Kudina Lydia P, Andreeva Regina E
Institute for Information Transmission Problems, Kharkevich Institute, Russian Academy of Sciences, Moscow, Russia
Institute for Information Transmission Problems, Kharkevich Institute, Russian Academy of Sciences, Moscow, Russia.
J Neurophysiol. 2017 Aug 1;118(2):1355-1360. doi: 10.1152/jn.00233.2017. Epub 2017 Jun 14.
Excitability of motor axons is critically important for realizing their main function, i.e., transmitting motoneuron firing to muscle fibers. The present study was designed to explore excitability recovery and firing behavior in single slow axons transmitting human motoneuron firing during voluntary muscle contractions. The abductor digiti minimi, flexor carpi ulnaris, and tibialis anterior were investigated during threshold stimulation of corresponding motor nerves. Motor unit (MU) firing index in response to testing volleys evoking M-responses was used as a physiological measure of axonal excitability and its changes throughout a target interspike interval (ISI) were explored. It was shown that axons displayed an early irresponsive period (within the first ~2-5 ms of a target ISI) that was followed by a responsive period (for the next 5-17 ms of the ISI), in which MUs fired axonal doublets, and a later irresponsive period. At the beginning of the responsive period, M-responses showed small latency delays. However, since at that ISI moment, MUs displayed excitability recovery with high firing index, slight latency changes may be considered as a functionally insignificant phenomenon. The duration of axonal doublet ISIs did not depend on motoneuron firing frequencies (range 4.3-14.6 imp/s). The question of whether or not traditionally described axonal recovery excitability cycle is realistic in natural motor control is discussed. In conclusion, the present approach, exploring, for the first time, excitability recovery in single slow axons during motoneuron natural activation, can provide further insight into axonal firing behavior in normal states and diseases. Excitability of single slow axons was estimated by motor unit firing index in response to motor nerve stimulation, and its changes throughout a target interspike interval were explored during transmitting human motoneuron natural firing. It was found that axons exhibited early irresponsive, responsive, and later irresponsive periods. Findings question whether the traditionally described axonal excitability recovery cycle is realistic in natural motor control.
运动轴突的兴奋性对于实现其主要功能（即将运动神经元的放电传递至肌纤维）至关重要。本研究旨在探究在自愿性肌肉收缩过程中，传递人类运动神经元放电的单个慢轴突的兴奋性恢复及放电行为。在对相应运动神经进行阈刺激时，对小指展肌、尺侧腕屈肌和胫骨前肌进行了研究。将运动单位（MU）对引发M反应的测试刺激串的放电指数用作轴突兴奋性的生理指标，并探究其在整个目标峰间间隔（ISI）内的变化。结果表明，轴突表现出一个早期无反应期（在目标ISI的最初约2 - 5毫秒内），随后是一个反应期（在ISI接下来的5 - 17毫秒内），在此期间运动单位发放轴突双脉冲，以及一个后期无反应期。在反应期开始时，M反应显示出较小的潜伏期延迟。然而，由于在该ISI时刻，运动单位以高放电指数表现出兴奋性恢复，轻微的潜伏期变化可被视为功能上无显著意义的现象。轴突双脉冲ISI的持续时间并不取决于运动神经元的放电频率（范围为4.3 - 14.6次/秒）。讨论了传统描述的轴突恢复兴奋性周期在自然运动控制中是否现实的问题。总之，本方法首次探究了在运动神经元自然激活过程中单个慢轴突的兴奋性恢复，能够为正常状态和疾病下的轴突放电行为提供进一步的见解。通过运动单位对运动神经刺激的放电指数来估计单个慢轴突的兴奋性，并在传递人类运动神经元自然放电过程中探究其在整个目标峰间间隔内的变化。结果发现轴突表现出早期无反应期、反应期和后期无反应期。这些发现对传统描述的轴突兴奋性恢复周期在自然运动控制中是否现实提出了质疑。
