Mechanism of analgesia is by

The degree to which a person reacts to pain varies tremendously. This variation results paly from a capability of the brain itself to suppress input of pain signals to thenervous system by activating a pain control system called an analgesia system. The analgesia system consists of three major components: (1) The periaqueductal gray and periventricular areas of the mesencephalon and upper pons surround the aqueduct of Sylvius and poions of the third and fouh ventricles. Neurons from these areas send signals to (2) the raphe magnus nucleus, a thin midline nucleus located in the lower pons and upper medulla, and the nucleus reticularis paragigantocellularis, located laterally in the medulla. From these nuclei, second-order signals are transmitted down the dorsolateral columns in the spinal cord to (3) a pain inhibitory complex located in the dorsal horns of the spinal cord. At this point, the analgesia signals can block the pain before it is relayed to the brain. Ref: guyton and hall textbook of medical physiology 12 edition page number:698,699,700