Brain, Vol. 117, No. 2, 225-234, 1994
© 1994 Oxford University Press
research-article |
Differences in behaviour of sensory and motor axons following release of ischaemia
1Sobell Department of Neurophysiology, Institute of Neurology, The National Hospital London, UK 2Department of Clinical Neurophysiology, The Prince Henry and Prince of Wales Hospitals and Prince of Wales Medical Research Institute, University of New South Wales Sydney, Australia
Correspondence to:
Dr H. Bostock, Sobell Department of Neurophysiology, Institute of Neurology, The National Hospital, Queen Square, London WC1 3BG, UK
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 520 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;441:53757). The notch on the threshold plot for sensory axons lasted 2040 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 sensory axons to discharge ectopically cannot be attributed to a single factor.
ischaemia; paraesthaesiae; motor axon; sensory axon; nerve excitability
Received October 5, 1993. Accepted December 3, 1993.
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