Interactions between neural and muscle systems during the early stage of motor learning

本研究探讨了视觉引导力跟踪性能短期改善可能的神经肌肉机制。18名健康志愿者在视觉引导的力跟踪条件下进行缓慢斜坡收缩并食指外展（5% MVC/秒，1% MVC/秒）。随着试验次数的增加，第一骨间肌肌电（EMG）活跃量呈逐渐下降趋势。相比之下，经颅磁刺激（TMS）引起的运动诱发电位（MEP）的大小没有明显变化。用一对肌内电极记录的单运动单元（MU）活动的调查表明，在30 MU的51中，MU单元的阈值力（F-th）随着试验次数的增加而逐渐增加。单MU的F-th在2 MU中显著降低。在运动任务过程中，小波神经元的放电率没有系统性变化，但F-th的放电率有所增加。为了确定肌肉力量产生机制可能发生的变化，通过峰值触发平均技术评估单个MU的抽搐力变化。在11个实验组中，有26个实验组的抽动力在运动任务后比对照组增加了20%以上。运动任务后，h反射和皮肤反射的强度与运动任务前相比均无显著变化。这些发现表明，减少招募的mu数量和增强肌纤维可能在提高性能中起关键作用。这可能是因为大脑有效地检索了与运动任务相关的肌肉同步变化的信息，然后重新组织运动程序，以便根据肌肉的特性优化下行运动输出。

The present study investigated possible neuromuscular mechanisms underlying short-term improvement of visually-guided force tracking performance. Eighteen healthy volunteers performed slow ramp contractions with index finger abduction (5 % MVC,1 % MVC/sec) under a visually-guided force tracking condition. The amount of electromyographic(EMG) activity in the first interosseus muscle(FDI) showed a gradual decrease with the increasing trial number. In contrast, the magnitude of the motor evoked potential(MEP) elicited by trascranial magnetic stimulation(TMS) did not show a significant change. Investigation of single motor unit(MU) activity recorded with a pair of intramuscular electrodes demonstrated that in 51 of 30 MUs threshold force (F-th) of the MU unit progressively increased with the increasing trial number. A significant decrease in F-th of single MU was detected in 2 MU. There were no systematic changes in the firing rate of MUs which showed an increase in the F-th during the motor task was found. To determine possible changes in muscular force producing mechanisms, changes in the twitch force of single MU was assessed by a spike-triggered averaging technique. In twenty-six of eleven MUs, the twitch force increased more than 20 % after the motor task compared to that under the control condition. Neither the magnitude of H-reflex nor cutaneous reflexes showed a significant change after the motor task compared to those before. These findings suggest that a decrease in number of recuited MUs and potentiation of muscle fibers could play a crucial role in improvement of the performance. It may be that the brain effectively retrieves information concerning concurrent changes in the muscle involved in the motor task, then re-organize the motor program in order to optimize the descending motor output regarding the properties of the muscles.