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Understanding the anatomy and physiology of pain transmission systems is important for the pain management specialist. Injuries to these areas may cause the common pain syndromes for which patients seek help (e.g., diabetic neuropathy, postherpetic neuralgia). Interventions at distinct anatomic sites may provide the relief the patient seeks (nerve blocks, implantable devices). And ongoing research may reveal new modalities or pharmaceutical agents to provide relief, such as the cyclooxygenase-2 (COX-2) inhibitors. This chapter is designed as an overview for the clinician and not as a comprehensive review of the anatomy and physiology of the nervous system, about which textbooks have been written.

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This chapter reviews the transmission of a nociceptive or pain impulse from the site of stimulus in the periphery to the central nervous system. The basic anatomic pathways of nociceptive transmission and of descending nociceptive modulations are described. Some of the basics of physiology also are discussed, with the assumption that the reader is knowledgeable about the fundamentals of neuronal transduction, such as action potential propagation and the relationship of the cell body to the dendrites and axon. Although some mention of pathophysiology and disease states is made, for further discussion, please see the specific chapters in this book.

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This chapter focuses on the ability of the nervous system to transmit and modulate nociceptive stimuli. The studies reviewed here likely apply more to acute pain than to chronic pain. Although there is intense interest in developing an animal model of chronic pain, most of the experimental paradigms used are more closely analogous to the injury of acute pain than chronic pain. The latter is both a physical and emotional entity and may result from plasticity in the nervous system.1 The complexities of chronic pain syndromes, including such theories as chronic pain as a variant of depressive disorders,2 are discussed elsewhere.

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Knowledge of nociceptive transmission has advanced considerably since Descartes outlined his concept of the nerve as tubes with delicate threads to convey a painful impulse.3 It is now widely believed that stimulation of a primary afferent neuron in the peripheral nervous system results in activation of neurons in the dorsal horn of the spinal cord and then in transmission rostrally to the brain.

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Sensory neurons, called primary afferents, have a cell body in the dorsal root ganglia (DRG) of the spinal cord or in the ganglia of the cranial nerves. Cranial nerves V, VII, IX, and X receive inputs from primary afferents and, thus, sensory information from the head, face, and throat. Cells in the DRG are a heterogenous population of size and function; a reflection of the heterogeneity of the sensory inputs processed by the central nervous system.4

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This heterogeneity is reflective of the multiple functions of the peripheral sensory nervous systems, as well as the supportive function of the glial system. The classic nomenclature of peripheral neurons relies on the size of the axon ...

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