[Physiology and pathophysiology of pain]

Pain is described as an unpleasant sensory and emotional experience associated with actual or potential tissue damage. A painful stimulus elicits an activation of receptors in the periphery which transduce the stimulus into an electrical impulse. This electrical impulse is transmitted to the dorsal horn of the spinal cord and subsequently to the brain. Here the stimulus is perceived as a painful experience in the context of the specific situation and earlier experiences. In its physiological function, pain serves the prevention of tissue injury. If tissue injury occurs, pain becomes persistent leading to changes in the neuroplasticity of the periphery, spinal cord and the brain. This results in an enhanced sensitivity of the nervous system against both noxious and unnoxious stimuli. In parallel, endogenous control mechanisms, in which predominantly the opioid but also other systems are involved, counteract these pathological changes. Both the initiation of pain and its control can be regarded as the body's response to prevent further injury, to support wound healing, and to return to a normal function as quickly as possible. The transition from acute to chronic pain may critically depend on disturbances of the intrinsic pain control mechanisms as well as on effective therapeutic measures, initiated at the earliest possible time.