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Page: Scientific Theories and Mechanisms of Action
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Many hypotheses have been proposed to address the physiological mechanisms of action of acupuncture. To date, more than 10,000 scientific research studies have been published on acupuncture as cataloged by the National Library of Medicine database.
Gate-control theory of pain
The "gate control theory of pain" (developed by Ronald Melzack and Patrick Wall in 1962 and in 1965) proposed that pain perception is not simply a direct result of activating pain fibers, but modulated by interplay between excitation and inhibition of the pain pathways. The "gating of pain" is controlled by the inhibitory action on the pain pathways. That is, the perception of pain can be altered (gated on or off) by a number of means physiologically, psychologically and pharmacologically. The gate-control theory was developed in neuroscience independent of acupuncture, which later was proposed as a mechanism to account for the analgesic action of acupuncture in the brainstem reticular formation by a German neuroscientist in 1976. (With the advance in modern-day technology, stimulation of these pathways can be demonstrated to alter pain perception using electrical stimulations or magnetic stimulations, such as transcranial magnetic stimulation (TMS) or pulsed electromagnetic field (PEMF) therapy for pain.)
It is well-documented in neuroscience that pain blockade can be achieved at multiple levels in the central nervous system (i.e., the brain and spinal cord). At the spinal cord level, pain transmission via the pain fibers can be blocked by surround inhibition of the neighboring nerve fibers that merge at the substantia gelatinosa in the spinal cord. That is to say, stimulation of the surrounding neurons can cause a reduction of pain when the center excitatory pain fibers are inhibited by the surrounding cutaneous (touch) fibers. This phenomenon is demonstrated in the all-too-common experience that, when we bump our head, pain can be relieved by rubbing the surrounding skin area (activating the surround inhibitory neural circuitry physiologically). Blockade of pain at this level suppresses pain by blocking the pain signal from the periphery. Furthermore, pain blocking by this cutaneous stimulus only lasts for a short period of time, whereas it is claimed that the effect of pain relief by acupuncture lasts for an extended period of time, sometimes months after the needle was removed.
This leads to the theory of central control of pain gating, i.e., pain blockade at the brain (i.e., central to the brain rather than at the spinal cord or periphery) via the release of endogenous opioid (natural pain killers in the brain) neurohormones, such as endorphins and enkephalins (naturally occurring morphines).
Neurohormonal theory
Pain transmission can also be modulated at many other levels in the brain along the pain pathways, including the periaqueductal gray, thalamus, and the feedback pathways from the cortex back to the thalamus. Each of these brain structure processes different aspect of the pain — from experiencing emotional pain to the perception of what the pain feels like to the recognition of how harmful the pain is to localizing where the pain is coming from. Pain blockade at these brain locations are often mediated by neurohormones, especially those that bind to the opioid receptors (pain-blockade site). Pain relief by morphine drug (exogenous opioid) is acting on the same opioid receptor (where pain blockade occurs) as endorphins (endogenous opioids) that the brain produces and releases.
Some studies suggest that the Analgesic (pain-killing) action of acupuncture is mediated by stimulating the release of natural endorphins in the brain. This can be proven scientifically by blocking the action of endorphins (or morphine) using a drug called naloxone. When naloxone is administered to the patient, the analgesic effects of morphine can be reversed, causing the patient to feel pain again. When naloxone is administered to an acupunctured patient, the analgesic effect of acupuncture can also be reversed, causing the patient to report an increased level of pain. This demonstrates that the site of action of acupuncture may be mediated through the natural release of endorphins by the brain, which can be reversed by naloxone. Similar results were also obtained in experiments with animals showing that the analgesic effect is not due to subjective psychological placebo effect, but real physiological phenomena. Such analgesic effect can also be shown to last more than an hour after acupuncture stimulation by recording the neural activity directly in the thalamus (pain processing site) of the monkey's brain. Furthermore, there is a large overlap between the nervous system and acupuncture trigger points (points of maximum tenderness in myofascial pain syndrome).
The sites of action of acupuncture-induced analgesia are also confirmed to be mediated through the thalamus (where emotional pain/suffering is processed) using modern-day powerful non-invasive fMRI (functional magnetic resonance imaging) and PET (positron emission tomography) brain imaging techniques, and via the feedback pathway from the cerebral cortex (where cognitive feedback signal to the thalamus distinguishing whether the pain is noxious (painful) or innocuous (non-harmful)) using electrophysiological recording of the nerve impulses of neurons directly in the cortex, which shows inhibitory action when acupuncture stimulus was applied.
