Pathophysiology
Clinical meaning
Pain is a complex sensory and emotional experience that serves as a protective warning system alerting the body to actual or potential tissue damage. The process of converting a noxious stimulus into the conscious experience of pain is called nociception and involves four distinct phases: transduction, transmission, perception, and modulation. Transduction is the first phase and occurs at the peripheral nerve endings (nociceptors) in the skin, muscles, joints, and visceral organs. When tissue damage occurs (from mechanical, thermal, or chemical stimuli), injured cells release inflammatory mediators including prostaglandins, bradykinin, histamine, substance P, and potassium ions. These chemical mediators activate nociceptors by depolarizing the nerve ending, converting the noxious stimulus into an electrical nerve impulse (action potential). Non-steroidal anti-inflammatory drugs (NSAIDs) work at this phase by inhibiting cyclooxygenase enzymes and blocking prostaglandin production. Transmission is the second phase, during which the electrical impulse travels from the peripheral nociceptor along afferent nerve fibers to the spinal cord and then to the brain. Two types of primary afferent fibers carry pain signals: A-delta fibers are thinly myelinated, conduct rapidly (5-30 meters per second),...