Acupuncture Mechanisms

Revised 25-05-2019

Introduction

Since the 1970s there has been a good deal of research to find out how acupuncture works. The early studies mostly focused on the endogenous opioids, which had been discovered not long before and seemed to provide a rational basis for acupuncture analgesia. While these are still part of the story there is much interest these days in other ideas, including the use of brain imaging techniques to study central changes in acupuncture.

In outline, there seem to be five types of mechanism involved in pain modulation by acupuncture.

Local changes in the tissues
Myofascial trigger points
Segmental analgesia (spinal cord)
Extrasegmental analgesia (spinal cord)
Central regulation (brain stem, thalamus, limbic system, cerebral cortex)

There is a good discussion of all this in Section 2 of Medical Acupuncture: A Western Scientific Approach, edited by Jacqueline Filshie, Adrian White, and Mike Cummings (Elsevier 2016). Rather than try to summarise it here I shall look at some aspects of neurophysiology that seem to me to be of particular importance and interest. These mainly concern pain.

The modern view of pain

Our understanding of pain has undergone a major transformation since the middle of the twentieth century. To put it at its simplest, we have seen that pain processing is dynamic and active, not passive and static. It depends on a balance between excitation and inhibition. To modify pain we can either decrease excitation or increase inhibition. Acupuncture for pain relief is largely a means of increasing inhibition.

Another way of thinking about chronic pain is as a form of faulty learning.

Pain memory

In this context 'memory' refers to a change in an organism that affects its subsequent behaviour. It is not necessarily a question of conscious memory, so acquired immunity is an example of memory in this sense.

Not all kinds of memory are as useful as is acquired immunity. Sometimes it's just the opposite—think of autoimmune disease. Chronic pain is due to unwanted memory that has some resemblance to autoimmunity. It can be thought of a form of faulty learning, in which case pain relief by acupuncture is produced by erasure of the unwanted memory. The mechanism by which this happens is still not entirely clear but it may be connected with a phenomenon known as LTP.

LTP

Long-term potentiation (LTP) was discovered in 1968 in the rabbit hippocampus. It is now known to be widespread throughout the central nervous system and is thought to be related to memory formation. It depends on the strengthening of synaptic links between neurons. It occurs in the pain pathways, where C-fibre inputs increase LTP in the posterior horn cells.

Acupuncture may be a means of counteracting LTP in cases of chronic pain.

Pain and the limbic system

The limbic system is involved in memory formation and this part of the brain is affected by acupuncture, so here is another link with memory. But parts of this system, particularly the anterior cingulate cortex, are concerned with the emotional response to pain. Acupuncture has been shown to reduce activity in the anterior cingulate, which may help to explain the clinical observation that acupuncture sometimes makes pain less unpleasant even when it does not eliminate it completely.

'Just a placebo?'

The principal objection raised by critics of acupuncture is that it is simply an elaborate placebo. One answer to this is to point out that placebos must ultimately work by modifying the way the nervous works (what else could they do?) and acupuncture is a means of modifying the activity of the nervous system, so to call acupuncture a placebo doesn't mean that its effects are any sense unreal. But we need more than this.

Acupuncture is normally done in conscious patients. (There is not much evidence to show that it works in patients who are unconscious.) Pain can be modulated by attention and expectation.

For example, Benedetti and colleagues induced pain by the injection of capsaicin into several body areas simultaneously while appling a placebo cream to just one of these areas. This produced analgesia only in the area to which the cream was applied; the effect was blocked by naloxone.

Most important, this specific effect is mediated by endogenous opioids, indicating that placebo-activated opioids do not act on the entire body but only on the part where expectancy is directed. This suggests that a highly organized and somatotopic network of endogenous opioids links expectation, attention, and body schema. [emphasis added] [Benedetti F, Arduino C, Amanzio M. Somatotopic activation of opioid systems by target-directed expectations of analgesia. The Journal of Neuroscience 1999;19(9):3639-3648.

The important thing to keep in mind is that there is more to acupuncture than the needles. An acupuncture session usually involves a preliminary examination for myofascial trigger points, for example—in other words touching the patient. There may also be the sight of the needles being inserted. And of course there is the information aspect—what the patient understands about what is happening. All these features, as well as others that neither the acupuncturist nor the patient may be fully aware of, influence the outcome. This means that acupuncture should be thought of as a form of multisensory analgesia.

Multisensory analgesia

[Campbell A. Seeing the body: a new mechanism for acupuncture analgesia. Acupunct Med 2013 Sep;31(3):315-8. doi: 10.1136/acupmed-2013-010357.]

This is a relatively new idea although the first example, the gate theory of pain, which gave rise to treatments such as transcutaneous electrical nerve stimulation (TENS), dates from the 1960s. But we now know that other sensory inputs can also modulate pain, including stimulation of the cochlear vestibular apparatus and vision. These observations have led to the concept of multisensory analgesia, whose relevance to acupuncture seems likely to be far-reaching.

It is difficult if not impossible to isolate the contribution of the needles from that of the other elements of acupuncture in the clinical setting. Still, there is no doubt that needle stimulation has particular effects that are not produced, or not so reliably produced, by other kinds of sensory stimulation. Moreover, not all needle stimulation is of equal effect; for example, periosteal (bone) needling and subcutaneous needling are not equivalent.

What this amounts to is that there is nothing magic about the needles. Acupuncture is a form of manual therapy that has much in common with osteopathy, chiropractic, and physiotherapy, all of which involve touching the patient and have effects which are often quite similar to acupuncture effects. But needles can produce responses that are difficult or sometimes impossible to produce in other ways.

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