Gorassini M A, Bennett D J, Yang J F
Division of Neuroscience, Faculty of Medicine, University of Alberta, Edmonton, Canada.
Neurosci Lett. 1998 May 8;247(1):13-6. doi: 10.1016/s0304-3940(98)00277-8.
Motoneurons of invertebrates and vertebrates can continue to fire repetitively after being activated by a brief, excitatory synaptic input (self-sustained firing). This firing behavior is due to the activation of intrinsic, voltage-gated currents which produce sustained regenerative depolarizations (plateau potentials) of the cell. Examination of these intrinsic cellular properties has been performed in reduced animal preparations and it is unknown if such self-sustained firing occurs in motoneurons of the intact human. In this paper, we present evidence of this in the human by using a technique of dual motor unit recordings. Subjects were instructed to maintain a constant dorsiflexion effort, and the common synaptic input (e.g. descending drive) onto the tibialis anterior (TA) motoneuron pool was monitored by recording the firing frequency of a low threshold 'control' unit. Once the firing rate of the control unit was constant, vibration of the TA tendon recruited a second 'test' unit which continued to fire after the vibration (i.e. synaptic input) was removed, even though the firing rate of the control unit (and thus, the common drive) remained the same or decreased. Self-sustained firing of motoneurons such as this may reduce the need for prolonged synaptic input when constant muscle activation is required (e.g. for postural tone).
无脊椎动物和脊椎动物的运动神经元在受到短暂的兴奋性突触输入激活后，能够持续重复放电（自持放电）。这种放电行为是由于内在的电压门控电流被激活，这些电流会产生细胞的持续再生性去极化（平台电位）。对这些内在细胞特性的研究是在简化的动物标本中进行的，完整人类的运动神经元是否会出现这种自持放电尚不清楚。在本文中，我们通过使用双运动单位记录技术，提供了人类中存在这种现象的证据。受试者被要求保持恒定的背屈用力，通过记录低阈值“对照”单位的放电频率，监测胫前肌（TA）运动神经元池上的共同突触输入（如下行驱动）。一旦对照单位的放电率恒定，TA肌腱的振动会募集第二个“测试”单位，即使对照单位的放电率（以及因此的共同驱动）保持不变或下降，该测试单位在振动（即突触输入）去除后仍会继续放电。当需要持续的肌肉激活（例如维持姿势张力）时，这样的运动神经元自持放电可能会减少对长时间突触输入的需求。
