Institute of Neurosciences, Newcastle University, Newcastle upon Tyne, UK.
J Physiol. 2019 May;597(10):2729-2739. doi: 10.1113/JP277462. Epub 2019 Mar 20.
Pairing stimulation of a finger flexor or extensor muscle at the motor point with transcranial magnetic stimulation (TMS) of the motor cortex generated plastic changes in motor output. Increases in output were greater in intrinsic hand muscles than in the finger flexor. No changes occurred in the finger extensor. This gradient was seen irrespective of which muscle was stimulated paired with transcranial magnetic stimulation. Intermittent theta-burst stimulation also produced increases in output, although these were similar across muscles. We suggest that intrinsic hand and flexor muscles have a higher potential to show plasticity than extensors, although only when plasticity is induced by sensory input. This may relate to differences seen in recovery of function in these muscles after injury, such as post-stroke.
The ability of the motor system to show plastic change underlies skill learning and also permits recovery after injury. One puzzling observation is that, after stroke, upper limb flexor muscles show good recovery but extensors remain weak, with this being a major contributor to residual disability. We hypothesized that there might be differences in potential for plasticity across hand and forearm muscles. In the present study, we investigated this using two protocols based on transcranial magnetic brain stimulation (TMS) in healthy human subjects. Baseline TMS responses were recorded from two intrinsic hand muscles: flexor digitorum superficialis (FDS) and extensor digitorum communis (EDC). In the first study, paired associative stimulation (PAS) was delivered by pairing motor point stimulation of FDS or EDC with TMS. Responses were then remeasured. Increases were greatest in the hand muscles, smaller in FDS and non-significant in EDC, irrespective of whether stimulation of FDS or EDC was used. In the second study, intermittent theta-burst rapid rate TMS was applied instead of PAS. In this case, all muscles showed similar increases in TMS responses. We conclude that the potential to show plastic changes in motor cortical output has the gradient: hand muscles > flexors > extensors. However, this was only seen in a protocol that requires integration of sensory input (PAS) and not when plasticity was induced purely by cortical stimulation (rapid rate TMS). This observation may relate to why functional recovery tends to favour flexor and hand muscles over extensors.
在运动点刺激手指屈肌或伸肌的同时进行经颅磁刺激（TMS），可使运动输出产生塑性变化。内在手部肌肉的输出增加大于手指屈肌。手指伸肌没有变化。无论刺激哪块肌肉与 TMS 配对，都会出现这种梯度。间歇性 theta 爆发刺激也会增加输出，尽管各肌肉之间的变化相似。我们认为，内在手部和屈肌比伸肌具有更高的产生可塑性的潜力，尽管只有在通过感觉输入诱导可塑性时才会如此。这可能与这些肌肉在受伤后（如中风后）功能恢复方面的差异有关。
运动系统在技能学习和损伤后恢复方面表现出可塑性变化的能力。一个令人费解的观察结果是，中风后，上肢屈肌恢复良好，但伸肌仍然较弱，这是导致残障的主要原因。我们假设手部和前臂肌肉的可塑性潜力可能存在差异。在本研究中，我们使用两种基于健康人类受试者经颅磁脑刺激（TMS）的方案来研究这一点。记录了两个内在手部肌肉的基线 TMS 反应：指浅屈肌（FDS）和指总伸肌（EDC）。在第一项研究中，通过将 FDS 或 EDC 的运动点刺激与 TMS 配对来进行联合刺激（PAS）。然后重新测量反应。手部肌肉的增加最大，FDS 较小，EDC 无显著变化，无论使用 FDS 还是 EDC 刺激。在第二项研究中，取而代之的是间歇性 theta 爆发快速重复 TMS。在这种情况下，所有肌肉的 TMS 反应都显示出相似的增加。我们得出结论，运动皮质输出产生塑性变化的潜力具有梯度：手部肌肉>屈肌>伸肌。然而，这仅在需要整合感觉输入（PAS）的方案中可见，而不是在仅通过皮质刺激（快速重复 TMS）诱导可塑性时可见。这种观察结果可能与为什么功能恢复往往有利于屈肌和手部肌肉而不是伸肌有关。
