Pain

Pain is an unpleasant experience associated with actual or potential tissue damage. It is a syndrome spanning from paper cuts to back pain, from sunburn to heartache. Pain is a normal human experience. It is a sensation designed to alert us to injury or disease, there by motivating us to protect our body part as it heals. Such pain, called nociceptive or inflammatory pain, dissipates as we heal. The cellular origin involves free nerve endings that express specialized ion channels and receptors that sense temperature, mechanical stimulation, and a large variety of chemicals released by injured tissue. Their activation causes signals to travel via nociceptive neurons to the brain where they are interpreted as aversive and painful. By contrast, one-fifth of the world’s population lives in chronic pain, which is often unrelated to an actual injury. This so-called neuropathic pain develops long after the healing process is complete, and it may even affect a body part is no longer there (phantom limb). The pathways and brain centers activated are seemingly similar to those involved in nociceptive and inflammatory pain, yet neuropathic pain serves no obvious biological role. The body has its own defense mechanism against pain in the form of the opioid system comprised of opioid receptors and endogenously released opioids. This system explains how our mind can modulate the pain experience by positive thought, distraction, and expectation as is the case with placebo analgesics. Opioid receptors are the target of natural opioids such as morphine or codeine, which are powerful analgesics, but they mediate the analgesic effect of chemically manufactured opioids and substances of abuse such as heroin. Opioids are very effective for some pain conditions, notably cancer pain, yet their therapeutic benefit must be weighed against their risk for addiction.