Opioid Analgesics

A painful stimulus — heat, pressure, a chemical from injured tissue — is picked up by nociceptors, the free nerve endings of pain fibers. Two kinds of fiber carry the message toward the spinal cord: fast, myelinated A-delta fibers (the sharp, immediate "first" pain) and slow, unmyelinated C fibers (the dull, burning "second" pain that lingers).

These fibers hand the signal off in the dorsal horn of the spinal cord, releasing excitatory transmitters (glutamate and substance P) onto a second neuron. That neuron crosses the cord and climbs the spinothalamic tract to the thalamus and on to the cortex, where the signal finally becomes the conscious *experience* of pain. Running the other direction is a descending pain-suppression system from the brainstem — and it is soaked in opioid receptors, ready to dampen the volume.

The body already makes its own painkillers: endorphins, enkephalins, and dynorphins. They act on opioid receptors — G-protein-coupled receptors that come in three main types, mu (µ), kappa (κ), and delta (δ). The µ receptor is the one that matters most clinically: it drives analgesia, but also euphoria, sedation, respiratory depression, pupil constriction, and constipation.

When an opioid activates the µ receptor, the neuron is quieted three ways at once: it closes calcium channels at the nerve terminal (so less pain transmitter is released), opens potassium channels on the receiving neuron (hyperpolarizing it, making it harder to fire), and lowers cAMP inside the cell. The pain signal is turned down at the spinal cord and again in the brain, and the emotional "this is unbearable" component is blunted in the limbic system.

µ receptors are not only in the pain pathway. The reason a single mechanism produces both the relief and the danger is that µ receptors sit in several tissues, and the drug hits all of them. In the brainstem respiratory centers, µ activation blunts the drive to breathe in response to rising CO₂ — this is respiratory depression, the effect that kills in overdose. In the gut, µ activation slows motility to a crawl — constipation that never lets up. In the mesolimbic reward pathway, µ activation floods the nucleus accumbens with dopamine — euphoria, the seed of misuse.

With repeated dosing the nervous system adapts. Receptors desensitize and the cell ramps its cAMP machinery back up, so the same dose does less — tolerance — and the patient needs more for the same relief. The system now *depends* on the drug to stay balanced, so stopping abruptly unmasks a rebound of everything the opioid was suppressing — withdrawal. Two ideas students must keep separate: physical dependence (an expected physiologic adaptation — withdrawal on stopping) is *not* the same as addiction (compulsive use despite harm). A hospitalized patient can be physically dependent without being addicted.