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The Pain Gate Control Theory
The Gate Control Theory was initially proposed in 1965 by Melzack and Wall based on the fact that small diameter nerve fibres carry pain stimuli through a 'gate mechanism' but larger diameter nerve fibres going through the same gate can inhibit the transmission of the smaller nerves carrying the pain signal. Chemicals released as a response to the pain stimuli also influence whether the gate is open or closed for the brain to receive the pain signal. This lead to the theory that the pain signals can be interfered with by stimulating the periphery of the pain site, the appropriate signal-carrying nerves at the spinal cord, or particular corresponding areas in the brain stem or cerebral cortex. Complementary Therapists need to concern themselves with the first 2 options in order to effectively modify the pain signal.
It is generally recognised that the 'Pain gate' can be shut by stimulating nerves responsible for carrying the touch signal (mechaoreceptors) which enables the relief of pain through massage techniques, rubbing, and also the application of wheat bags and ice packs.
The Gate can also be shut by stimulating the release of endogenous opioids which are opioid (pain-relieving) type chemicals released by the body in response to pain stimuli. Acupuncture and electrical analgesia (TENS) is thought to stimulate their release as a response to stimulation, the opioids then inhibiting the transmission of pain signals in the substantia gelatinosa part of the spinal cord - what is often referred to as the spinal root part of the nerve.
References
Doody, S.B., Smith, C., & Webb, J. (1991). Nonpharmacologic interventions for pain management. Critical Care Nursing Clinics of North America, 3(1), 69-75.
Melzack R, Wall PD (1996) The challenge of pain (2nd ed) London: Penguin books
Micklovich (no date) Gate Control Theory http://mancstra.tec.nh.us/deathcare/presentations/cogpain/tsld001.htm and http://www.painforum.com/en/1/hcpmjgate.html
Taylor, A.G., West, B.A., Simon, B., Skeleton, J., & Rowlingson. (1983). How effective is TENS for acute pain? American Journal of Nursing, 83, 1171-1174.
Turchaninov R (1996) Medical Massage (Volume 1) Phoenix: Aesculapius
Books