Pain and the Sympathetic Nervous System

Clinical observations and experimentation on humans and animals indicate that the sympathetic nervous system is potentially involved in the generation of pain, hyperalgesia, and inflammation. These functions of the sympathetic nervous system are different from its conventional functions, namely to transmit impulses from the brain by distinct pathways to peripheral effector cells. Pain, hyperalgesia, and inflammation generated by the sympathetic nervous system occur via multiple mechanisms, including direct or indirect sympathetic–afferent coupling following trauma, with or without nerve lesion leading to excitation or sensitization of nociceptors; nociceptor sensitization independent of excitation and noradrenaline release; nociceptor sensitization by nerve growth factor and cytokines; and nociceptor sensitization involving the sympathoadrenal system.