Scientific Method and the Assessment of Chinese Medical Theory and Techniques
Views of proponents
Criticism of TCM theory hinges on the question of how to assess 'intangible' concerns. There is an assumption that all knowledge can be tested by randomly-controlled double-blind studies, and that anything not susceptible to this method of assessment must be jettisoned as unverifiable. Yet the difficulty is not in the methodology, but rather that the nature of Traditional Chinese Medicine itself makes it difficult to subject it as a whole, or subsets of the medical theory, to this type of assessment.
The theory, practice and techniques of Chinese medicine evolved over many thousands of years, well in advance of a formal articulation of the scientific method. Nevertheless, the principles of the scientific method have been used throughout the development of Chinese medical knowledge. Documentation of developments allowed practitioners to evaluate each other's theoretical and practical hypotheses, and what was shown to be effective and/or consistent with observable phenomena was kept, and the remainder discarded over time.
Chinese medicine is inherently individually applied. Given that the health of the entire individual is taken into account for each patient, any two patients, even with the same diagnosis, will receive different treatments based on their constitutional differences, their pattern of response to treatment, and so on. In addition, each treatment may vary from the previous one, in the same way that a masseur might use strokes in a different order, or different strokes, to treat exactly the same condition, from one treatment to the next.
Thus the very complexity and flexibility of this medical system makes it extremely difficult to run clinical trials – a cohort of many thousands would have to be evaluated in order to even begin to assess any claims made for or against the medicine. Clinical trials are still a valuable exercise, but they are not sufficient to determine conclusively whether either the individual constituents of the medical theory (e.g. acupuncture points), or the medical theory as a whole, are valid.
Views of critics
One of the major criticisms of studies which purport to find that acupuncture is anything more than a placebo is that most such studies are not (in the view of critics) properly conducted. Many are not double blinded and are not randomised. However, double-blinding is not a trivial issue in acupuncture: since acupuncture is a procedure and not a pill, it is difficult to design studies in which the person providing treatment is blinded as to the treatment being given. The same problem arises in double-blinding procedures used in biomedicine, including virtually all surgical procedures, dentistry, physical therapy, etc.; the NIH Consensus Statement notes such issues with regard to sham acupuncture, a technique often used in studies purporting to be double-blinded. See also Criticism of evidence-based medicine. Tonelli, a prominent critic of EBM, argues that complementary and alternative medicine (CAM) cannot be EBM-based unless the definition of evidence is changed. Tonelli also says "the methods of developing knowledge within CAM currently have limitations and are subject to bias and varied interpretation. CAM must develop and defend a rational and coherent method for assessing causality and efficacy, though not necessarily one based on the results of controlled clinical trials."
In China, placebo-controlled studies are often not performed as it believed to be unethical to pretend to give patients bonafide treatment.
Some researchers argue that there is no evidence that acupuncture has any affect on the pathogenesis of viruses and microorganisms, or on human physiology, with the exception of the neurological pathways associated with the nerve cells that were stimulated by them. Thus, the most promising clinical application of acupuncture is in the area of pain control.
Some researchers argue that to date there is no conclusive scientific evidence indicating that the procedure has any effectiveness beyond that of a placebo. They argue that studies on acupuncture that meet scientific standards of experimentation have concluded two things: acupuncture is usually more effective than no treatment or a placebo in pill form, and that there is no significant difference in the effectiveness of acupuncture and “sham” acupuncture (needling performed superficially a/o at non-acupuncture sites), which is often used as a control. These researchers therefore conclude that acupuncture's effect is either caused by the tendency of extended, invasive procedures to generate more powerful placebo effects than pills or by the general stimulation of afferent nerve endings at the surface of the skin, causing the release of pain relieving biochemical compounds such as endorphins (this can also be done with jalapeno peppers, electricity, and various other form of stimulation). It may also be a combination of these two effects.
The vast majority of research on acupuncture is conducted by researchers in China, and Ernst et al. argue that there exist major flaws in the design of the experiments, as well as selective reporting of results, and conclude that no conclusions can be drawn from them Some researchers argue that numerous experimental difficulties have prevented the conclusive establishment of a causative relationship (if it exists) between pain relief and the administration of acupuncture. These include the subjective nature of pain measurement and the pervasive influence of psychological factors such as suggestion, confirmation bias, and the distraction of being poked by a needle. Also, they argue, the tendency of chronic pain to ebb and flow on its own without any external intervention leads people to falsely perceive that the last measure they took before the pain subsided was the cause of the relief. This is a logical fallacy known as post hoc ergo propter hoc.
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