A noninvasive strategy for identifying and quantifying innocuous and nociceptive peripheral afferent activity evoked by nerve stimulation.

This study evaluated the utility of the compound nerve action potential (CAP) and spinal nociceptive withdrawal (R3) reflex in identifying and quantifying peripheral afferent activity evoked by sural nerve stimulation in humans. The results of this work demonstrate that currents less than or equal to that which elicits a just-maximal CAP can be considered purely innocuous; provide further evidence that the R3 is an objective means of identifying noxious stimulus levels; and suggest that current provides the best noninvasive quantitative estimate of afferent activity throughout the innocuous and noxious range. This work also demonstrates that some of the individual variability in the psychophysical function can be attributed to peripheral factors that affect the amount of current reaching the nerve. It is important, therefore, that these peripheral factors be considered when studying individual differences in the psychophysical function generated by electrical stimulation.