nHow can it work?
Melzack and Wall's Gate Control Theory of Pain argues acupuncture stimulates the myelinated A-delta fibres that can close the gate to higher centres from pain information conveyed in the small unmyelinated C fibres.
Release of neuropeptides (enkephalins and endorphins) and monoamines (5HT and noradrenaline) by needling plays a significant role. Research showing naloxone could reverse the effects of acupuncture gave weight to this theory. Trigger points are tender areas in a taut band of muscle that can cause referred pain. Firm palpation of a trigger point can be very painful, whereas needling into the trigger point can relax the muscle band and gives instant relief.
Many trigger and acupuncture points are co-located: a trigger point often found in the upper free border of trapezius is precisely at acupuncture point gallbladder 21.