Temperature dependence of excitability indices of human cutaneous afferents

The temperature dependence of different indices of axonal excitability (thr eshold, latency, refractoriness, supernormality, strength-duration time con stant, and rheobase) was studied for cutaneous afferents of 8 healthy human volunteers using threshold tracking. Cooling from similar to 32-similar to 22 degrees C dramatically increased the threshold for a conditioned potent ial evoked during the relatively refractory period (average increase 573%) but had little effect on the threshold for unconditioned potentials (increa sed by 4% with 0.1-ms test stimuli), strength-duration time constant (incre ased by 18%), or rheobase (decreased by 12%). Cooling increased the latency of the unconditioned test potential by 41%, but this slowing was small com pared with the effect of cooling on the latency slowing attributable to ref ractoriness. This measure of refractoriness was initially 0.17 ms at a cond itioning-test interval of 2 ms, and increased with cooling to 1.30 ms at th e same interval. With cooling, refractoriness was both greater at any one c onditioning-test interval and longer in duration, extending into intervals normally associated with supernormality. It is concluded that, although coo ling affects all excitability indices to some extent, the most prominent fe ature is the increase in refractoriness. By contrast, strength-duration tim e constant is influenced little by temperature. (C) 1999 John Wiley & Sons, Inc.