Department of Clinical Neurophysiology, Kuopio University Hospital, Kuopio, Finland.
Department of Psychiatry, UT Southwestern Medical Center, Dallas, U.S.A.
J Clin Neurophysiol. 2019 May;36(3):229-235. doi: 10.1097/WNP.0000000000000568.
Transcranial magnetic stimulation-induced motor responses have been considered to mainly reflect the electrophysiological characteristics of the central motor system. However, certain motor phenomena, such as the magnitude of repetition suppression measured with motor evoked potentials (MEPs), could also in part be influenced by metabolic processes in the peripheral muscles and in both the peripheral and central nervous system. Repetition suppression is an inhibitory phenomenon in which the amplitude of MEP decreases in comparison to that of the first MEP in a train of transcranial magnetic stimulation pulses. This study aimed to identify possible metabolic processes influencing repetition suppression.
The metabolic profiles from serum samples and repetition suppression from the right abductor pollicis brevis muscle were measured in 73 subjects (37 female subjects). Repetition suppression was measured using trains of transcranial magnetic stimulation stimuli consisting of 4 identical single pulses at 1-second intervals. The trains were repeated every 20 seconds, and 30 trains were given with a stimulation intensity of 120% of the resting motor threshold of the abductor pollicis brevis. Thus, a total of 120 stimuli were administered.
The main finding was a significant negative relationship between serum creatine levels and the magnitude of repetition suppression (standardized β coefficient (β) = -0.43; P < 0.001). In other words, higher creatine levels corresponded to a smaller decrement in the MEP amplitude in response to repetition. When MEPs were not repeated, no relationship was observed (β = 0.09; P = 0.454). Creatine is used to form phosphocreatine, which in turn is needed to resynthesize adenosine triphosphate from adenosine diphosphate in situations requiring high amounts of energy in muscles and neural cells.
For the first time, this study demonstrated a connection between repeated MEPs and peripheral serum metabolites linked to muscle function. These findings could explain some of the intersubject variability commonly observed in MEPs when the pulses are repeated.
经颅磁刺激诱导的运动反应被认为主要反映中枢运动系统的电生理特征。然而，某些运动现象，如运动诱发电位（MEPs）测量的重复抑制程度，也可能部分受到外周肌肉以及外周和中枢神经系统代谢过程的影响。重复抑制是一种抑制现象，即在一串经颅磁刺激脉冲中，MEP 的幅度与第一个 MEP 相比减小。本研究旨在确定可能影响重复抑制的代谢过程。
在 73 名受试者（37 名女性）中测量血清样本的代谢谱和右侧拇指外展短肌的重复抑制。使用由 1 秒间隔的 4 个相同单脉冲组成的经颅磁刺激刺激串来测量重复抑制。刺激串每 20 秒重复一次，给予 30 个刺激串，刺激强度为拇指外展短肌静息运动阈值的 120%。因此，总共给予 120 个刺激。
主要发现是血清肌酸水平与重复抑制程度之间存在显著负相关（标准化β系数（β）=-0.43；P<0.001）。换句话说，较高的肌酸水平对应于 MEPs 幅度在重复刺激时的较小下降。当不重复 MEPs 时，观察到没有关系（β=0.09；P=0.454）。肌酸用于形成磷酸肌酸，磷酸肌酸又在肌肉和神经细胞需要大量能量的情况下从二磷酸腺苷重新合成三磷酸腺苷。
本研究首次证明了重复 MEPs 与与肌肉功能相关的外周血清代谢物之间存在联系。这些发现可以解释在重复脉冲时经常观察到的 MEPs 中一些个体间变异性。
