AN INVESTIGATION OF THE GATE CONTROL-THEORY OF PAIN USING THE EXPERIMENTAL PAIN STIMULUS OF POTASSIUM IONTOPHORESIS

This study investigated a prediction derived from gate control theory- that there would be a pulse of pain as a pain stimulus was being rampe d off due to the rapidly transmitting, inhibitory large fiber activity falling away sooner at the spinal level than the excitatory activity of the slow-transmitting, small nociceptive afferents. A further predi ction was that the more distant the peripheral stimulus was from the s pine, the greater the pain pulse would be. Fourteen subjects had the p ain stimulus df ion-tophoretically applied potassium ions (K+) applied to an upper and a lower site on the dominant arm. In a threshold dete ction task using the double random staircase method, subjects were ask ed to indicate whether they could detect a pulse of additional pain du ring this ramp-off phase. The average rate of stimulus ramp-off in ord er to detect a pain pulse was statistically greater for the upper-arm site (14.3 mu g K+/sec) than for the lower-arm site (9.4 mu g K+/sec). These results were consistent with gate control theory. Alternative e xplanations, including intrinsic differences in nociceptive responding for different dermatomes and anode break, were considered. It was con cluded that the detection of a pain pulse during the ramping off of a peripheral pain stimulus potentially provides a quantitative measure o f the spinal modulation of pain.