Mitsuhashi Kosei, Seki Kazunori, Akamatsu Chizuru, Handa Yasunobu
Department of Restorative Neuromuscular Rehabilitation, Tohoku University Graduate School of Medicine, Sendai, Japan.
Tohoku J Exp Med. 2007 Jul;212(3):221-8. doi: 10.1620/tjem.212.221.
The forearm rotation changes sensory inputs to the central nervous system, thereby providing orientation of the hand for grasping an object. Electrical activities of the muscles, induced by transcranial magnetic stimulation to the brain, i.e., motor evoked potentials (MEPs), are used for estimation of the excitability of motor neurons in the brain and spinal cord. It is well known that rotational positioning of the forearm influences MEPs of forearm muscles through modulation of excitability in the central nervous system. In the present study, we investigated whether such a posture-dependent change of MEPs could be found in upper arm and intrinsic hand muscles at three different rotational forearm positions: the most internal (pronation), neutral, and most external (supination) positions of rotation. MEPs were simultaneously recorded from the four muscles, biceps brachii (BB), triceps brachii (TB), abductor digiti minimi (ADM), and abductor pollicis brevis (AbPB). MEP amplitudes and latencies in BB, TB and ADM were significantly larger and shorter, respectively, in supination compared to the values in other positions. By contrast, MEP of AbPB in supination was lower in amplitude and longer in latency. Importantly, muscle lengths of TB, ADM and AbPB are constant in any rotational forearm positions, excluding the possibility of the muscle-length dependent change of spinal reflex. Therefore, these results might be attributable to the posture-dependent modulation of the motor cortex activity for the upper limb. The motor cortex probably changes the control strategy for the upper limb muscles in accordance with the sensory input from the forearm.
前臂旋转会改变传入中枢神经系统的感觉输入，从而为抓握物体提供手部的方位信息。经颅磁刺激大脑所诱发的肌肉电活动，即运动诱发电位（MEP），被用于评估大脑和脊髓中运动神经元的兴奋性。众所周知，前臂的旋转定位通过调节中枢神经系统的兴奋性来影响前臂肌肉的MEP。在本研究中，我们调查了在三种不同的前臂旋转位置：最内旋（旋前）、中立和最外旋（旋后）位置，上臂和手部固有肌是否能发现MEP的这种姿势依赖性变化。同时从肱二头肌（BB）、肱三头肌（TB）、小指展肌（ADM）和拇短展肌（AbPB）这四块肌肉记录MEP。与其他位置的值相比，旋后时BB、TB和ADM的MEP波幅显著更大，潜伏期显著更短。相比之下，旋后时AbPB的MEP波幅更低，潜伏期更长。重要的是，在任何前臂旋转位置，TB、ADM和AbPB的肌肉长度都是恒定的，排除了脊髓反射的肌肉长度依赖性变化的可能性。因此，这些结果可能归因于上肢运动皮层活动的姿势依赖性调节。运动皮层可能根据来自前臂的感觉输入改变上肢肌肉的控制策略。
