Cortical control of human soleus muscle during volitional and postural activities studied using focal magnetic stimulation
Cortical control of human soleus muscle during volitional and postural activities studied using focal magnetic stimulation
复制标题
使用聚焦磁刺激研究意志和姿势活动期间人类比目鱼肌的皮质控制
作者：
B. Lavoie;Frederick W.J. Cody;C. Capaday
在健康人类受试者中研究了运动皮层的局灶性经颅磁刺激在踝关节肌肉组织中引起的表面记录肌电图 (EMG) 反应。这种比目鱼肌诱发运动反应（EMR）的特征在于广泛的运动活动背景水平和使用不同的刺激强度。主要在 (1) 意志任务和 (2) 姿势任务期间记录 EMR。在前一项任务中，受试者坐着，并根据肌电图的视觉监测仪自愿产生规定水平的比目鱼肌激活。在后一项任务中，受试者在没有肌电图反馈的情况下采取站立姿势。在这些任务期间，比目鱼肌（传统上被认为是缓慢的反重力伸肌）的 EMR 比较被用来评估其在主要意志活动与主要姿势活动中的皮层控制。单一磁皮质刺激产生的比目鱼肌 EMR 形式包括 EMG 的初始（约 30 毫秒）增加和随后（约 50 毫秒）的降低。皮质刺激可引起显着的兴奋性比目鱼肌肌电图反应；例如，当受试者完全自主地进行跖屈肌力时，由轻微的磁刺激（在放松的肌肉中诱发反应的阈值为 125%）引起的反应平均几乎是最大 M 波的 20%，而最大 M 波可以通过对胫后神经的电刺激引起。大多数受试者在坐着时，自愿放松的比目鱼肌会引发兴奋性 EMR。由放松状态阈值刺激或大约 125% 阈值强度引起的比目鱼肌反应幅度，在较宽的意志收缩水平范围内随背景肌电图近似线性增加。相比之下，随着自愿努力的增加，兴奋性比目鱼肌 EMR 的潜伏期没有发生系统性变化。通过磁刺激在坐位受试者自愿放松的比目鱼肌中引起的兴奋反应，通常通过对侧踝伸肌的渐进式自愿收缩来促进。然而，当受试者自愿激活引发 EMR 的肌肉时，并没有观察到这种反应的促进作用。在姿势任务期间，磁刺激在比目鱼肌中引起的反应模式通常与意志任务期间发现的相似。当受试者安静地站立、前倾或用脚趾站立以产生不同程度的踝伸肌收缩时，在给定的刺激强度下获得的兴奋性比目鱼肌EMR的振幅随着背景EMG的增加而增加。总体而言，在匹配刺激强度的个体受试者中观察到的皮质诱发比目鱼肌反应大小与运动活动强直水平之间的关系，在姿势任务和意志任务之间并没有一致的差异。目前的结果表明，运动皮层可能能够对比目鱼肌以及其他踝伸肌进行快速调节，不仅当肌肉参与意志任务时，而且当它参与姿势维持时。
The surface-recorded electromyographic (EMG) responses evoked in the ankle musculature by focal, transcranial, magnetic stimulation of the motor cortex were studied in healthy human subjects. Such soleus evoked motor responses (EMRs) were characterised over a wide range of background levels of motor activity and using different stimulus intensities. EMRs were recorded during predominantly (1) volitional and (2) postural tasks. In the former task subjects were seated and voluntarily produced prescribed levels of soleus activation by reference to a visual monitor of EMG. In the latter task subjects assumed standing postures without EMG feedback. Comparison of the EMRs of soleus, traditionally considered a slow anti-gravity extensor muscle, during these tasks was used to evaluate its cortical control in primarily volitional versus primarily postural activities. The form of soleus EMRs produced by single magnetic cortical stimuli comprised an initial (approx. 30 ms) increase and subsequent (approx. 50 ms) depression of EMG. Cortical stimulation could elicit substantial excitatory soleus EMG responses; for example, responses evoked by mild, magnetic stimuli (125% threshold for inducing a response in the relaxed muscle) as subjects exerted full voluntary plantarflexor effort averaged almost 20% of the maximum M-wave which could be elicited by an electrical stimulus to the posterior tibial nerve. Excitatory EMRs could be elicited in the voluntarily relaxed soleus muscle of the majority of subjects during sitting. The amplitude of soleus responses, induced by threshold stimuli for the relaxed state or approximately 125% threshold intensity, increased approximately linearly with background EMG over a wide range of volitional contraction levels. By contrast, there was no systematic change in the latency of excitatory soleus EMRs with increasing voluntary effort. The excitatory responses evoked in the voluntarily relaxed soleus of seated subjects by magnetic stimulation were regularly facilitated by incremental, voluntary contraction of the contralateral ankle extensors in a graded manner. However, such facilitation of responses was not observed when subjects voluntarily activated the muscle in which EMRs were elicited. The pattern of the responses elicited in soleus by magnetic stimulation during the postural task generally resembled that found during the volitional task. The amplitudes of excitatory soleus EMRs at a given stimulus intensity, obtained when subjects stood quietly, leaned forwards or stood on their toes to produce differing levels of ankle extensor contraction, increased with background EMG. Overall, the relationship between the size of cortically evoked soleus responses and the tonic level of motor activity, observed in individual subjects at matched stimulus intensities, did not consistently differ between postural and volitional tasks. The present results suggest that the motor cortex is potentially capable of exerting rapid regulation of the soleus muscle, and presumably other ankle extensors, not only when the muscle participates in volitional tasks but also when it is engaged in postural maintenance.