Schematic illustrations of endogenous opioid-mediated electro-acupuncture analgesia (EAA). (A) EA at 2 Hz or 100 Hz releases different endogenous opioids, which act on different opioid receptors to induce synergic analgesic effects. (B) Impulses elicited by EA at 2 Hz or 100 Hz project to different brain areas and induce analgesic effects through descending inhibition. Em: endomorphine; Enk: encephalin: β-End: β-endorphin; Dyn: dynorphin; μ: mu-opioid receptors; δ: delta opioid receptor; κ: kappa opioid receptor; PAG: periaqueductal grey matter. Modified from Han 2003 reference .