Testing Assumptions in Human Pain Models: Psychophysical Differences Between First and Second Pain.

Acute pain arises from activation of myelinated (A delta) and unmyelinated (C) nociceptive afferents, leading to first (A-fiber) or second (C-fiber) pain sensations. The current study sought to investigate first and second pain within glabrous and hairy skin sites in human upper limbs. Fifty healthy adults (25 male/25 female, 18-30 years old, mean = 20.5 ± 1.4 years) participated in a psychophysical study investigating electronically rated, thermal first and second pain sensations within the glabrous skin at the palm and hairy skin of the forearm. Repeated measures analysis of variance indicated that the threshold for first pain was lower (more sensitive) than for second pain (P = .004), for glabrous as well as hairy skin, and thresholds at glabrous skin were higher than for hairy skin (P = .001). Hairy skin presented a steeper slope for testing, whereas there were no differences in slope between first and second pain. The study findings support assumptions associated with mechanistic differences between first and second pain sensations, while offering a novel method for producing first and second pain with the same thermal stimulus. Efforts to understand abnormalities among people with clinical pain and development of new therapeutic agents will benefit from specific psychophysical methods. PERSPECTIVE: This article presents a novel method for directly comparing first and second pain within the same thermal stimulus. The ability to directly compare first and second pain sensations can aid in understanding pain abnormalities in clinical pain and development of therapeutic aids.