Brain, Vol. 112, No. 1, 147-164, 1989
© 1989 Oxford University Press
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CHANGES IN EXCITABILITY OF HUMAN CUTANEOUS AFFERENTS FOLLOWING PROLONGED HIGH-FREQUENCY STIMULATION
Unit of Clinical Neurophysiology, Department of Neurology, The Prince Henry Hospital and School of Medicine, University of New South Wales Sydney, Australia
Correspondence to:
Correspondence to Professor David Burke, Department of Neurology, The Prince Henry Hospital, Little Bay, NSW 2036, Australia.
Prolonged high-frequency stimulation of cutaneous nerves can result in paraesthesiate that begin 20 to 30 s after the end of the train and last for 5 to 10 min In the present experiments the effects of such stimulation on the excitability of human cutaneous afferents and on their refractory and supernormal periods were measured to determine whether these changes could explain the postactivation paraesthesiae. Attention was focused on the axons of lowest threshold (1.0–1.5 T) in the compound sensory action potential evoked by stimulating the digital nerves of the index or middle fingers. Repetitive activation produced two opposing effects on the excitability of low-threshold cutaneous afferents. Following stimulus trains of short duration (1–5 min) the dominant effect was a long-lasting decrease in excitability, such that the amplitude of a test afferent volley was always less than before stimulation With these trains, no subject experienced paraesthesiate. For 10 min after stimulus trains lasting longer than 7 to 12 min the dominant effect was an increase in excitability such that the amplitude of the test volley was greater than before stimulation. Within this interval, following such trains, subjects experienced paraesthesiae. The extent and duration of supernormality induced by a supramaximal conditioning stimulus were greatly increased by stimulation for 1 min. Following stimulation for 10 min, the degree of supernormality of the enhanced test volley was much the same as before stimulation, but was inappropriately high for the size of the test volley. The sum total of the excitability change and the change in supernormality resulted in a larger potential after stimulation, whether the train lasted 1 min or 10 min It is concluded that the postactivation changes in axonal excitability could predispose the most excitable axons to generate ectopic impulses and, thereby, to produce paraesthesiae.
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Received July 27, 1987. Revised March 18, 1988. Accepted April 15, 1988.
* Current address Department of Neurology and Neurological Surgery, Washington University School of Medicine, 660 South Euclid Avenue, St Louis, Missouri 63110, USA.
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