Quantitative description of the voltage dependence of axonal excitability in human cutaneous afferents

The voltage dependence of indices of axonal excitability were quantified for cutaneous afferents in eight normal subjects, using the threshold for a target compound sensory action potential as a measure of membrane potential. The membrane potential was altered using subthreshold depolarizing and hyperpolarizing currents of various sizes (-50% to +50% of threshold). Refractoriness and supernormality were determined as the threshold change required to produce the target potential when preceded by a supramaximal stimulus at appropriate conditioning-test intervals. The strength-duration time constant (tau(SD)) was calculated from the threshold currents using unconditioned test stimuli of 0.1 and 1 ms. There was a near-linear relationship between each of these indices and the reciprocal of threshold (a measure of 'excitability'). It is argued that the voltage dependencies of refractoriness and tau(SD) largely reflect the behaviour of transient and persistent Na+ channels, respectively, and that the present data therefore quantify aspects of Na+ channel behaviour in human nerves.