Ever since Chinese doctors first poked their patients with sharp objects 4,000 years ago, and charged them for the pleasure, acupuncture has been shrouded in mystery. Tradition has it that the procedure works by improving the flow of "qi" along invisible energy channels called meridians, but research published today points to a less mystical explanation for the painkilling claims of acupuncture. The answer, according to a team of scientists in New York, follows an extraordina