Traditionally, pain is thought to be a manifestation of blockages of Qi within the meridian network; acupuncture needling removes these Qi blockages, restoring the smooth flow of Qi throughout the body, thereby relieving pain.
From a modern, scientific perspective, Qi can be thought of as energy, matter, and information, and the acupuncture meridians can be thought of as a composite roadmap of multiple biological signaling systems, including the neural, vascular, and connective tissue networks. Read more about Qi and meridians here.
Using these definitions, Qi blockages can then be thought of as disruptions in the movement of biological energy, matter, and information throughout the body's networks, causing various symptoms, including pain.
That being said, the scientific mechanisms of acupuncture are not yet fully understood; however, research has uncovered several key mechanisms of how acupuncture works for pain. These include:
Acupuncture needling sensation (known as De-Qi) deactivates the pain center of the brain (link to study). Read more.
Acupuncture triggers the release of endorphins, which are the body's natural pain killers (link to study).
Acupuncture triggers the local release of adenosine, which has antinociceptive and anti-inflammatory properties (link to study).
Acupuncture triggers the release of serotonin and other neurochemicals, which have pain-modulating properties (link to study).
Acupuncture leverages the Gate Control Theory of Pain by introducing competing stimuli (link to study).
Because of the incredible complexity of the human body, studying the mechanisms of acupuncture is inherently difficult.
So, while we have not yet uncovered all of the antinociceptive pathways of acupuncture, what we can say with certainty is that acupuncture works for pain by stimulating both systemic and local reactions within the body that produce a pain-relieving effect.