导 I 波的神经元机制。用腕部的电刺激正中神经，然后用 TMS 刺激对侧 M1，在不同的电流方向上测试 SAI。表面肌电图和单个运动单位记录自第一背侧骨间肌。与 PA 电流方向相比，AP 方向的 SAI 较弱。随着正中神经刺激强度的增加，PA 方向的 SAI 增加，但 AP 方向的 SAI 呈 U 形关系。与 AP 电流方向相比，PA 电流方向产生的晚期 I 波受到的抑制作用更大。我们的结论是，PA 和 AP 电流方向产生的晚期 I 波由不同的神经元机制介导。
