Adaptation of thermal pain in the skin

¹ Aviation Medical Acceleration Laboratory, U. S. Naval Air Development Center, Johnsville; and Department of Physiology, University of Pennsylvania, Philadelphia, Pennsylvania

Cutaneous pain thresholds were determined on blackened skin of foreheads and forearms of human subjects over areas of 16 cm² by recording skin temperature during exposure to thermal radiation for periods up to 50 min. Intensity of stimulus was controlled by the subject so that threshold pain was maintained throughout the exposure. After the initial period of adjustment by the subject, radiation intensity was generally maintained constant although skin temperature for the pain threshold decreased from 44.9 C to 43.8 C. By using an intensity as low as 22 mcal/cm²/sec, threshold pain was evoked in 29 min at a skin temperature of 42.2 C. In both groups, once pain had been established it did not disappear. It is inferred from these observations that thermal pain does not adapt for near-threshold stimulation in the period between onset of pain at 30 sec and termination of stimulation.

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