fibers - a fast, relatively thick, myelinated "Aδ" fiber that carries messages quickly with intense pain, and a small, unmyelinated, slow "C" fiber that carries the longer-term throbbing and chronic pain. Large-diameter Aβ fibers are nonnociceptive (do not transmit pain stimuli) and inhibit the effects of firing by Aδ and C fibers. When it was first proposed in 1965, the theory was met with considerable skepticism. Despite having to undergo several modifications, its basic conception remains unchanged. Ronald Melzack and Patrick Wall introduced their "gate control" theory of pain in the 1965 Science article "Pain Mechanisms: A New Theory". The authors proposed that both thin (pain) and large diameter (touch, pressure, vibration) nerve fibers carry information from the site of injury to two destinations in the spinal cord: transmission cells that carry the pain signal up to the brain, and inhibitory interneurons that impede transmission cell activity. Activity in both thin and large diameter fibers excites transmission cells. Thin fiber activity impedes the inhibitory cells (tending to allow the transmission cell to fire) and large diameter fiber activity excites the inhibitory cells (tending to inhibit transmission cell activity). So, the more large fiber (touch, pressure, vibration) activity relative to thin fiber activity at the inhibitory cell, the less pain is felt. The authors had drawn a neural "circuit diagram" to explain why we rub a smack. They pictured not only a signal traveling from the site of injury to the inhibitory and transmission cells and up the spinal cord to the brain, but also a signal traveling from the site of injury directly up the cord to the brain (bypassing the inhibitory and transmission cells) where, depending on the state of the brain, it may trigger a signal back down the spinal cord to modulate inhibitory cell activity (and so pain intensity). The theory offered a physiological explanation for the previously observed effect of psychology on pain perception. In 1968, three years after the introduction of the gate control theory, Ronald Melzack concluded that pain is a multidimensional complex with numerous sensory, affective, cognitive, and evaluative components. Melzack's description has been adapted by the International Association for the Study of Pain in a contemporary definition of pain. Despite flaws in its presentation of neural architecture, the theory of gate control is currently the only theory that most accurately accounts for the physical and psychological aspects of pain. The gate control