Pain is a complex physiological-psychological activity, an unpleasant sensation and emotional feeling, including the pain sensation caused by injurious stimuli acting on the organism, and the nociceptive response of the organism to injurious stimuli. The injurious stimulus that causes pain can be mechanical stimuli such as knife, stick, or physicochemical factors such as electric current, high temperature, strong acid, and strong alkali. The site of the injurious stimulus shows vasodilation, tissue edema, and the production of local pain-causing chemicals. These chemicals act on free nerve endings in the skin and/or related tissues, generating nerve impulses that travel along the slow-conducting afferent nerve fibers, through the dorsal root ganglion of the spinal cord to the relevant neurons in the posterior horn of the spinal cord or the spinal nucleus of the trigeminal nerve, and then through the contralateral ventral lateral cord to the pain center of the brain – the thalamus – the cerebral cortex, etc., causing pain sensation and response. At the same time, non-painful information such as touch and pressure transmitted along the fast-conducting afferent nerve fibers has reached the relevant brain areas in the CNS first and interacts with the nociceptive information transmitted by the fine fibers. Any factor that increases the activity of the fine fibers and/or decreases the activity of the coarse fibers can cause pain. It is more ambiguous for a person to be able to accurately localize an injurious stimulus acting on the body surface. Sometimes, pain sensations can also be induced for psychological reasons, but there is no actual clear objective cause. In general, pain is susceptible to moods such as attention, suggestion, and anticipation, and a person’s past experiences and prevailing emotions can also influence pain perception.