Lemon R N, Johansson R S, Westling G
Sobell Department of Neurophysiology, Institute of Neurology, London, U.K.
Can J Physiol Pharmacol. 1996 Apr;74(4):547-58.
Transcranial magnetic brain stimulation (TMS) was used to investigate corticospinal influences during a task in which human subjects had to reach out and grasp and lift an object. TMS applied to the hand area of the motor cortex was delivered during eight different phases of the task. There was a striking phase-related modulation in the amplitude of the short-latency EMG responses elicited by TMS in six arm and hand muscles. Although several mechanisms probably contribute to this modulation, one result of their operation is a potentially greater influence of the cortex during particular phases of the task. Evidence is produced that one factor contributing to this modulation is a phase-related change in corticospinal excitability. The results are consistent with a strong excitatory corticospinal drive throughout the reach to brachioradialis and anterior deltoid, which contribute to hand transport, and to the extrinsic hand muscles, which orientate the hand and fingertips. In contrast, the intrinsic hand muscles appear to receive their strongest cortical input as the digits close around and first touch the object. TMS just before contact delayed the isometric parallel increase in load and grip forces necessary to lift the object. The particularly strong EMG and behavioral effects seen at touch may reflect a powerful interaction, at the cortical level, between cutaneous inputs signalling contact with the object and the effects of TMS. Central interactions between tactile afferent input and TMS were tested by delivering TMS at different times relative to the application of an unexpected load to an object held between the fingertips. The largest responses occurred when TMS was applied 60-80 ms after load onset. THe enhanced corticospinal influence that this represents probably contributes to the powerful, short-latency boosting in grip force observed when the object was suddenly subjected to an external load. Recording of corticospinal cells in the primary motor cortex of the awake monkey suggests that the phasic modulation observed with TMS may reflect the phasic-tonic pattern of corticomotoneuronal cell discharge during the task. Since the activation of corticospinal cells by low-intensity TMS is dependent upon their level of excitability, EMG responses evoked by TMS during the performance of skilled tasks in man may, in part, reflect changes in the excitability of these cells.
经颅磁刺激（TMS）被用于研究人类受试者在伸手抓取和举起物体任务过程中皮质脊髓的影响。在该任务的八个不同阶段，对运动皮质的手部区域施加TMS。TMS诱发的短潜伏期肌电图反应在六条手臂和手部肌肉中的幅度呈现出与阶段相关的显著调制。尽管可能有多种机制导致这种调制，但它们作用的一个结果是在任务的特定阶段皮质可能产生更大的影响。有证据表明，促成这种调制的一个因素是皮质脊髓兴奋性与阶段相关的变化。结果表明，在整个伸手过程中，桡侧腕短伸肌和三角肌前部（它们参与手部移动）以及手部外在肌（它们对手部和指尖进行定位）都受到强烈的皮质脊髓兴奋性驱动。相比之下，当手指合拢并首次接触物体时，手部内在肌似乎接收到最强的皮质输入。接触前的TMS延迟了举起物体所需的等长平行增加的负载和握力。在接触时观察到的特别强烈的肌电图和行为效应可能反映了在皮质水平上，信号表明与物体接触的皮肤输入和TMS效应之间的强大相互作用。通过在相对于向指尖间握持的物体施加意外负载的不同时间施加TMS，测试了触觉传入输入与TMS之间的中枢相互作用。当在负载开始后60 - 80毫秒施加TMS时，反应最大。这所代表的增强的皮质脊髓影响可能有助于解释当物体突然受到外部负载时观察到的握力的强大、短潜伏期增强。对清醒猴子初级运动皮质中皮质脊髓细胞的记录表明，TMS观察到的相位调制可能反映了任务期间皮质运动神经元细胞放电的相位 - 紧张模式。由于低强度TMS对皮质脊髓细胞的激活取决于它们的兴奋性水平，因此在人类执行熟练任务期间TMS诱发的肌电图反应可能部分反映了这些细胞兴奋性的变化。
