EXCITABILITY CHANGES IN HUMAN CUTANEOUS AFFERENTS INDUCED BY PROLONGED REPETITIVE AXONAL ACTIVITY

1. The present study was undertaken to document the excitability chang es produced by prolonged high-frequency trains of impulses in cutaneou s afferents of six human subjects. 2. Trains of supramaximal stimuli a t 200 Hz for 2 min or less produced a prolonged depression in excitabi lity, consistent with activation of the electrogenic Na+-K+ pump. Trai ns of longer duration resulted in an initial period of hyperexcitabili ty which, with 10 min trains, was associated with the sensation of par aesthesiae in all subjects. This transient hyper excitability graduall y gave way to a long-lasting period of hypoexcitability. 3. The excita bility changes were reproducible, and were accompanied by correspondin g changes in supernormality, refractoriness, strength-duration time co nstant and rheobase current, suggesting that the changes in axonal. ex citability reflected a change in membrane potential. 4. The transient increase in excitability that follows tetanic trains of 10 min had qua litatively similar effects on cutaneous axons as ischaemia or applicat ion of a depolarizing current. The post-tetanic changes in the superno rmal period of sensory axons were those expected from the changes in e xcitability, without evidence of a gross distortion in its time course , as has been previously demonstrated in a hyperstimulated human motor axon. 5. It is concluded that the post-tetanic hyperexcitability of h uman sensory axons is probably driven by increased K+ accumulation in the restricted diffusion space under the myelin sheath, much as in mot or axons, the differences in behaviour of sensory and motor axons bein g explicable by greater inward rectification in sensory axons.