large-diameter fibers. However, activation of the inhibitory interneurons varies: large-diameter fibers excite the interneuron, which ultimately reduces transmission cell firing, whereas small-diameter fibers inhibit the inhibitory interneuron which lessens the inhibitory input to the transmission cell. Therefore, less pain is felt (via reduced transmission cell activity) when more activity in large-diameter fibers (touch-, pressure-, and vibration- transmitting) occurs relative to the activity in small-diameter (pain-transmitting) fibers. 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 GABAA receptors (not through the α1 GABAA receptor and not through the activation of glycine receptors which are also absent from these types of terminals). Thus, certain GABAA receptor subtypes but not glycine receptors can presynaptically regulate nociception and pain transmission. 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. History and legacy 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