According to the old model of pain perception, which goes back as far as Descartes for its ultimate inspiration, the nervous system is something like a telephone system. If you tread on a drawing pin, say, a pain impulse travels up the nerves from your foot to your spinal cord and thence to your brain, where in some wholly mysterious way it gives rise to a pain in consciousness. This model is a passive one, in that transmission of the painful stimulus is supposed to happen automatically provided the nervous pathways are intact. Melzack and Wall (1992) have pointed out that there are serious difficulties with this scheme. Sometimes a severe injury causes little pain, or a relatively trivial injury may cause agonizing pain. Again, pain may persist for months or years after the original injury has healed completely.
The new model proposed by Melzack and Wall is based on the idea that the brain does not just attend to single messages coming along specific nerve fibres but instead monitors all the information at its disposal before registering pain. This is the basis of the ‘gate theory theory’. To describe this in the sketchiest possible outline, the spinal cord and brain stem is supposed to contain ‘gates’ which can open or close to allow pain impulses to travel to the brain or not, as the case may be. Afferent impulses from the periphery can open or close the gates, according to the type of nerve fibre involved: large diameter fibres close the gates, small diameter fibres open them. This explains why rubbing the site of an injury can relieve pain.
The gates are also supposed to be influenced by efferent or descending impulses from higher centres in the brain, including those concerned with consciousness. This helps to explain how psychological factors alter our perception of pain and why patients who are afraid of acupuncture or unwilling to have it seldom do well.
Melzack has himself applied these ideas to acupuncture. One difficulty with such theories is to explain how the brief insertion of a needle can cause pain relief lasting for days, weeks, or even permanently. One suggestion is that the nervous system is continually bombarded by impulses arising from the persisting microtrauma inflicted by the needle; another is that the initially temporary relief of pain allows the patient to use the part more freely and hence to provide a more normal input of impulses into the central nervous system. Repeated acupuncture would enhance this effect and so set up a ‘virtuous circle’ of progressive relief of pain.
Whether the gate theory is correct in detail or not is not critically important for the working acupuncturist. What matters is the idea of the nervous system as a dynamic, constantly changing and evolving interplay of patterns, in which it is not surprising to find that altering the input by inserting needles can produce quite profound alterations in function. The following diagram indicates some of the factors involved.
The opioid peptides and the gate theory theory, though of importance as providing a theoretical underpinning for acupuncture, don’t have a great deal of relevance to everyday practice. There are however two other ideas which do have a great deal of practical relevance: pain memory and trigger points points (TPs).