Neurophysiological bases of the counterirritation phenomenon: diffuse control inhibitors induced by nociceptive stimulation.

To define the counterirritation phenomenon, we might refer to the Hippocrat ic aphorism: 'If two sufferings take place at the same time, but at differe nt points, the stronger one makes the weaker silent'. On the basis of this clinically common observation, often used advantageously by the patients th emselves, a number of therapeutic methods have been developed which are gro uped under the terms counterirritation or counterstimulation. This phenomen on has not been scientifically analysed until recent years. Experimental re sults gathered during the last decade have shown that counterirritation phe nomena have a well-defined neural substrate both in animals and in man. In particular; they have proved not to rely on segmental mechanisms, but rathe r imply spino-bulbo-spinal loops involving ascending pathways in the antero lateral spinal columns, integration in the lower brain stern, and descendin g influences reaching dorsal horn neurons via the dorsolateral quadrant. Th e results also suggest that the study of counterirritation is essential for accessing the physiology of nociception and pain control. The very existen ce of the counterirritation phenomenon is the easiest demonstrable index of a specific system for pain modulation in man. Besides its scientific inter est, the elucidation of its neurophysiological bases has clinical importanc e, in as much as it may ameliorate our understanding of certain pain syndro mes and contribute to the development of new investigative and therapeutic procedures. (C) 1999 Editions scientifiques et medicales Elsevier SAS.