Gorassini Monica, Yang Jaynie F, Siu Merek, Bennett David J
Division of Neuroscience, Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, Alberta T6G 2S2, Canada.
J Neurophysiol. 2002 Apr;87(4):1859-66. doi: 10.1152/jn.00025.2001.
The main purpose of this study was to examine whether facilitation of human motor unit recruitment by repeated voluntary contractions is mediated, in part, by time and activity-dependent increases in the intrinsic excitability of the parent motoneuron. To do this, pairs of tibialis anterior or soleus motor units were recorded during slowly increasing and then decreasing voluntary contractions. The firing rate of the lower-threshold motor unit of the pair (control unit) was used as a measure of effective synaptic excitation (i.e., drive) to the motoneurons. This rate was used to estimate the recruitment threshold of the higher-threshold unit of the pair (test unit). The test unit was repeatedly recruited and de-recruited in a series of contractions, and the interval between the de-recruitment and re-recruitment of the test unit (interactivation interval) was systematically varied between 0.6 and 60 s. An increase in intrinsic excitability of a unit was considered to have occurred if the level of estimated synaptic input (as measured by the firing rate of the control motor unit) needed to recruit a unit was reduced. At short interactivation intervals (1-2 s), the control unit firing frequency was significantly lower when the test unit was recruited on the second contraction, compared with the first (by 3.9 Hz or a 64% reduction). This suggested that the intrinsic excitability of the test motor unit had increased during the second contraction because it could be recruited at a much lower level of estimated synaptic drive. Longer interaction intervals (2-6 s) produced less recruitment facilitation. At even longer interactivation intervals (>6 s) there was no significant facilitation (time constant of effect was 4.8 s). In some motor units, the effect of this short-term facilitation appeared to be so pronounced that it resulted in reversing the order of de-recruitment with the other initially lower-threshold motor units. Such reversals were occasionally observed for orderly re-recruitment. The time course and behavior of the observed short-term facilitation of motor unit discharge was qualitatively similar to the warm-up phenomenon of plateau potentials seen in motoneurons of reduced preparations (e.g., 4-6 s). The possibility of warm-up contributing to the time and activity-dependent facilitation of human motor unit recruitment is discussed.
本研究的主要目的是检验反复的自主收缩对人类运动单位募集的促进作用是否部分由支配该运动单位的运动神经元内在兴奋性随时间和活动的增加所介导。为此，在缓慢增加然后减少的自主收缩过程中，记录胫前肌或比目鱼肌运动单位对。将该对中阈值较低的运动单位（对照单位）的放电频率用作对运动神经元有效突触兴奋（即驱动）的度量。该频率用于估计该对中阈值较高的单位（测试单位）的募集阈值。在一系列收缩中，测试单位被反复募集和去募集，并且测试单位去募集和重新募集之间的间隔（激活间隔）在0.6至60秒之间系统地变化。如果募集一个单位所需的估计突触输入水平（通过对照运动单位的放电频率测量）降低，则认为该单位的内在兴奋性增加。在短激活间隔（1 - ２秒）时，与第一次收缩相比，第二次收缩时募集测试单位时对照单位的放电频率显著降低（降低3.9赫兹或64％）。这表明测试运动单位的内在兴奋性在第二次收缩期间增加，因为它可以在低得多的估计突触驱动水平下被募集。较长的激活间隔（2 - 6秒）产生的募集促进作用较小。在更长的激活间隔（>6秒）时，没有明显的促进作用（效应的时间常数为4.8秒）。在一些运动单位中，这种短期促进作用似乎非常明显，以至于导致与其他最初阈值较低的运动单位的去募集顺序相反。在有序的重新募集中偶尔会观察到这种反转。观察到的运动单位放电短期促进作用的时间进程和表现与在离体标本运动神经元中看到的平台电位热身现象（例如4 - 6秒）在质量上相似。讨论了热身对人类运动单位募集的时间和活动依赖性促进作用的可能性。
