Diffuse noxious inhibitory controls (DNIC). II. Lack of effect on non-convergent neurones, supraspinal involvement and theoretical implications.

(1) Diffuse noxious inhibitory controls (DNIC) were tested for their effect on noxious only, non-noxious and proprioceptive cells in the dorsal horn of the intact anaesthetized rat. Unlike convergent neurones, as described in the previous paper, there was no effect of DNIC on these neurones. It is concluded that convergent neurones are specifically inhibited by DNIC. (2) The effect of DNIC could not be demonstrated for convergent neurones in the spinal animal. Thus the neuronal substrate for DNIC must involve supraspinal structures. (3) Because of the level of firing in convergent neurones induced by hair and touch receptors, presumably constantly and randomly activated in the freely moving animal, a noxious message arriving at higher centres may be partly masked by this background noise. On the basis of the known role of convergent neurones in nociception, we propose the following mechanism which may interpret this paradoxical convergence: two pools of convergent neurones are influenced by a painful peripheral stimulation, one segmental pool being activated whilst the remaining population of cells is inhibited; the "contrast" between the messages from these two pools may well produce a significant pain signalling output from the convergent dorsal horn cells. (4) These results and their theoretical implications are discussed with regard to the concept of the "analgesic system", certain clinical observations and the paradoxical pain relieving effects of counterirritation and some forms of acupuncture.