NeuroMuscular Research Center, 19 Deerfield St., Boston, MA 02215, USA.
J Neurophysiol. 2012 Jan;107(1):178-95. doi: 10.1152/jn.00961.2010. Epub 2011 Oct 5.
For the past five decades there has been wide acceptance of a relationship between the firing rate of motor units and the afterhyperpolarization of motoneurons. It has been promulgated that the higher-threshold, larger-soma, motoneurons fire faster than the lower-threshold, smaller-soma, motor units. This relationship was based on studies on anesthetized cats with electrically stimulated motoneurons. We questioned its applicability to motor unit control during voluntary contractions in humans. We found that during linearly force-increasing contractions, firing rates increased as exponential functions. At any time and force level, including at recruitment, the firing rate values were inversely related to the recruitment threshold of the motor unit. The time constants of the exponential functions were directly related to the recruitment threshold. From the Henneman size principle it follows that the characteristics of the firing rates are also related to the size of the soma. The "firing rate spectrum" presents a beautifully simple control scheme in which, at any given time or force, the firing rate value of earlier-recruited motor units is greater than that of later-recruited motor units. This hierarchical control scheme describes a mechanism that provides an effective economy of force generation for the earlier-recruited lower force-twitch motor units, and reduces the fatigue of later-recruited higher force-twitch motor units-both characteristics being well suited for generating and sustaining force during the fight-or-flight response.
在过去的五十年中，运动单位的放电率与运动神经元的后超极化之间的关系已经被广泛接受。已经公布的是，较高阈值、较大胞体的运动神经元比较低阈值、较小胞体的运动单位放电更快。这种关系是基于对电刺激运动神经元的麻醉猫的研究得出的。我们质疑它是否适用于人类自愿收缩期间的运动单位控制。我们发现，在线性力增加收缩期间，放电率呈指数函数增加。在任何时间和力水平，包括募集时，放电率值与运动单位的募集阈值呈反比。指数函数的时间常数与募集阈值直接相关。根据 Henneman 大小原则，放电率的特征也与胞体的大小有关。“放电率谱”呈现出一种非常简单的控制方案，即在任何给定的时间或力下，较早募集的运动单位的放电率值大于较晚募集的运动单位的放电率值。这种分层控制方案描述了一种机制，为较早募集的较低力颤搐运动单位提供了有效的力量产生经济性，并减少了较晚募集的较高力颤搐运动单位的疲劳——这两种特性都非常适合在战斗或逃跑反应中产生和维持力量。
