Chinese theory indicates that pain is frequently associated with stagnation or obstruction of "Qi," and the application of acupuncture stimulation unblocks this obstruction and stagnation resulting in the resolution of this pain (Liu and Akira 1994). The Western physiology of pain perception and modulation describes a multilevel system that is activated once an injury occurs under normal circumstances. The peripheral activation leads to a series of events processing toward the central nervous system. This leads to a sequence ofevents including signal processing along neural pathways, immunologic, hormonal release, and psychobehavioral responses (Wang et al. 2008). Pain perception and inhibition accept a dynamic, malleable, and complex set of interacting neurons with gene regulation and expression producing a variety of neuropeptides and cytokines at both the peripheral and the central nervous systems (Besson 1999, Melzack and Wall 1965, Bolay and Moskowitz 2002). The recognition of the plasticity of the nervous system has revolutionized the understanding of pain, especially chronic pain. The neuroanatomy of nociception can be organized into three major domains. The periphery is composed of small-fiber sensory axons that respond to various types of noxious input called nociceptors. At the spinal cord level, the interneurons, which receive nociceptive and non-nociceptive afferent information, act on the side of dynamic neurons and others ascend the order neuron to alter the retrograde processing of "pain" signals from the periphery. In the brain, these signals are mediated further by norepinephrine, serotonin, and acetylcholine and endogenous opioids. The well-identified opioid receptor sites of the brain and spinal cord include the hypothalamus, limbic system, basal ganglia and periaqueductal gray area, nucleus raphe magnus, reticular activating system, and dorsal horn of the spinal cord (Besson 1999).
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