Lemon R N, Johansson R S, Westling G
Sobell Department of Neurophysiology, Institute of Neurology, London, United Kingdom.
J Neurosci. 1995 Sep;15(9):6145-56. doi: 10.1523/JNEUROSCI.15-09-06145.1995.
Transcranial magnetic brain stimulation (TMS) was used to assess the influence of the corticospinal system on motor output in seven human subjects during a task in which they had to reach out, grasp, and lift an object. Stimuli, directed at the hand area of the motor cortex, were delivered at eight defined points during the task: during reach, at grip closure, during object manipulation, during the parallel isometric increase in grip and load forces, during the lifting movement, and while the object was held in air. The amplitudes of short-latency EMG responses evoked by TMS in six arm and hand muscles showed a striking modulation across the different phases of the task. This modulation may well reflect phasic changes in corticospinal excitability because: (1) it did not simply reflect phasic changes in muscular activity associated with task performance, (2) it could vary inversely with the amplitude of the background EMG, and (3) it was only obtained with weak TMS intensities, below threshold for evoking responses in hand muscles of the relaxed subject. Our results suggest that the cortical representations of extrinsic hand muscles, which act to orientate the hand and finger tips, were subjected to a strong excitatory drive throughout the reach. This drive was also observed for brachioradialis and anterior deltoid, which contribute to transport of the hand. In contrast, the intrinsic hand muscles appear to receive their strongest cortical input as the digits closed around the object, and just after the subject first touched the object at the onset of manipulation. The isometric parallel increase in load and grip forces necessary to lift the object, which is normally triggered by tactile contact, was delayed by TMS delivered late during the reach. TMS at this time may disrupt processing necessary to control this critical phase of the task.
经颅磁刺激（TMS）被用于评估七名人类受试者在伸手、抓握并举起一个物体的任务过程中皮质脊髓系统对运动输出的影响。刺激针对运动皮层的手部区域，在任务的八个特定时间点施加：在伸手过程中、抓握闭合时、物体操作过程中、握力和负载力等长平行增加过程中、举起动作过程中以及物体在空中保持时。TMS在六块手臂和手部肌肉中诱发的短潜伏期肌电图反应的幅度在任务的不同阶段呈现出显著的调制。这种调制很可能反映了皮质脊髓兴奋性的阶段性变化，因为：（1）它并非简单地反映与任务执行相关的肌肉活动的阶段性变化；（2）它可能与背景肌电图的幅度呈反比变化；（3）它仅在弱TMS强度下获得，该强度低于在放松受试者手部肌肉中诱发反应的阈值。我们的结果表明，用于使手部和指尖定向的外在手部肌肉的皮质表征在整个伸手过程中受到强烈的兴奋性驱动。在肱桡肌和三角肌前部也观察到这种驱动，它们有助于手部的移动。相比之下，内在手部肌肉似乎在手指围绕物体闭合时以及受试者在操作开始时首次接触物体后不久接收最强的皮质输入。举起物体所需的负载和握力等长平行增加，通常由触觉接触触发，在伸手过程后期施加的TMS使其延迟。此时的TMS可能会干扰控制任务这一关键阶段所需的处理过程。
