Kiernan M C, Bostock H
Sobell Department of Neurophysiology, Institute of Neurology, Queen Square, London WC1N 3BG, UK.
Brain. 2000 Dec;123 Pt 12:2542-51. doi: 10.1093/brain/123.12.2542.
Multiple nerve excitability measurements have been proposed for clinical testing of nerve function, since excitability measures can provide evidence of altered axonal membrane properties and are complementary to conventional nerve conduction studies. An important determinant of excitability is membrane potential, and this study was undertaken to determine the changes in a range of excitability properties associated with alterations in membrane potential. Membrane potential was varied directly using DC polarizing currents and indirectly by ischaemia. The median nerve was stimulated at the wrist and the resultant compound muscle action potentials recorded from abductor pollicis brevis. Stimulus-response behaviour, strength-duration time constant (tau(SD)), threshold electrotonus to 100-ms polarizing currents, a current-threshold relationship and the recovery of excitability following supramaximal activation were each followed in four normal subjects during the two manoeuvres, using a recently described protocol. Membrane depolarization and ischaemia produced an increase in axonal excitability, an increase in the slope of the current-threshold relationship, a 'fanning in' of responses during threshold electrotonus, a decrease in super-excitability, and increases in both tau(SD) and the refractory period. Changes in the opposite direction occurred with membrane hyperpolarization and during the post-ischaemic period. One excitability parameter differentiated between the direct and indirect changes in membrane potential: late subexcitability was sensitive to polarizing currents but relatively insensitive to ischaemia, probably because of compensatory changes in extracellular potassium ions. These results should enable multiple excitability measurements to be used as a tool to identify changes in axonal membrane potential in neuropathy.
由于兴奋性测量可以提供轴突膜特性改变的证据，并且是传统神经传导研究的补充，因此已经提出了多种神经兴奋性测量方法用于神经功能的临床测试。兴奋性的一个重要决定因素是膜电位，本研究旨在确定与膜电位改变相关的一系列兴奋性特性的变化。膜电位通过直流极化电流直接改变，通过缺血间接改变。在腕部刺激正中神经，并从拇短展肌记录由此产生的复合肌肉动作电位。在两名受试者进行的两次操作过程中，使用最近描述的方案，对四名正常受试者的刺激-反应行为、强度-持续时间时间常数（tau(SD)）、对100毫秒极化电流的阈下电紧张、电流-阈值关系以及超强激活后兴奋性的恢复进行了跟踪。膜去极化和缺血导致轴突兴奋性增加、电流-阈值关系斜率增加、阈下电紧张期间反应“扇形散开”、超兴奋性降低以及tau(SD)和不应期增加。膜超极化和缺血后期间则出现相反方向的变化。一个兴奋性参数区分了膜电位的直接和间接变化：晚期亚兴奋性对极化电流敏感，但对缺血相对不敏感，这可能是由于细胞外钾离子的代偿性变化。这些结果应能使多种兴奋性测量用作识别神经病变中轴突膜电位变化的工具。
