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Department of Clinical Neurophysiology, Price Henry Hospital, University of New South Wales, Sydney, Australia.

1. The depression in axonal excitability that follows short trains of impulses (H1) may lead to spike frequency adaptation to a sustained stimulus, and has been attributed to a slow K+ conductance. The present experiments sought indirect evidence for slow K+ channels at the node of Ranvier of human cutaneous afferents based on the demonstration of post-tetanic changes in excitability typical of H1. 2. The excitability changes in low-threshold cutaneous afferents in the digital nerves of the index finger were explored using a submaximal test pulse conditioned by trains of supramaximal stimuli, containing up to 100 impulses. Changes in the amplitude of the compound sensory action potential set up by a constant test stimulus were used as a measure of the changes in excitability. These changes in amplitude were paralleled by inverse changes in latency. 3. When the conditioning stimulus was a single supramaximal pulse, excitability was enhanced at conditioning-test intervals of 4-40 ms, with a peak at 6-8 ms. When the conditioning stimulus consisted of a train of ten pulses delivered at 200 Hz, the recovery cycle was dominated by subnormality that was maximal at 20 ms and subsided gradually over 50 ms. 4. The post-train depression in excitability increased as the number of pulses in the conditioning train increased to ten but changed little with further increases in train duration. The degree of depression increased with the pulse frequency within the train. Cooling the hand from a skin temperature of 35 to 25 degrees C slowed the recovery processes but did not alter the magnitude of the post-train depression. 5. These characteristics are typical of the H1 phase of post-tetanic depression in axonal excitability. The extent of the depression in excitability suggests, first, that there may be a significant K+ conductance at the nodes of human cutaneous afferents and, secondly, that H1 may play a significant role in limiting repetitive discharge in normal and pathological afferents.

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