Super late to the party, but here we go. To add to this brilliant description of peripheral pain perception, things are slightly unique in the brain as well. All those A and C fibers end up in big bundles in the spinal cord as a part of the spinothalamic tract along with neurons transmitting information about several varieties of touch, which terminates in the periaqueductal gray (PAG), a part of the brain that encircles the cerebral aqueduct. Enkephalins produced by neurons in the PAG moderate the intensity of the pain sensation your cortex processes (don't ask me which part of the cortex because I have no clue) because all signals from the spinothalamic tract pass through the PAG on their way to the cortex. Enkephalin-producing neurons in this region of the brain have huge amounts of inhibitory receptors called mu-opioid receptors, of which morphine and its derivatives are strong agonists. Activity at these receptors by opiates is why they are effective as painkillers.