Refractory period of human motor nerve fibres.

Using a collision technique, the ulnar nerve was made refractory for a shorter distance normally covered in 0.5 ms or a longer distance covered in 1.5 ms. Studying the shorter refractory segment with paired shocks of maximal intensity, the test response first appeared (more than 5% of unconditioned response) at an interstimulus interval of 1.16 +/- 0.18 ms (mean +/- SD in 20 ulnar nerves). The conduction velocity of the test impulse then was 55.3% +/- 19.2% of normal. Recovery in amplitude of the test response was nearly complete (more than 95%) at 2.11 +/- 0.50 ms, when it was conducting at a speed of 81.2 +/- 17.4% of normal. The conduction velocity recovered to a level above 95% of normal at 2.65 +/- 0.65 ms. Whereas recovery in amplitude of the test response was unrelated to the length of the refractory segment, change in latency was greater with the longer refractory segment, although not in proportion to the distance.