Abstract
Pain stems from a noxious stimulus such as tissue damage, and plays an important role in transmitting hazardous signals to the body. Nociceptors excited by a noxious stimulus have no specific receptor structures, and are considered primary afferent free nerve endings of Aδ and C fibers. Primary afferent fibers release transmitter substances in the posterior horn of the spinal cord, for example glutamic acid or substance P, and thereby excite secondary nociceptive neurons. The excitation ascends the spinal cord, is projected on to specific or non-specific nuclei of the thalamus, and is then projected on sensory areas of the cerebral cortex or the limbic system. Sense of pain, emotional responses, autonomic responses, and defence responses are then provoked. Inhibitory (relieving) system for pain is also observed in a living body.
