Responses of thoracic dorsal horn interneurons to cutaneous stimulation and to the administration of algogenic substances into the mesenteric artery in the spinal cat.

The effects of the injection of algogenic substances (bradykinin, acetylcholine) into the inferior mesenteric artery were studied at the thoracic level on 47 dorsal horn interneurons responding to cutaneous stimulation. Each unit was characterized by its electrophysiological properties and carefully located within the cord by extracellular injection of pontamine sky blue. Twenty cells, driven only by non-noxious cutaneous stimulation and mainly located in lamina IV, were not affected by the administration of algogenic substances. The activity of 25/27 cells, excited by both non-noxious and noxious cutaneous stimulation and mainly located in lamina V, was strongly modified by nociceptive visceral stimulation, induced by bradykinin and acetylcholine: 8/27 cells were activated, 14/27 were inhibited, 3/27 had a mixed inhibitory-excitatory response. From our study it clearly appears that nociceptive visceral messages only project on dorsal horn cells receiving noxious cutaneous afferents. Thus viscerosomatic convergence seems only to concern nociceptive messages; the existence of this kind of convergence reinforces the hypothesis suggested by several authors to explain referred pain from a neurophysiological point of view.