According to the National Center for Complimentary and Integrative Health (a branch of the NIH), acupuncture is associated with multiple biological responses. These responses occur both at the point of insertion as well as at at other places in the body. Responses are related to the stimulation of sensory neurons that connect to structures within the central nervous system. This leads to activation of pathways affecting various physiological systems in the brain as well as in the periphery.
One of the major actions of acupuncture's effectiveness is in decreasing inflammation. Physiologically, acupuncture has been found to downregulate pro-inflammatory cells known as M1 macrophages. It also upregulates anti-inflammatory M2 macrophages, which are a source of anti-inflammatory interleuken-10.
Considerable evidence also shows that opioid peptides are released during acupuncture and that the analgesic effects of acupuncture are at least partially explained by their actions. In addition, stimulation with acupuncture activates the hypothalamus and the pituitary gland, resulting in a broad spectrum of systemic effects. Alteration in the secretion of neurotransmitters and neurohormones and changes in the regulation of blood flow, both centrally and peripherally, have been documented. There is also evidence that there are alterations in immune function produced by acupuncture.
Extensive research has been completed about acupuncture's mechanism of action in the treatment of a variety of specific conditions. For example, below is an illustrated summary of the specific biochemical effects of acupuncture when treating for pain modulation in the gastrointestinal system.
Credit: Zhang, F.L., Wu, L., Zhao, J., Lv, T., Hu, Z., Weng, Z., Wang, S., Wu, H., & Liu, H. (2017). Neurobiological Mechanism of Acupuncture for Relieving Visceral Pain of Gastrointestinal Origin. Gastroenterology research and practice.