Differences in behaviour of sensory and motor axons following release of ischaemia

Abstract

The changes in excitability and supernormality of sensory and motor axons of the median (or ulnar) nerve were tracked during and following ischaemia at the wrist for periods of 5-20 min in normal human volunteers. Supernormality was defined as the fractional increase in excitability produced by a maximal conditioning stimulus, 10 ms before the test stimulus. With relatively brief periods of ischaemia (< 10 min), sensory and motor axons behaved similarly, with an increase in excitability (producing a decrease in threshold) and a decrease in supernormality during ischaemia and a long-lasting decrease in excitability (and increase in supernormality) following release of ischaemia. Most subjects reported paraesthesiae during brief periods of ischaemia but not after its release. No one experienced fasciculation. The threshold changes were generally similar during longer periods of ischaemia, but in the post-ischaemic phase the behaviour of sensory and motor axons diverged. After a rapid post-ischaemic increase, the threshold of sensory axons decreased, approaching the pre-ischaemic level, before rising again and then slowly returning to the control level. Sensory axons of different threshold behaved in a qualitatively similar manner, with no evidence of a bimodal distribution of thresholds in the post-ischaemic phase (as occurs with motor axons when the ischaemia is sufficient to produce fasciculation; see Bostock et al. J. Physiol (Lond) 1991; 441: 537-57). The 'notch' on the threshold plot for sensory axons lasted 20-40 min and was accompanied by a relatively small but appropriate change in supernormality. No such 'notch' was seen with motor axons. The changes in latency were generally similar for sensory and motor axons, largely paralleling the supernormality plots, except at the time of the 'notch'. To test the hypothesis that the differences in behaviour of sensory and motor axons resulted from differences in inward rectification activated by hyperpolarization, the changes in threshold produced by long-lasting (300 ms) depolarizing and hyperpolarizing current pulses were compared for sensory and motor axons. In seven of eight subjects, there was evidence of more inward rectification in sensory axons. In the eighth subject, motor axons behaved similarly to sensory axons. It is concluded that a difference in inward rectification contributes to but is insufficient by itself to account for the differences in behaviour of sensory and motor axons and that the greater propensity of sensor y axons to discharge ectopically cannot be attributed to a single factor.