Kimura Toshitaka, Yamanaka Kentaro, Nozaki Daichi, Nakazawa Kimitaka, Miyoshi Tasuku, Akai Masami, Ohtsuki Tatsuyuki
Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, 153-8902 Tokyo, Japan.
Exp Brain Res. 2003 Sep;152(1):123-32. doi: 10.1007/s00221-003-1518-1. Epub 2003 Jul 17.
Many studies have demonstrated that the firing behavior of single motor units varies in a nonlinear manner to the exerted torque during gradual muscle contraction and relaxation. However, it is unclear whether corticospinal excitability has such a hysteresis-like feature. In this study, we examined corticospinal excitability using transcranial magnetic stimulation (TMS) during gradual muscle contraction and relaxation for torque regulation in elbow flexor muscles. Eight healthy male subjects performed two different isometric elbow flexion tasks, namely, sinusoidal and tonic torque exertion tasks. In the sinusoidal task, the subjects sinusoidally increased and decreased the isometric elbow flexion torque (range of 0-15% of maximum voluntary contraction) at three different frequencies (0.33, 0.17, and 0.08 Hz). For each ascending (contraction: CON) and descending (relaxation: REL) period of the exerted torque, a single TMS was applied at 5 phases. In the tonic task, the elbow flexion torque was tonically exerted at 7 levels in a similar range as that in the sinusoidal task. EMG activities were recorded from the agonists, the biceps brachii (BB) and brachioradialis (BRD) muscles, and an antagonist, the triceps brachii (TB) muscle. The results demonstrated that the EMG activities of both the agonists and antagonist were larger in the CON period than the REL period, even when the exerted torque was the same. However, there were no significant differences in EMG activation profiles among the different frequencies of contraction. In BB and BRD, the motor-evoked potential (MEP) elicited by the TMS was also greater in the CON period than in the REL period. This CON-REL difference of MEP amplitudes was still observed when corrections were made for the increased EMG activities; that is, the MEP amplitudes to the identical EMG activities were greater in the CON period than in the REL period, and this phenomenon was more pronounced at higher frequencies. In addition, the degree to which sinusoidal MEPs exceeded tonic MEPs in the CON period and were smaller than tonic MEPs in the REL period became more pronounced at higher frequencies. On the other hand, there were significant correlations between the BB and BRD MEP amplitudes and the rate of change of elbow flexion/extension torque. These results indicate that corticospinal excitability during muscle contraction and relaxation has a neural hysteresis to the muscle activity, i.e., spinal motoneuronal activity, according to the rate of change of the exerted torque, i.e., muscle tension. This suggests that corticospinal excitability modulation depends not only on concurrent spinal motoneuronal activity and muscle tension but also on the time-series pattern of their changes during muscle contraction and relaxation.
许多研究表明，在肌肉逐渐收缩和放松过程中，单个运动单位的放电行为随所施加的扭矩呈非线性变化。然而，尚不清楚皮质脊髓兴奋性是否具有这种类似滞后的特征。在本研究中，我们在肘部屈肌逐渐收缩和放松以调节扭矩的过程中，使用经颅磁刺激（TMS）来检测皮质脊髓兴奋性。八名健康男性受试者进行了两项不同的等长肘部屈曲任务，即正弦和强直扭矩施加任务。在正弦任务中，受试者以三种不同频率（0.33、0.17和0.08Hz）正弦式地增加和减少等长肘部屈曲扭矩（最大自主收缩的0-15%范围）。对于所施加扭矩的每个上升（收缩：CON）和下降（放松：REL）阶段，在5个相位施加单次TMS。在强直任务中，肘部屈曲扭矩在与正弦任务类似的范围内以7个水平进行强直施加。从主动肌肱二头肌（BB）和肱桡肌（BRD）以及拮抗肌肱三头肌（TB）记录肌电图活动。结果表明，即使所施加的扭矩相同，主动肌和拮抗肌的肌电图活动在CON阶段也比REL阶段更大。然而，不同收缩频率之间的肌电图激活模式没有显著差异。在BB和BRD中，TMS诱发的运动诱发电位（MEP）在CON阶段也比REL阶段更大。当对增加的肌电图活动进行校正时，仍可观察到MEP幅度的这种CON-REL差异；也就是说，在相同肌电图活动下，CON阶段的MEP幅度比REL阶段更大，并且这种现象在较高频率下更明显。此外，在CON阶段正弦MEP超过强直MEP且在REL阶段小于强直MEP的程度在较高频率下变得更加明显。另一方面，BB和BRD的MEP幅度与肘部屈伸扭矩的变化率之间存在显著相关性。这些结果表明，肌肉收缩和放松过程中的皮质脊髓兴奋性对肌肉活动（即脊髓运动神经元活动）具有神经滞后现象，这取决于所施加扭矩（即肌肉张力）的变化率。这表明皮质脊髓兴奋性调制不仅取决于同时存在的脊髓运动神经元活动和肌肉张力，还取决于它们在肌肉收缩和放松过程中的变化的时间序列模式。
