Department of Exercise and Sport Sciences, University of Copenhagen, Copenhagen, Denmark.
J Physiol. 2010 Jul 1;588(Pt 13):2391-402. doi: 10.1113/jphysiol.2010.190678. Epub 2010 May 10.
Flexion and extension movements are organized reciprocally, so that extensor motoneurones in the spinal cord are inhibited when flexor muscles are active and vice versa. During and just prior to dorsiflexion of the ankle, soleus motoneurones are thus inhibited as evidenced by a depression of the soleus H-reflex. It is therefore surprising that soleus motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) have been found not to be reduced and even facilitated during a voluntary dorsiflexion. The objective of this study was to investigate if MEPs, evoked by TMS, show a similar facilitation prior to and at the onset of contraction of muscles that are antagonists to the muscle in which the MEP is evoked and if so, examine the origin of such a facilitatory motor programme. Eleven seated subjects reacted to an auditory cue by contracting either the tibialis anterior (TA) or soleus muscle of the left ankle. TMS was applied to the hotspot of TA and soleus muscles on separate days. Stimuli were delivered prior to and at the beginning of contraction. Soleus MEPs were significantly facilitated when TMS was applied 50 ms prior to onset of plantar flexion. Surprisingly, soleus MEPs were also facilitated (although to a lesser extent) at a similar time in relation to the onset of dorsiflexion. TA MEPs were facilitated 50 ms prior to onset of dorsiflexion and neither depressed nor facilitated prior to plantar flexion. No difference was found between the facilitation of the soleus MEP and motor evoked responses to cervicomedullary stimulation prior to dorsiflexion, suggesting that the increased soleus MEPs were not caused by changes at a cortical level. This was confirmed by the observation that short-latency facilitation of the soleus H-reflex by subthreshold TMS was increased prior to plantar flexion, but not prior to dorsiflexion. These findings suggest that voluntary contraction at the ankle is accompanied by preceding facilitation of antagonists by a subcortical motor programme. This may help to ensure that the direction of movement may be changed quickly and efficiently during functional motor tasks.
弯曲和伸展运动是相互组织的，因此当屈肌活跃时，脊髓中的伸肌运动神经元被抑制，反之亦然。在踝关节背屈期间和之前，比目鱼肌运动神经元被抑制，这可以从比目鱼肌 H 反射的抑制中得到证明。因此，令人惊讶的是，通过经颅磁刺激（TMS）诱发的比目鱼肌运动诱发电位（MEPs）在自愿背屈时并未减少，甚至得到增强。本研究的目的是探讨 TMS 诱发的 MEPs 是否在与诱发 MEP 的肌肉相拮抗的肌肉收缩之前和开始时表现出类似的易化，如果是这样，检查这种易化运动程序的起源。11 名坐姿受试者听到听觉提示后，收缩左侧踝关节的胫骨前肌（TA）或比目鱼肌。TMS 分别在 TA 和比目鱼肌的热点上施加于不同的日子。刺激在收缩之前和开始时给予。当 TMS 在跖屈开始前 50 毫秒给予时，比目鱼肌 MEPs 显著增强。令人惊讶的是，当与背屈开始相关时，比目鱼肌 MEPs 也以相似的时间被易化（尽管程度较小）。在背屈开始前 50 毫秒，TA MEPs 被易化，而在跖屈之前既不被抑制也不被易化。在背屈之前，比目鱼肌 MEP 和对颈髓刺激的运动诱发电响应的易化之间没有发现差异，这表明增加的比目鱼肌 MEPs 不是由皮质水平的变化引起的。这一点通过观察到在跖屈之前，阈下 TMS 对比目鱼肌 H 反射的短潜伏期易化增加，但在背屈之前没有增加得到证实。这些发现表明，踝关节的自愿收缩伴随着由皮质下运动程序引起的拮抗剂的预先易化。这有助于确保在功能性运动任务中可以快速有效地改变运动方向。
