Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Amager and Hvidovre, Hvidovre, Denmark.
Division of Physical and Health Education, Graduate School of Education, The University of Tokyo, Bunkyo City, Tokyo, Japan.
J Neurophysiol. 2023 Feb 1;129(2):410-420. doi: 10.1152/jn.00263.2022. Epub 2023 Jan 11.
Single-pulse transcranial magnetic stimulation (TMS) of the precentral hand representation (M1) can elicit indirect waves in the corticospinal tract at a periodicity of ∼660 Hz, called I-waves. These descending volleys are produced by transsynaptic excitation of fast-conducting corticospinal axons in M1. Paired-pulse TMS can induce short-interval intracortical facilitation (SICF) of motor evoked potentials (MEPs) at interpulse intervals that match I-wave periodicity. This study examined whether short-latency corticospinal facilitation engages additional mechanisms independently of I-wave periodicity. In 19 volunteers, one to four biphasic TMS pulses were applied to left M1 with interpulse intervals adjusted to the first peak or trough of the individual SICF curve at different intensities to probe the intensity-response relationship. Multipulse TMS at individual peak latency facilitated MEP amplitudes and reduced resting motor threshold (RMT) compared with single pulses. Multipulse TMS at individual trough latency also produced a consistent facilitation of MEPs and a reduction of RMT. Short-latency facilitation at trough latency was less pronounced, but the relative difference in facilitation decreased with increasing stimulus intensity. Increasing the pulse number had only a modest effect. Two mechanisms underlie short-latency facilitation caused by biphasic multipulse TMS. One intracortical mechanism is related to I-wave periodicity and engages fast-conducting direct projections to spinal motoneurons. A second corticospinal mechanism does not rely on I-wave rhythmicity and may be mediated by slower-conducting indirect pyramidal tract projections from M1 to spinal interneurons. The latter mechanism deserves more attention in studies of the corticomotor system and its link to manual motor control using the MEP. TMS pairs evoke SICF at interpulse intervals (IPIs) that match I-wave periodicity. Biphasic bursts with IPIs at the latency of the first peak facilitate MEPs and reduce corticomotor threshold. Bursts at the latency of the first trough facilitate MEPs and reduce corticomotor threshold to a lesser extent. TMS bursts facilitate corticomotor excitability via two mechanisms: SICF-dependently via fast-conducting direct projections from M1 to spinal motoneurons and SICF-independently, probably through slower-conducting indirect pyramidal tract projections.
单次经颅磁刺激（TMS）于中央前手部代表区（M1）可诱发皮质脊髓束中约 660 Hz 的周期性的间接波，称为 I 波。这些下行的爆发是通过 M1 中快速传导的皮质脊髓轴突的突触后兴奋产生的。成对的 TMS 脉冲可以在与 I 波周期性匹配的脉冲间隔内诱导运动诱发电位（MEP）的短潜伏期皮质脊髓易化（SICF）。本研究探讨了短潜伏期皮质脊髓易化是否独立于 I 波周期性而涉及其他机制。在 19 名志愿者中，用不同强度的单个双相 TMS 脉冲以个体 SICF 曲线的第一峰或波谷的脉冲间隔施加于左侧 M1，以探测强度-反应关系。与单个脉冲相比，个体峰潜伏期的多脉冲 TMS 增加了 MEP 幅度并降低了静息运动阈值（RMT）。个体波谷潜伏期的多脉冲 TMS 也产生了一致的 MEP 易化和 RMT 的降低。与波谷潜伏期的短潜伏期易化相比，这种易化作用不太明显，但随着刺激强度的增加，易化的相对差异减小。增加脉冲数的效果也只是适度的。双相多脉冲 TMS 引起的短潜伏期易化有两种机制。一种是皮质内机制，与 I 波周期性有关，涉及快速传导的直接投射到脊髓运动神经元。第二种皮质脊髓机制不依赖于 I 波的节律性，可能是由 M1 到脊髓中间神经元的较慢传导的间接锥体束投射介导的。在后一种机制中，在使用 MEP 研究皮质运动系统及其与手动运动控制的联系时，应该给予更多的关注。TMS 对在与 I 波周期性匹配的脉冲间隔（IPI）处诱发 SICF。在第一个峰的潜伏期处具有 IPI 的双相爆发促进 MEP 并降低皮质运动阈值。在第一个波谷的潜伏期处的爆发在较小程度上促进 MEP 并降低皮质运动阈值。TMS 爆发通过两种机制促进皮质运动兴奋性：SICF 依赖性通过 M1 到脊髓运动神经元的快速传导的直接投射，以及 SICF 独立性，可能通过较慢传导的间接锥体束投射。
