Pruszynski J Andrew, Kurtzer Isaac, Lillicrap Timothy P, Scott Stephen H
Centre for Neuroscience Studies, Queen's University, Kingston, Ontario K7L 3N6, Canada.
J Neurophysiol. 2009 Aug;102(2):992-1003. doi: 10.1152/jn.00085.2009. Epub 2009 May 13.
The earliest neural response to a mechanical perturbation, the short-latency stretch response (R1: 20-45 ms), is known to exhibit "automatic gain-scaling" whereby its magnitude is proportional to preperturbation muscle activity. Because gain-scaling likely reflects an intrinsic property of the motoneuron pool (via the size-recruitment principle), counteracting this property poses a fundamental challenge for the nervous system, which must ultimately counter the absolute change in load regardless of the initial muscle activity (i.e., show no gain-scaling). Here we explore the temporal evolution of gain-scaling in a simple behavioral task where subjects stabilize their arm against different background loads and randomly occurring torque perturbations. We quantified gain-scaling in four elbow muscles (brachioradialis, biceps long, triceps lateral, triceps long) over the entire sequence of muscle activity following perturbation onset-the short-latency response, long-latency response (R2: 50-75 ms; R3: 75-105 ms), early voluntary corrections (120-180 ms), and steady-state activity (750-1250 ms). In agreement with previous observations, we found that the short-latency response demonstrated substantial gain-scaling with a threefold increase in background load resulting in an approximately twofold increase in muscle activity for the same perturbation. Following the short-latency response, we found a rapid decrease in gain-scaling starting in the long-latency epoch ( approximately 75-ms postperturbation) such that no significant gain-scaling was observed for the early voluntary corrections or steady-state activity. The rapid decrease in gain-scaling supports our recent suggestion that long-latency responses and voluntary control are inherently linked as part of an evolving sensorimotor control process through similar neural circuitry.
对机械扰动的最早神经反应，即短潜伏期牵张反应（R1：20 - 45毫秒），已知表现出“自动增益缩放”，即其幅度与扰动前的肌肉活动成正比。由于增益缩放可能反映了运动神经元池的内在特性（通过大小募集原则），抵消这一特性对神经系统构成了一项基本挑战，因为神经系统最终必须抵消负荷的绝对变化，而不管初始肌肉活动如何（即不表现出增益缩放）。在此，我们在一个简单的行为任务中探究增益缩放的时间演变，在该任务中，受试者使手臂抵抗不同的背景负荷以及随机出现的扭矩扰动。我们在扰动开始后的整个肌肉活动序列中，对四块肘部肌肉（肱桡肌、肱二头肌长头、肱三头肌外侧头、肱三头肌长头）的增益缩放进行了量化，包括短潜伏期反应、长潜伏期反应（R2：50 - 75毫秒；R3：75 - 105毫秒）、早期自主校正（120 - 180毫秒）以及稳态活动（750 - 1250毫秒）。与先前的观察结果一致，我们发现短潜伏期反应表现出显著的增益缩放，背景负荷增加三倍会导致相同扰动下肌肉活动增加约两倍。在短潜伏期反应之后，我们发现从长潜伏期阶段（扰动后约75毫秒）开始增益缩放迅速下降，以至于在早期自主校正或稳态活动中未观察到显著的增益缩放。增益缩放的迅速下降支持了我们最近的观点，即长潜伏期反应和自主控制通过相似的神经回路作为不断演变的感觉运动控制过程的一部分内在地联系在一起。
