Hyperexcitability of cutaneous afferents during the supernormal period. Relevance to paraesthesiae

The excitability of cutaneous afferents in the median nerve of 20 normal human subjects was investigated using a supramaximal conditioning stimulus and a submaximal test stimulus that produced an afferent volley of 20 to 40% of maximum. The intensity of the test stimuli was 1.1 to 1.25 times threshold for the first recruited axons (defined as a compound action potential of approximately 0.5 microV). The amplitude of the test volley did not reach the control amplitude until conditioning-test intervals of 3.4 ms. This was followed by a period of increased axonal excitability which reached a peak at intervals of 5 to 8 ms and declined gradually into a late phase of subnormality at 15 to 20 ms. The amplitude of the submaximal test potential was potentiated by, on average, 47% (range 9-146%) during the supernormal period. There were no significant changes in latency. The time course and the degree of potentiation were reproducible in the same subject on separate occasions. When the limb was cooled, the latency of the onset of supernormality was increased and the degree of supernormality was decreased and ultimately abolished. There was an inverse relationship between the degree of potentiation and the size of the test volley. High-frequency repetitive stimulation of the digital nerves for 10 min modified axonal excitability and produced spontaneous paraesthesiae that lasted for 20 min after the stimulation. Following this prolonged stimulus train, single submaximal stimuli evoked larger cutaneous afferent potentials than before the train, and the extent of the potentiation during the supernormal period was increased. It is concluded that prolonged repetitive stimulation of cutaneous afferents has two effects, one on axonal threshold, the second on the supernormal period. It is suggested that, in this experimental model, paraesthesiae result from ectopic impulses generated when axonal excitability is highest, namely at the peak of the supernormal period.