McKay W B, Tuel S M, Sherwood A M, Stokić D S, Dimitrijević M R
Baylor College of Medicine, Division of Restorative Neurology and Human Neurobiology, Houston, TX 77030, USA.
Exp Brain Res. 1995;105(2):276-82. doi: 10.1007/BF00240963.
Motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (TES) of the motor cortex were recorded in separate sessions to assess changes in motor cortex excitability after a fatiguing isometric maximal voluntary contraction (MVC) of the right ankle dorsal flexor muscles. Five healthy male subjects, aged 37.4 +/- 4.2 years (mean +/- SE), were seated in a chair equipped with a load cell to measure dorsiflexion force. TMS or TES was delivered over the scalp vertex before and after a fatiguing MVC, which was maintained until force decreased by 50%. MEPs were recorded by surface electrodes placed over quadriceps, hamstrings, tibialis anterior (TA), and soleus muscles bilaterally. M-waves were elicited from the exercised TA by supramaximal electrical stimulation of the peroneal nerve. H-reflex and MVC recovery after fatiguing, sustained MVC were also studied independently in additional sessions. TMS-induced MEPs were significantly reduced for 20 min following MVC, but only in the exercised TA muscle. Comparing TMS and TES mean MEP amplitudes, we found that, over the first 5 min following the fatiguing MVC, they were decreased by about 55% for each. M-wave responses were unchanged. H-reflex amplitude and MVC force recovered within the 1st min following the fatiguing MVC. When neuromuscular fatigue was induced by tetanic motor point stimulation of the TA, TMS-induced MEP amplitudes remained unchanged. These findings suggest that the observed decrease in MEP amplitude represents a focal reduction of cortical excitability following a fatiguing motor task and may be caused by intracortical and/or subcortical inhibitory mechanisms.
在单独的实验环节中记录经颅磁刺激（TMS）和经颅电刺激（TES）运动皮层所诱发的运动诱发电位（MEP），以评估右踝背屈肌进行疲劳性等长最大自主收缩（MVC）后运动皮层兴奋性的变化。五名健康男性受试者，年龄37.4±4.2岁（均值±标准误），坐在配备有测力传感器的椅子上以测量背屈力。在疲劳性MVC前后，于头皮顶点施加TMS或TES，该疲劳性MVC持续至力量下降50%。通过双侧置于股四头肌、腘绳肌、胫前肌（TA）和比目鱼肌上的表面电极记录MEP。通过对腓总神经进行超强电刺激从受锻炼的TA引出M波。在另外的实验环节中还分别独立研究了疲劳性、持续性MVC后的H反射和MVC恢复情况。MVC后20分钟内，TMS诱发的MEP显著降低，但仅在受锻炼的TA肌肉中。比较TMS和TES的平均MEP波幅，我们发现，在疲劳性MVC后的前5分钟内，它们各自下降了约55%。M波反应未改变。H反射波幅和MVC力量在疲劳性MVC后的第1分钟内恢复。当通过对TA进行强直肌点刺激诱发神经肌肉疲劳时，TMS诱发的MEP波幅保持不变。这些发现表明，观察到的MEP波幅降低代表了疲劳性运动任务后皮层兴奋性的局灶性降低，可能是由皮质内和/或皮质下抑制机制引起的。
