Kennedy David S, McNeil Chris J, Gandevia Simon C, Taylor Janet L
Neuroscience Research Australia, Randwick, NSW, Australia.
University of New South Wales, Kensington, NSW, Australia.
Exp Physiol. 2016 Dec 1;101(12):1552-1564. doi: 10.1113/EP085753. Epub 2016 Oct 31.
What is the central question of this study? Do group III and IV muscle afferents act at the spinal or cortical level to affect the ability of the central nervous system to drive quadriceps muscles during fatiguing exercise? What is the main finding and its importance? The excitability of the motoneurone pool of vastus lateralis was unchanged by feedback from group III and IV muscle afferents. In contrast, feedback from these afferents may contribute to inhibition at the cortex. However, the excitability of the corticospinal pathway was not directly affected by feedback from these afferents. These findings are important for understanding neural processes during fatiguing exercise. In upper limb muscles, changes in afferent feedback, motoneurone excitability, and motor cortical output can contribute to failure of the central nervous system to recruit muscles fully during fatigue. It is not known whether similar changes occur with fatigue of muscles in the lower limb. We assessed the corticospinal pathway to vastus lateralis during fatiguing sustained maximal voluntary contractions (MVCs) of the knee extensors and during firing of fatigue-sensitive group III/IV muscle afferents maintained by postexercise ischaemia after fatiguing MVCs of the knee extensors and, separately, the flexors. In two experiments, subjects (n = 9) performed brief knee extensor MVCs before and after 2-min sustained MVCs of the knee extensors (experiment 1) or knee flexors (experiment 2). During MVCs, motor evoked potentials (MEPs) were elicited by transcranial magnetic stimulation over the motor cortex and thoracic motor evoked potentials (TMEPs) by electrical stimulation over the thoracic spine. During the 2-min extensor contraction, the size of vastus lateralis MEPs normalized to the maximal M-wave increased (P < 0.05), but normalized TMEPs were unchanged (P = 0.16). After the 2-min MVC, maintained firing of group III/IV muscle afferents had no effect on vastus lateralis MEPs or TMEPs (P = 0.18 and P = 0.50, respectively). Likewise, after the 2-min knee flexor MVC, maintained firing of these afferents showed no effect on vastus lateralis MEPs or TMEPs (P = 0.69 and P = 0.34, respectively). Motoneurones of vastus lateralis do not become less excitable during fatiguing isometric MVCs. Moreover, fatigue-sensitive group III/IV muscle afferents fail to affect the overall excitability of vastus lateralis motoneurones during MVCs.
本研究的核心问题是什么？Ⅲ类和Ⅳ类肌肉传入神经是在脊髓水平还是皮质水平发挥作用，以影响中枢神经系统在疲劳运动期间驱动股四头肌的能力？主要发现及其重要性是什么？股外侧肌运动神经元池的兴奋性并未因Ⅲ类和Ⅳ类肌肉传入神经的反馈而改变。相比之下，这些传入神经的反馈可能有助于在皮质水平产生抑制作用。然而，皮质脊髓通路的兴奋性并未直接受到这些传入神经反馈的影响。这些发现对于理解疲劳运动期间的神经过程具有重要意义。在上肢肌肉中，传入反馈、运动神经元兴奋性和运动皮质输出的变化可导致中枢神经系统在疲劳期间无法充分募集肌肉。目前尚不清楚下肢肌肉疲劳时是否会发生类似变化。我们在膝关节伸肌进行疲劳性持续最大自主收缩（MVC）期间，以及在膝关节伸肌和屈肌进行疲劳性MVC后通过运动后缺血维持疲劳敏感的Ⅲ/Ⅳ类肌肉传入神经放电时，评估了至股外侧肌的皮质脊髓通路。在两个实验中，受试者（n = 9）在膝关节伸肌进行2分钟持续MVC之前和之后（实验1），或在膝关节屈肌进行2分钟持续MVC之前和之后（实验2），进行短暂的膝关节伸肌MVC。在MVC期间，通过对运动皮质进行经颅磁刺激诱发运动诱发电位（MEP），通过对胸椎进行电刺激诱发胸段运动诱发电位（TMEP）。在2分钟的伸肌收缩期间，股外侧肌MEP相对于最大M波的大小增加（P < 0.05），但标准化的TMEP未改变（P = 0.16）。在2分钟MVC之后，Ⅲ/Ⅳ类肌肉传入神经的持续放电对股外侧肌MEP或TMEP均无影响（分别为P = 0.18和P = 0.50）。同样，在2分钟膝关节屈肌MVC之后，这些传入神经的持续放电对股外侧肌MEP或TMEP均无影响（分别为P = 0.69和P = 0.34）。在疲劳性等长MVC期间，股外侧肌的运动神经元兴奋性不会降低。此外，在MVC期间，疲劳敏感的Ⅲ/Ⅳ类肌肉传入神经无法影响股外侧肌运动神经元的整体兴奋性。
