Acupuncture is one of the key components of the system of traditional Chinese medicine (TCM). In the TCM system of medicine, the body is seen as a delicate balance of two opposing and inseparable forces: yin and yang. The body's vital energy (qi) is carried by yin and yang. Yin represents the cold, slow, or passive principle, while yang represents the hot, excited, or active principle. Among the major assumptions in TCM are that health is achieved by maintaining the body in a "balanced state" and that disease is due to an internal imbalance of yin and yang. This imbalance leads to blockage in the flow of qi (vital energy) along pathways known as meridians. It is believed that acupuncture needles, when placed at the appropriate site, alleviate symptoms by restoring normal flow within the meridians.
Preclinical studies have documented acupuncture's effects, but they have not been able to fully explain how acupuncture works. It is proposed that acupuncture produces its effects through regulating the nervous system, thus aiding the activity of pain-killing biochemicals such as endorphins and immune system cells at specific sites in the body. In addition, studies have shown that acupuncture may alter brain chemistry by changing the release of neurotransmitters and neurohormones and, thus, affecting the parts of the central nervous system related to sensation and involuntary body functions, such as immune reactions and processes that regulate a person's blood pressure, blood flow, and body temperature.
One widely accepted medical explanation of acupuncture's claimed efficacy is that when needles prick the skin, they cause the brain to release a variety of pain-killing neutral chemicals, such as endorphins, encephalin and other opioids. Modern imaging techniques have shown that acupuncture does stimulate certain areas within the brain and suppress others.
Other theories are also proposed on how acupuncture works especially related to its effects on pain. Two commonly cited theories are called Gate Theory of Pain, and Double Gate Control Theory. These are described elsewhere.
