Pain modulation triggered by high-intensity stimulation: implication for acupuncture analgesia?

The spinal transmission of nociceptive signals is modulated by both spinal and supraspinal systems. Spinal networks underlie segmental inhibitions, which are mediated via myelinated cutaneous afferent fibres activated by mechanical stimuli (e.g. rubbing, etc.) Diffuse noxious inhibitory controls (DNIC) are triggered by peripheral Adelta- and C-fibres, involve brain structures confined to the caudal-most part of the medulla including the subnucleus reticularis dorsalis (SRD) and are mediated by descending pathways in the dorsolateral funiculi. Combined psychophysical measurements and recordings of nociceptive reflexes (RIII) in both normal volunteers and patients with lesions in the spinal cord or brain suggest the existence of identical or equivalent inhibitory processes in man. Both these mechanisms could be triggered to produce hypoalgesia by acupuncture needles or moxibustion. When applied near a painful focus, the techniques have similarities to high-frequency, low-intensity transcutaneous nerve stimulation, which brings segmental inhibitory mechanisms into play. When applied to remote areas, the techniques might be related to "counter-irritation" or "counter-stimulation" phenomena which, in addition to involving complex autonomic functions, seem to involve DNIC. Experimental data in both animals and humans support these conclusions. (C) 2002 Elsevier Science B.V. All rights reserved.