THE PERIPHERAL UNIT FOR PAIN

Abstract

The area usually referred to as a pain spot in the skin has been investigated by stimulation with very brief electrical shocks. Such spots were first located in a region of the forearm, and the nerve twigs innervating the region were then anaesthetised by novocaine one at a time. The unit so exhibited in the region of overlap between the distributions of adjacent twigs, where one anaesthetised spot may be entirely surrounded by normal ones, consists of a very sensitive point surrounded by a field of decrementing sensitivity. Such units appear to overlap but slightly, and the region of overlap is the least sensitive part of such an area. Evidence is presented that such a "unit" spot must be innervated by more than one fiber. Stimulation of 2 spots by simultaneous or alternate bursts of repetitive shocks reveals that summation and 2-point discrimination between them are both influenced by the intensity and frequency of stimulation. Increase of quantity of stimulation decreases the distance at which two-point discrimination for pain is possible, and likewise that beyond which summation of intensity is experienced. There thus appears to be mutual exclusion between these 2 functions for the conditions of these expts. This circumstance suggests that central qualitative interpretation depends on peripheral factors entirely quantitative.