When an acupuncture point is stimulated various neurological and neurohumoral changes occur in the body. In decerebrate cats, stimulation of the skin inhibits the passage of painful stimuli.7 Work on small animals also shows that stimulating a specific acupuncture point changes the nerve transmission in the painful area, the spinal cord and the thalamus.8 Furthermore, some of the detailed neurophysiological studies completed at the Shanghai Institute of Physiology suggests that there is a supraspinal centre that inhibits viscerosomatic reflexes and that this is stimulated via acupuncture points; there is considerable evidence which would support the mediation of painful stimuli at a thalamic level as being one of the important mechanisms through which acupuncture can have an effect.9
The gate theory of pain10 also goes some way to explain the mechanism of acupuncture in pain. All pain input enters the spinal cord via the substantia gelatinosa, pain impulses travelling along the small nerve fibres. The large myelenated nerve fibres have an inhibitory effect on pain, by closing the gate to pain at cordal level, within the substantia gelatinosa. If pain is not transmitted to the brain, no pain is perceived. Melzack has suggested that acupuncture stimulates large myelenated nerve fibres, thereby closing the gate to pain.11 However, there are a large number of problems with the gate theory of pain, particularly as it is used to explain the mechanism of acupuncture. It is probable that acupuncture does work partially through the gate control theory, although this cannot be seen as a complete explanation of its mechanism.
Endorphins, or naturally occurring morphine-like substances, have recently been found in a wide variety of body tissues. In people suffering from chronic pain the endorphin level in the fluid around the brain, the cerebrospinal fluid, is low. Acupuncture increases the endorphin level in various parts of the central nervous system and beta-endorphin can be shown to attenuate chronic pain.12 This analgesic effect can sometimes be blocked by naloxone, a morphine antagonist13 although other studies suggest that the analgesic effect of acupuncture cannot be reversed by naloxone. Therefore, although the endorphin theory is another very useful idea through which acupuncture can be shown to have an effect, it does not seem to explain the complete physiological mechanism of this therapeutic technique.
These two ideas, the gate control theory of pain and endorphins, are not mutually exclusive. They go some way towards explaining the possible mechanism of acupuncture in pain, but the clinical application of acupuncture is much wider than its use in pain, so these ideas leave a great deal unexplained.
During our course we had a considerable number of lectures on the physiological basis of acupuncture, and some interesting ideas were discussed. As well as having an analgesic effect acupuncture seems to have a sedative effect. Puncturing Zusanli (St 36) bilaterally causes the alpha rhythm in the brain to predominate, and its amplitude to increase. Acupuncture also has a regulatory and anti-shock effect, which has been clearly demonstrated by creating massive blood loss in dogs; the dogs receiving acupuncture show a significantly increased survival rate.14 Acupuncture also affects the immune system; needling increases the white cell count, raises the titres of all groups of immunoglobulins, increases the activity of the reticulo-endothelial system and raises the level of serum complement. The claims that acupuncture can affect the immune system have been made primarily by the Chinese, but the author has also conducted some preliminary research which would seem to confirm this hypothesis. Furthermore, acupuncture has been shown, quite clearly, to cause changes in many of the chemical messengers (neurotransmitters) within many different areas of the brain.15