Let's not discard “Noxious” and blur the distinction between nociception and pain

Abstract

W hat is a noxious stimulus? The answer is straightforward when, as Sherrington [1] intended, the domain of inquiry is restricted to the skin. That the same cutting, crushing, and burning stimuli, when applied to a viscus, do not produce pain has been known since before Sherrington, as noted by Cervero and Merskey. To help resolve the seeds of potential trouble sown by Sherrington, Cervero and Merskey donate a significant portion of their article to the meaning of stimulus; they do not address the key issue of adequate stimulus. The external integument is the barrier between environmental challenge and internal comfort (not to put too fine a point on it). When skin is damaged or in threat of being damaged, it is appropriate, if not indeed crucial to our survival, that a nociceptive system be activated. Should we expect that deeper tissue (joints and muscle), and particularly the viscera, be responsive to the same stimuli? Cervera and Merskey point out that cutting the skin is painful, whereas cutting the colon is not, but that does not mean that the stimulus (i.e., the physical event of cutting the colon) did not give rise to an afferent discharge or a response. It is just that the response does not lead to pain. And why should it? It is to be expected that receptors in the skin would be exposed to all variety of potentially damaging stimuli to which they should respond, whereas receptors in the viscera would not be expected to be exposed to the same stimuli. Other visceral stimuli, however, do have analogous significance to the organism: overdistension, ischemia, an inflammatory reaction to invasion or infection, traction on the mesentery, etc. When these stimuli