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See OLFACTION/SMELL, THEORIES OF.

GATE-CONTROL THEORY. = pain theory. The American psychologist Ronald Mel-zack (1929- ) and the American-based English biologist and anatomist Patrick D. Wall (1925- ) formulated the gate-control theory of pain, which states that the spinal cord contains a type of neurological "gate" that either blocks or allows pain signals to pass on to higher centers in the brain. The spinal cord contains small nerve fibers ("C-fibers") that conduct most pain signals and larger fibers ("A-delta fibers") that transmit most other sensory information. When some bodily tissue is injured, the small fibers activate and open the "neural gate," and the person feels "pain." Larger fiber activity, on the other hand, serves to close the pain gate and turns pain off. Thus, according to the gate-control theory of pain, one way to treat chronic pain is to stimulate (via small electrical currents or by acupuncture) the "gate-closing" activity in the large neural fibers. This is called the "counterirri-tant" method for reducing pain. For example, ice applied to an arm bruise not only controls swelling but also triggers "cold messages" that close the gate on the pain signals. Patients suffering from arthritis may carry a small, portable electrical stimulation unit next to a chronically painful area, and when the unit stimulates nerves in that area, the individual feels a vibrating sensation rather than pain. The effectiveness of acupuncture (acupuncture theory of pain) may be adequately explained by gate-control theory where inserting needles into the large neural fibers transmits sensory signals that compete with pain signals of the small neural fibers and, thus, close the

"pain gate." The gate-control theory of pain has been revised recently to account for the importance of several brain mechanisms in the perception of pain, such as one's current emotional state interacting with the onset of a painful stimulus, which may alter the pain intensity one feels. Thus, the brain itself may affect pain perception by sending messages that either close the spinal gate (as when one relaxes) or keep it open (as when one is anxious). Opiate-like chemicals, called endor-phins and enkephalins, that are produced in the body naturally may interact with the spinal gate, also, to lessen the sensations of pain. Certain areas of the spinal cord are rich in opiate receptors and endorphin-loaded neurons, and these substances may close the spinal gate by inhibiting the release of excitatory substances for neurons transmitting signals about pain. It has been suggested that social and cognitive-behavioral factors can strongly affect pain, even though pain is a basic sensory experience, and the specific mechanisms accounting for such factors are still being researched. In the illusory sensation called phantom limb phenomenon or phantom extremity effect (which is experienced by an amputee of the limb still being attached to the body even though all sensory nerve fibers associated with the limb have been removed) the allied sensations include pain perception experienced by a majority of amputees; the accompanying sensations of tingling, itching, or burning may be due to stimulation of the nerve ends or due to psychological reactions; such an experience may occur, also, in phantom breast effect occasioned by a woman's breast(s) having been amputated due to cancer or disease. See also COGNITIVE THERAPY, THEORIES OF. REFERENCES

Melzack, R., & Wall, P. (1965). Pain mechanism: A new theory. Science, 150, 971-979.

Melzack, R. (1976). Pain: Past, present, and future. In M. Weisenberg & B. Tur-sky (Eds.), Pain: New perspectives in therapy and research. New York: Plenum.

Nathan, P. (1976). The gate control theory of pain. Brain, 99, 123-158.

Melzack, R., & Wall, P. (1982). The challenge of pain. New York: Basic Books. Weisenberg, M. (1984). Cognitive aspects of pain. In P. Wall & R. Melzack (Eds.), Textbook of pain. Edinburgh: Churchill Livingstone. Melzack, R. (1992). Phantom limbs. Scientific American, 266, 120-126.

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