Ketamine is a phencyclidine derivative that acts primarily as a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) glutamate receptor. At sub-anesthetic doses (0.1-0.5 mg/kg IV), ketamine produces profound analgesia, anti-hyperalgesic effects, and antidepressant properties through mechanisms distinct from its anesthetic effects at higher doses. The NMDA receptor is a ligand-gated ion channel that, when activated by glutamate and glycine co-agonist binding, allows calcium influx into postsynaptic neurons. In chronic pain states, persistent nociceptive input causes sustained NMDA receptor activation, leading to central sensitization -- a pathological amplification of pain signaling in the dorsal horn of the spinal cord. Central sensitization manifests clinically as allodynia (pain from normally non-painful stimuli), hyperalgesia (exaggerated pain response), and wind-up (progressively increasing pain from repeated stimuli). Ketamine blocks the NMDA receptor channel in a use-dependent manner (it enters the open channel and lodges in the pore), effectively resetting the sensitized pain circuitry. Ketamine also interacts with opioid receptors (mu and delta, providing additive analgesia), monoaminergic transporters (inhibiting serotonin and norepinephrine reuptake, contributing to antidepressant effects), sigma receptors, and voltage-gated sodium channels (weak local anesthetic effect). The...