Scientific theories regarding acupuncture

have been conducted concerning the hypothesized principles underlying the proposed mechanism of action of acupuncture.

Nerve-reflex theory

The nerve-reflex theory proposed the reflex interactions between the periphery and the autonomic nervous system. This theory states that the skin surface and internal organs are intimately connected by these reflexes — "viscera-cutaneous reflex" and "cutaneous-viscera reflex."

These reflexes can be related to the neuroendocrine-autonomic responses, which is mediated partly by the hypothalamic-pituitary-adrenal axis . HPA axis is a complex set of feedback interactions between the hypothalamus , the pituitary and the adrenal glands . The HPA axis is a major part of the neuroendocrine system that regulates stress responses and maintains the homeostatic condition of autonomic responses directly or indirectly, such as circulation regulation, breathing regulation, feeding behavior, weight control and digestion, immune responses, pain responses, es and stresses, states, sexual/reproductive responses, growth, fluid balance and metabolic energy balances.

In 2006, a broad sense hypothalamus-pituitary-adrenal model was proposed to explain the hypothesized analgesic effect of acupuncture based on observed neuroimaging results using fMRI technique. The model incorporated the stress-induced HPA axis model together with neuro-immune interaction including the cholinergic anti-inflammatory model.

Gate-control theory of pain

The "gate control theory of pain" 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. According to the theory, the "gating of pain" is controlled by the inhibitory action on the pain pathways. That is, the perception of pain can be altered 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 hypothesized analgesic action of acupuncture in the brainstem reticular formation by a German neuroscientist in 1976.

This leads to the theory of central control of pain gating, i.e., pain blockade at the brain via the release of endogenous opioid neurohormones, such as endorphins and enkephalins .

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 back to the thalamus. Pain blockade at these brain locations are often mediated by neurohormones, especially those that bind to the opioid receptors .

Analgesic action of acupuncture was demonstrated to be mediated by stimulating the release of natural endorphins in the brain. This effect can be inferred scientifically by blocking the action of endorphins 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, leaving the patient with intense pain again. This demonstrates that the site of action of acupuncture is mediated through the natural release of endorphins by the brain, which can be reversed by naloxone. 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 of the monkey's brain. It should be noted, however, that studies using similar methodological procedures, including the administration of naloxone, have suggested a role of endogenous opioids in the placebo response, demonstrating that this response is not unique to acupuncture. There is also a large overlap between the nervous system and acupuncture trigger points .

Research has also suggested that the sites of action of analgesia associated with acupuncture administration are mediated through the thalamus using fMRI and positron emission tomography brain imaging techniques, and via the feedback pathway from the cerebral cortex using electrophysiological recording of the nerve impulses of neurons directly in the cortex, which shows inhibitory action when acupuncture stimulus was applied. Similar effects have been observed in association with the placebo response. One study using fMRI found that placebo analgesia was associated with decreased activity in the thalamus, insula and anterior cingulate cortex .