Mechanisms of acupuncture analgesia

Abstract

The mechanisms involved in acupuncture analgesia are gradually being clarified. The old Chinese meridian theory has no scientific foundation. The explanation has to be sought in terms of neurophysiology and psychophysiology. The first step in this direction has come from Chinese physiologists in animal experiments who have recorded recently effects of acupuncture at several levels of the central nervous system. These results are described and attempts are made to place the observations within the framework of Western concepts of pain mechanisms. Experimental data have been provided by Chinese physiologists suggesting that a humoral analgesic agent is possibly liberated in the brain during prolonged acupuncture. No such substance is produced at the site of stimulation. If confirmed, this humoral factor(s) may account for the generalized and prolonged acupuncture effect, but it cannot explain why the effects are best when stimulating in the same segments as those of the site of pain. This segmental analgesia must have a neural explanation; the total degree of analgesia achieved therefore may represent a summation of several effects. The nature of the postulated humoral factor is unknown, although it has been shown that reserpine augments acupuncture analgesia. Acupuncture analgesia is dependent on impulses in sensory nerves, mainly from deep pressure and possibly stretch receptors; these are transmitted to the spinal cord chiefly by myelinated group II fibers. Acupuncture inhibits spinal transmission of impulses evoked by painful stimulation and reduces 'pain discharges' in cells of the medial thalamus and midbrain reticular formation. Caudate stimulation potentiates acupuncture analgesia. Transmission of 'pain impulses' through the spinal cord is also under tonic inhibition from supraspinal levels, an important mechanism which makes it possible for activities subserving attention, emotion and past experience to control sensory input. Of particular interest in this context is the recently discovered anti nociceptive or analgesic 'serotonin system' located in midline structures of the brain stem, which exerts its action through descending inhibition. Reserpine increases this effect. It is postulated by the author that this anti nociceptive system may be involved in acupuncture analgesia. Last but not least psychical factors which reduce anxiety, apprehension and tension undoubtedly contribute to the analgesic effect. The success of producing analgesia and pain relief by suggestion, placebos and acupuncture is to a great extent a matter of reducing anxiety. Confidence in the method and compliant behaviour are created in China by cultural, educational and propagandistic factors, strengthened by a close patient doctor relationship. Careful selection of the patients suitable for acupuncture and preoperative administration of sedatives and analgesics add considerably to the success. Diverted attention during surgery appears to be of less importance and does not explain the long lasting effect. In animals, the 'still reaction' or 'hypnosis', an inborn reflex evoked mainly by restraint from movements and characterized by immobility and unresponsiveness, may contribute to the analgesic effect achieved by acupuncture. 93 references are cited.