Arányi Z, Mathis J, Hess C W, Rösler K M
Department of Neurology, Inselspital, University of Bern, Switzerland.
Muscle Nerve. 1998 Oct;21(10):1309-16. doi: 10.1002/(sici)1097-4598(199810)21:10<1309::aid-mus10>3.0.co;2-x.
Task-dependent differences in the facilitation of motor evoked potentials (MEPs) following cortex stimulation were studied in a proximal (deltoid) and a distal muscle (abductor digiti minimi; ADM) in 23 healthy subjects during both dynamic and steady contractions of the target muscle under isometric and under nonisometric conditions. In the deltoid, MEP amplitudes were significantly greater if stimulation was performed during dynamic contractions than during steady contractions, despite equal background electromyographic levels just prior to the stimulus. The same task-specific extra facilitation of deltoid MEP amplitudes was also found with magnetic stimulation of the brain stem instead of the cortex in 3 subjects. In the ADM, no such task-dependent extra facilitation of MEPs during dynamic contractions was found. It is concluded that in the deltoid, during dynamic contractions, a greater proportion of the spinal motoneurons is close to depolarization threshold (greater "subliminal fringe") whereas the number of firing motoneurons is similar to that during steady contraction. The lack of task-dependent extra facilitation of MEPs in the ADM is explained by the predominant recruitment principle for force gradation in small hand muscles, which is in contrast to the predominant frequency principle used in proximal muscles.
在23名健康受试者中，研究了在等长和非等长条件下目标肌肉进行动态和稳定收缩时，刺激皮层后运动诱发电位（MEP）促进作用的任务依赖性差异，涉及近端肌肉（三角肌）和远端肌肉（小指展肌；ADM）。在三角肌中，尽管刺激前的背景肌电图水平相同，但在动态收缩期间进行刺激时，MEP振幅显著大于稳定收缩期间。在3名受试者中，用脑干而非皮层的磁刺激也发现了三角肌MEP振幅同样具有任务特异性的额外促进作用。在ADM中，未发现动态收缩期间MEP存在这种任务依赖性的额外促进作用。得出的结论是，在三角肌中，动态收缩期间，更大比例的脊髓运动神经元接近去极化阈值（更大的“阈下边缘”），而放电运动神经元的数量与稳定收缩期间相似。ADM中MEP缺乏任务依赖性的额外促进作用，这是由于手部小肌肉力量分级的主要募集原则，这与近端肌肉中使用的主要频率原则相反。
