Studies indicate there are multiple mechanisms by which the acupuncture stimulates the body. One, acupuncture increases pain threshold via the release of certain opioid peptides (1). Second, acupuncture stimulates the deeper parts of the brain, such as the limbic system, thus influencing the ways we perceive and respond to  pain (2-3). Third, studies have found that acupuncture increases blood flow to the local area (4). Fourth, acupuncture can block our pain or nociceptive pathway (5). Lastly, acupuncture stimulates myofascial trigger point release (6).

References:

  1. Han J, Terenius L. Neurochemical basis of acupuncture analgesia. Annual review of Pharmacology and Toxicology, 1982;22:193-220.
  2. Hui K, Liu J, et al. Acupuncture modulates the limbic system and subcortical grey structures of the human brain: Evidence from fMRI studies in normal subjects. Human Brain Mapp, 2000;9(1):13-25.
  3. Hui K, Liu J. et al. The integrated response of the human cerebro-cerebellar and limbic systems to acupuncture stimulation at ST 36 as evidenced by fMRI. Neuroimage, 2005 Sept;27(3):479-496.
  4. Sandberg M, Lundeberg T, et al. Effects of acupuncture on skin and muscle blood flow in healthy subjects. European Journal of Applied Physiology, 2003 Sept;90(1-2):114-119.
  5. Sandkuhler J. Learning and memory in pain pathways. Pain, 2000 Nov;88(2):113-118.
  6. Mense S, Simons D. Muscle Pain: Understanding Its Nature, Diagnosis and Treatment. Philadelphia: Lippincott Williams, & Wilkins, 1999.