<P> 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 dorsal horn of 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 . </P> <P> Gate control theory asserts that activation of nerves which do not transmit pain signals, called nonnociceptive fibers, can interfere with signals from pain fibers, thereby inhibiting pain . Afferent pain - receptive nerves, those that bring signals to the brain, comprise at least two kinds of 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 . </P> <P> The peripheral nervous system has centers at which pain stimuli can be regulated . Some areas in the dorsal horn of the spinal cord that are involved in receiving pain stimuli from Aδ and C fibers, called laminae, also receive input from Aβ fibers . The nonnociceptive fibers indirectly inhibit the effects of the pain fibers,' closing a gate' to the transmission of their stimuli . In other parts of the laminae, pain fibers also inhibit the effects of nonnociceptive fibers,' opening the gate' . This presynaptic inhibition of the dorsal nerve endings can occur through specific types of GABA receptors (not through the α1 GABA receptor and not through the activation of glycine receptors which are also absent from these types of terminals). Thus certain GABA receptor subtypes but not glycine receptors can presynaptically regulate nociception and pain transmission . </P> <P> An inhibitory connection may exist with Aβ and C fibers, which may form a synapse on the same projection neuron . The same neurons may also form synapses with an inhibitory interneuron that also synapses on the projection neuron, reducing the chance that the latter will fire and transmit pain stimuli to the brain (image on the right). The inhibitory interneuron fires spontaneously . The C fiber's synapse would inhibit the inhibitory interneuron, indirectly increasing the projection neuron's chance of firing . The Aβ fiber, on the other hand, forms an excitatory connection with the inhibitory interneuron, thus decreasing the projection neuron's chance of firing (like the C fiber, the Aβ fiber also has an excitatory connection on the projection neuron itself). Thus, depending on the relative rates of firing of C and Aβ fibers, the firing of the nonnociceptive fiber may inhibit the firing of the projection neuron and the transmission of pain stimuli . </P>

According to gate-control theory what type of fibers small or large open the pain gates