Slow Potentials, Receptive Field Plasticity and Pain

Abstract

A multitude of sensations are experienced as a consequence of the wide range of external stimuli that impinge upon the body surface. In spite of this diversity a simple distinction can readily be made for each individual sensation, is it painful or not? The relationship between stimuli that produce painless or painful sensations is not however fixed. After injury, stimuli that would normally not be expected to produce pain can begin to do so. This is the phenomenon of post injury pain hypersensitivity or hyperalgesia. If we examine the relationship between stimulus intensity and the sensory responses evoked (Fig. 1) we see firstly that beyond the threshold for detecting the stimulus (S_t) there is a range of stimulus intensities (b) that produce increasing but non-painful sensations. At a certain critical point the stimulus intensity reaches a level where the sensation changes from being painless to being painful (the pain threshold, S_p). Further increases in stimulus intensity ultimately injure the tissue (S_i). The range of stimuli that exceed Sp are effectively what we call noxious (c in Fig. 1) , and are characterized as being potentially or actually damaging to the tissue. Stimuli below these intensities (a and b) are innocuous and are never damaging. This relationship is sufficiently robust that we can normally easily predict from a potential stimulus, whether it is likely to be painful or not, and is a reflection of the specificity of function of primary sensory neurons.