‘Memory’ in this context does not refer to the conscious recollection of painful events, but to the persistence of functional and possibly structural changes in the central nervous system as a result of injury to distant parts of the body.
To most patients, and many doctors, to suggest that pain can persist without a ’cause’ in the ordinary sense of the word appears absurd. We are all familiar with the pain that arises from an acute injury, and it is natural to assume that when pain persists it must be because of some continuing lesion at the site of injury. But there are plenty of examples to the contrary: central (thalamic) pain, phantom limb pain, brachial plexus, for example. Nor is it only severe injury which causes this: sometimes it is as apparently trivial as a prick by a thorn.
It might seem that pain of this kind is a fortunately rare phenomenon. But conceivably it is really very common. It may be that many kinds of chronic pain, even most, are due to persisting changes within the nervous system. It is not possible to say exactly what these changes are, though it has been suggested that they may be reverberating neuronal circuits. There could also be biochemical changes at the cellular level. Those of us who are not experts in the field may be satisfied with a grossly over-simplified picture, and think in terms of analogies such as the loops that may occur in a computer program or even the eddies in a stream.
Whatever the mechanism of pain memory, if you practise acupuncture for any length of time you will soon encounter what seem to be instances of it. And many kinds of treatment seem to depend on it. Massage, manipulation, electrical stimulation stimulation, application of heat and cold, are all examples of ‘counter-irritation’, which appears to act by altering the input of impulses into the CNS and so changing the way in which it registers pain. Acupuncture, I suggest, should be viewed in the same way.
Acupuncture is one method among others for resetting the nervous system. Like the other physical methods I have mentioned, it provides a train of impulses for the central nervous system, and in some cases this can turn off the painful circuit. In many cases this seems to be best achieved by recreating the original painful stimulus in a modified form. Just as a trivial impulse can sometimes turn on pain for many months or even longer, so inserting a needle can sometimes have the opposite effect.