CHANGES IN EXCITABILITY OF HUMAN MOTOR AXONS UNDERLYING POSTISCHEMIC FASCICULATIONS - EVIDENCE FOR 2 STABLE STATES
CHANGES IN EXCITABILITY OF HUMAN MOTOR AXONS UNDERLYING POSTISCHEMIC FASCICULATIONS - EVIDENCE FOR 2 STABLE STATES
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DOI：
10.1113/jphysiol.1991.sp018766
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发表时间：
1991-09-01
影响因子：
5.5
通讯作者：
REID, G
中科院分区：
文献类型：
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作者：
BOSTOCK, H;BAKER, M;REID, G
1.我们通过记录缺血后(15-20分钟)足以诱发自发运动束的电兴奋性的变化，探讨了人类神经缺血后异位放电的起源。在上臂加压套下刺激尺神经，从小指展肌记录到复合运动动作电位。在缺血15分钟后松开袖带时，短电流脉冲的阈值在两个不同的阶段增加：缓慢阶段，然后快速上升到峰值阈值。快速上升的速度太快，难以跟踪(即在S不到4岁的时间内100%提高门槛)，有时在30-40岁的S之后，也会出现同样迅速的下降。小的极化电流影响阈值快速增加的时间，就好像它发生在特定的膜电位上一样。通过每隔几秒记录一次完整的刺激-反应曲线，我们发现阈值的快速变化与阈值的双峰分布有关。大多数纤维要么处于高阈值状态，要么处于低阈值状态，这两种状态在大约10分钟的时间内汇聚。只有在阈值迅速升高后，才能记录到自发的运动束，当纤维存在于两种阈值状态时。自发活动不是导致这两种状态的原因，因为它们也可以在没有自发活动的情况下被记录下来。建立了人类运动轴突节点和节间的计算机模型，包括在其他轴突中展示的通道类型，并调整通道密度以匹配人类轴突对去极化和超极化电流脉冲的反应。随着细胞外钾浓度的增加，在模型的电流-电压关系中出现了负斜率电导区域，并出现了两个稳定状态，同时电生钠泵的活性也随之增强。模型的两个稳定状态之间的转换可以定性地解释缺血后轴突记录到的快速阈值变化。在该模型中，自发动作电位发生在从高电位态到低电位态的一些转变之后。我们认为，人类缺血后运动束也涉及两种平衡状态之间的转换，发生在具有高细胞外钾和高生电泵活性的轴突中。
1. We have investigated the origin of post-ischaemic ectopic discharges in human nerve by recording changes in electrical excitability following periods of ischaemia (15-20 min) sufficient to induce spontaneous motor fasciculations. The ulnar nerve was stimulated beneath a pressure cuff on the upper arm, and compound motor action potentials recorded from abductor digiti minimi.2. On releasing the cuff after 15 min of ischaemia, thresholds to short current pulses increased in two distinct phases: a slow phase followed by a rapid rise to a peak threshold. The rapid rise was too fast to track (i.e. 100% threshold increase in less than 4 s), and was sometimes followed after 30-40 s by an equally rapid fall. Small polarizing currents affected the timing of the rapid threshold increase, as if it was occurring at a particular membrane potential.3. By recording complete stimulus-response curves every few seconds, we found that the rapid threshold changes were associated with a bimodal distribution of thresholds. Most fibres were found in either a high-threshold or low-threshold state, and these two states converged over a period of about 10 min.4. Spontaneous motor fasciculations were only recorded after the rapid rise in threshold and when the fibres existed in two threshold states. The spontaneous activity was not responsible for inducing the two states, since they could also be recorded in its absence.5. A computer model of a human motor axon node and internode was constructed, incorporating channel types demonstrated in other axons, and channel densities adjusted to match the responses of human axons to depolarizing and hyperpolarizing current pulses. An increase in extracellular potassium concentration produced a region of negative slope conductance in the current-voltage relationship of the model, and the appearance of two stable states with enhanced activity of the electrogenic sodium pump.6. Transitions between the two stable states of the model could account qualitatively for the rapid threshold changes recorded from post-ischaemic axons. In the model, spontaneous action potentials occurred following some transitions from the high potential state to the low potential state. We suggest that post-ischaemic motor fasciculations in man also involve transitions between two equilibrium states, occurring in axons with high extracellular potassium and high electrogenic pump activity.