Neuroscience Research Australia, Sydney, NSW, Australia.
University of New South Wales, Sydney, NSW, Australia.
Exp Physiol. 2019 Apr;104(4):546-555. doi: 10.1113/EP087472. Epub 2019 Feb 21.
What is the central question of this study? Corticospinal excitability to biceps brachii is known to modulate according to upper-limb posture. Here, cervicomedullary stimulation was used to investigate potential spinal contributions to elbow angle-dependent changes in corticospinal excitability at rest. What is the main finding and its importance? At more extended elbow angles, biceps responses to cervicomedullary stimulation were decreased, whereas cortically evoked responses (normalized to cervicomedullary-evoked responses) were increased. Results suggest decreased spinal excitability but increased cortical excitability as the elbow is placed in a more extended position, an effect that is unlikely to be attributable to cutaneous stretch receptor activation.
Corticospinal excitability to biceps brachii is known to modulate according to upper-limb posture. In study 1, our aim was to investigate potential spinal contributions to this modulation and the independent effect of elbow angle. Biceps responses to transcranial magnetic stimulation (motor evoked potentials; MEPs) and electrical cervicomedullary stimulation (cervicomedullary motor evoked potentials; CMEPs) were measured at five elbow angles ranging from full extension to 130 deg of flexion. In study 2, possible contributions of cutaneous stretch receptors to elbow angle-dependent excitability changes were investigated by eliciting MEPs and CMEPs in three conditions of skin stretch about the elbow (stretch to mimic full extension, no stretch or stretch to mimic flexion). Each study had 12 participants. Evoked potentials were acquired at rest, with participants seated, the shoulder flexed 90 deg and forearm supinated. The MEPs and CMEPs were normalized to maximal compound muscle action potentials. In study 1, as the elbow was moved to more extended positions, there were no changes in MEPs (P = 0.963), progressive decreases in CMEPs (P < 0.0001; CMEPs at 130 deg flexion ∼220% of full extension) and increases in the MEP/CMEP ratio (P = 0.019; MEP/CMEP at 130 deg flexion ∼20% of full extension). In study 2, there were no changes in MEPs (P = 0.830) or CMEPs (P = 0.209) between skin stretch conditions. Therefore, although results suggest a decrease in spinal and an increase in supraspinal excitability at more extended angles, the mechanism for these changes in corticospinal excitability to biceps is not cutaneous stretch receptor feedback.
本研究的核心问题是什么？已知肱二头肌的皮质脊髓兴奋性会根据上肢姿势进行调节。在这里，使用颈髓刺激来研究在休息时肘部角度依赖性变化中潜在的脊髓对皮质脊髓兴奋性的影响。主要发现及其重要性是什么？在肘部伸展角度更大时，肱二头肌对颈髓刺激的反应减弱，而皮质诱发反应（相对于颈髓诱发反应进行归一化）增加。结果表明，随着肘部处于更伸展的位置，脊髓兴奋性降低，而皮质兴奋性增加，这种效应不太可能归因于皮肤拉伸感受器的激活。
已知肱二头肌的皮质脊髓兴奋性会根据上肢姿势进行调节。在研究 1 中，我们的目的是研究这种调节的潜在脊髓贡献以及肘部角度的独立影响。在从完全伸展到 130 度屈曲的五个肘部角度下，测量经颅磁刺激（运动诱发电位；MEPs）和电颈髓刺激（颈髓运动诱发电位；CMEPs）对肱二头肌的反应。在研究 2 中，通过在肘部周围的三种皮肤拉伸状态下诱发 MEPs 和 CMEPs 来研究皮肤拉伸感受器对肘部角度依赖性兴奋性变化的可能贡献（模仿完全伸展的拉伸、无拉伸或模仿屈曲的拉伸）。每项研究都有 12 名参与者。在休息时、参与者坐姿、肩部弯曲 90 度和前臂旋前时获取诱发电位。MEPs 和 CMEPs 被归一化为最大复合肌肉动作电位。在研究 1 中，随着肘部移动到更伸展的位置，MEPs 没有变化（P=0.963），CMEPs 逐渐减少（P<0.0001；130 度屈曲时的 CMEPs 约为完全伸展时的 220%），MEP/CMEP 比值增加（P=0.019；130 度屈曲时的 MEP/CMEP 约为完全伸展时的 20%）。在研究 2 中，皮肤拉伸条件之间的 MEPs（P=0.830）或 CMEPs（P=0.209）没有变化。因此，尽管结果表明在更伸展的角度下，脊髓兴奋性降低，皮质兴奋性增加，但肱二头肌皮质脊髓兴奋性变化的机制不是皮肤拉伸感受器的反馈。
