Modulation of a Hoffmann (H)-reflex following transcranial magnetic stimulation (TMS) has been used to assess the nature of signals transmitted from cortical centers to lower motor neurons. Further characterizing the recruitment and time-course of the TMS-induced effect onto the soleus H-reflex adds to the discussion of these pathways and may improve its utility in clinical studies. In 10 healthy controls, TMS was used to condition the soleus H-reflex using TMS intensities from 65 to 110% of the resting motor threshold (RMT). Early facilitation [- 5 to - 3 ms condition-test (C-T) interval] was evident when TMS was 110% of RMT (P < 0.05). By comparison, late facilitation (+ 10 to + 20 ms C-T interval) was several times larger and observed over a wider range of TMS intensities, including 65-110% of RMT. The early inhibition (- 3 to - 1 ms C-T interval) had a low TMS threshold and was elicited over a wide range of intensity from 65% to 95% of RMT (all P < 0.05). A second inhibitory phase was seen ~ 4 ms later (+ 1 to + 4 ms C-T intervals) and was only observed for a TMS intensity of 95% of RMT (P < 0.05). The present findings reaffirm that subthreshold TMS strongly modulates soleus motor neurons and demonstrates that distinct pathways can be selectively probed at discrete C-T intervals when using specific TMS intensities.

中文翻译：

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表征低强度经颅磁刺激对静息时比目鱼肌 H 反射的影响。

经颅磁刺激 (TMS) 后霍夫曼 (H) 反射的调制已被用于评估从皮质中心传输到下运动神经元的信号的性质。进一步表征 TMS 诱导作用对比目鱼 H 反射的招募和时间过程增加了对这些途径的讨论，并可能提高其在临床研究中的效用。在 10 个健康对照中，TMS 用于调节比目鱼肌 H 反射，使用 TMS 强度从 65% 到 110% 的静息运动阈值 (RMT)。当 TMS 是 RMT 的 110% (P < 0.05) 时，早期促进 [- 5 到 - 3 ms 条件测试 (CT) 间隔] 很明显。相比之下，晚期易化（+ 10 到 + 20 ms CT 间隔）要大几倍，并且在更广泛的 TMS 强度范围内观察到，包括 65-110% 的 RMT。早期抑制（- 3 到 - 1 ms CT 间隔）具有较低的 TMS 阈值，并且在从 65% 到 95% RMT 的广泛强度范围内引发（所有 P < 0.05）。约 4 ms 后（+ 1 至 + 4 ms CT 间隔）出现第二个抑制阶段，仅在 TMS 强度为 RMT 的 95% 时观察到（P < 0.05）。目前的研究结果重申，阈下 TMS 强烈调节比目鱼肌运动神经元，并表明当使用特定的 TMS 强度时，可以在离散的 CT 间隔选择性地探测不同的通路。