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):1850-8. doi: 10.1152/jn.00024.2001.
The main purpose of this study was to estimate the contribution of intrinsic activation of human motoneurons (e.g., by plateau potentials) during voluntary and reflexive muscle contractions. Pairs of motor units were recorded from either the tibialis anterior or soleus muscle during three different conditions: 1) during a brief muscle vibration followed by a slow relaxation of a steady isometric contraction; 2) during a triangular isometric torque contraction; and 3) during passive sinusoidal muscle stretch superimposed on a steady isometric contraction. In each case, the firing rate of a tonically firing control motor unit was used as a measure of the effective synaptic excitation (i.e., synaptic drive) to a slightly higher-threshold test motor unit that was recruited and de-recruited during a contraction trial. The firing rate of the control unit was compared at recruitment and de-recruitment of the test unit. This was done to determine whether the estimated synaptic drive needed to recruit a motor unit was less than the amount needed to sustain firing as a result of an added depolarization produced from intrinsic sources. After test unit recruitment, the firing rate of the control unit could be decreased significantly (on average by 3.6 Hz from an initial recruitment rate of 9.8 Hz) before the test unit was de-recruited during a descending synaptic drive. Similar decreases in control unit rate occurred in all three experimental conditions. This represents a possible 40% reduction in the estimated synaptic drive needed to maintain firing of a motor unit compared with the estimated amount needed to recruit the unit initially. The firing rates of both the control and test units were modulated together in a highly parallel fashion, suggesting that the unit pairs were driven by common synaptic inputs. This tight correlation further validated the use of the control unit firing rate as a monitor of synaptic drive to the test motor unit. The estimates of intrinsically mediated depolarization of human motoneurons ( approximately 40% during moderate contractions) are consistent with values obtained for plateau potentials obtained from intracellular recordings of motoneurons in reduced animal preparations, although various alternative mechanisms are discussed. This suggests that similar intrinsic conductances provide a substantial activation of human motoneurons during moderate physiological activity.
本研究的主要目的是评估在自主和反射性肌肉收缩过程中，人类运动神经元内在激活（例如通过平台电位）的作用。在三种不同情况下，从胫骨前肌或比目鱼肌记录运动单位对：1）在短暂肌肉振动后，稳定等长收缩缓慢放松期间；2）在三角等长扭矩收缩期间；3）在叠加于稳定等长收缩的被动正弦肌肉拉伸期间。在每种情况下，持续放电的对照运动单位的放电频率被用作对一个阈值稍高的测试运动单位有效突触兴奋（即突触驱动）的度量，该测试运动单位在收缩试验中被募集和去募集。在测试单位募集和去募集时，比较对照单位的放电频率。这样做是为了确定募集一个运动单位所需的估计突触驱动是否小于由于内在来源产生的额外去极化而维持放电所需的量。在测试单位被募集后，在下行突触驱动期间测试单位去募集之前，对照单位的放电频率可能会显著降低（平均从初始募集频率9.8 Hz降低3.6 Hz）。在所有三种实验条件下，对照单位频率都有类似的降低。这表明与最初募集该单位所需的估计量相比，维持运动单位放电所需的估计突触驱动可能减少了40%。对照和测试单位的放电频率以高度平行的方式一起调制，表明单位对由共同的突触输入驱动。这种紧密的相关性进一步验证了使用对照单位放电频率作为测试运动单位突触驱动监测指标的有效性。人类运动神经元内在介导的去极化估计值（中等收缩期间约为40%）与从简化动物标本中运动神经元的细胞内记录获得的平台电位值一致，尽管也讨论了各种其他机制。这表明在中等生理活动期间，类似的内在电导为人类运动神经元提供了大量激活。
