Background

Pulses of transcranial magnetic stimulation (TMS) with a predominantly anterior-posterior (AP) or posterior-anterior (PA) current direction over the primary motor cortex appear to activate distinct excitatory inputs to corticospinal neurons. In contrast, very few reports have examined whether the inhibitory neurons responsible for short-interval intracortical inhibition (SICI) are sensitive to TMS current direction.

Objectives

To investigate whether SICI evaluated with AP and PA conditioning stimuli (CS_PA and CS_AP) activate different inhibitory pathways. SICI was always assessed using a PA-oriented test stimulus (TS_PA).

Methods

Using two superimposed TMS coils, CS_PA and CS_AP were applied at interstimulus intervals (ISI) of 1–5 ms before a TS_PA, and at a range of different intensities. Using a triple stimulation design, we then tested whether SICI at ISI of 3 ms using opposite directions of CS (SICI_CSPA3 and SICI_CSAP3) interacted differently with three other forms of inhibition, including SICI at ISI of 2 ms (SICI_CSPA2), cerebellum-motor cortex inhibition (CBI 5 ms) and short-latency afferent inhibition (SAI 22 ms). Finally, we compared the effect of tonic and phasic voluntary contraction on SICI_CSPA3 and SICI_CSAP3.

Results

CS_AP produced little SICI at ISIs = 1 and 2 ms. However, at ISI = 3 ms, both CS_AP and CS_PA were equally effective at the same percent of maximum stimulator output. Despite this apparent similarity, combining SICI_CSPA3 or SICI_CSAP3 with other forms of inhibition led to quite different results: SICI_CSPA3 interacted in complex ways with CBI, SAI and SICI_CSPA2, whereas the effect of SICI_CSAP3 appeared to be quite independent of them. Although SICI_CSPA and SICI_CSAP were both reduced by the same amount during voluntary tonic contraction compared with rest, in a simple reaction time task SICI_CSAP was disinhibited much earlier following the imperative signal than SICI_CSPA.

Conclusions

SICI_CSPA appears to activate a different inhibitory pathway to that activated by SICI_CSAP. The difference is behaviourally relevant since the pathways are controlled differently during volitional contraction. The results may explain some previous pathological data and open the possibility of testing whether these pathways are differentially recruited in a range of tasks.

中文翻译：

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人类运动皮层中两种形式的短间隔皮质内抑制

背景

经颅磁刺激 (TMS) 脉冲在初级运动皮层上具有主要的前-后 ( AP ) 或后-前 (PA) 电流方向，似乎激活了对皮质脊髓神经元的不同兴奋性输入。相比之下，很少有报告检查负责短间隔皮质内抑制 (SICI) 的抑制性神经元是否对 TMS 电流方向敏感。

目标

研究用AP和 PA 调节刺激（CS _PA和 CS _AP）评估的 SICI 是否激活不同的抑制途径。SICI 总是使用面向 PA 的测试刺激 (TS _PA )进行评估。

方法

使用两个叠加的 TMS 线圈，在 TS _PA之前以 1–5 ms 的刺激间隔 (ISI)和不同强度范围应用CS _PA和 CS _AP。使用三重刺激设计，然后我们测试了使用相反方向的 CS（SICI _CSPA3和 SICI _CSAP3）在 ISI 为 3 ms 的SICI_是否与其他三种形式的抑制相互作用不同，包括在 ISI 为 2 ms 的 SICI（SICI _CSPA2）、小脑-运动皮层抑制（CBI 5 ms）和短延迟传入抑制（SAI 22 ms）。最后，我们比较了强直和阶段性自主收缩对 SICI _CSPA3和 SICI _{CSAP3 的影响}。

结果

CS _AP在 ISI = 1 和 2 ms 时产生很少的 SICI。然而，在 ISI = 3 ms 时，CS _AP和 CS _PA在最大刺激器输出的相同百分比下同样有效。尽管这样明显的相似性，SICI组合_CSPA3或SICI _CSAP3与其他形式的抑制导致了相当不同的结果：SICI _CSPA3相互作用与CBI，SAI和SICI复杂的方式_CSPA2，而SICI的效果_CSAP3显得相当独立它们。虽然 SICI _CSPA和 SICI _CSAP与休息相比，在自愿强直收缩期间两者都减少了相同的数量，在一个简单的反应时间任务中，SICI _CSAP在命令信号之后比 SICI _CSPA更早地被解除抑制。

结论

SICI _CSPA似乎激活了一种与 SICI _CSAP激活的抑制途径不同的抑制途径。这种差异在行为上是相关的，因为在意志收缩期间这些途径受到不同的控制。结果可能解释了之前的一些病理数据，并开启了测试这些途径是否在一系列任务中被不同招募的可能性。