The skin senses include at least four sensations: pressure, heat, cold, and pain. If you unbend a paper clip and use an end to probe an area of your skin lightly, you will feel pressure at some points where the wire contacts your skin but not at every point. If you do the same thing with a cold wire, you will feel cold at other specific points. Try a warm wire, and you will feel warmth at still other points. In each case, specialized receptor cells in the skin are responsive to particular stimuli.
Although the skin also contains pain receptors, pain may be triggered by stimulation of a variety of nerve endings—not only in the skin but in other organs. Blinding lights, blaring noises, extremely high or low temperatures, and great pressure all yield pain sensations. Pain serves to warn us of tissue destruction. How do we experience pain? According to specificity theory, specific pain receptors send signals of pain to the spinal cord, which relays the signals directly to the pain centers of the brain.
Skin Senses And Pain
According to this theory, a person should feel pain exactly where the stimulation occurs, and the amount of pain felt should depend on the amount of stimulation at the pain site. But specificity theory does not account for the phenomenon of acupuncture, in which the insertion of needles into various sites on the body surface sometimes relieves pain in body regions quite distant from the needle site. Chinese physicians regularly use acupuncture as the only form of anesthesia in major operations.
Ronald Melzack and Patrick Wall (1989) proposed the gate-control theory of pain sensation, which may account for the effects of acupuncture. Their research suggested that the nervous system contains “gates” that permit or block the trans- mission of pain signals from sense receptors to the central nervous system. Stimulation of certain areas opens the gate to allow pain signals to pass; stimulation of other areas closes the gate, so pain signals cannot reach the pain reception areas of the brain.
In the case of acupuncture, Mclzack and Wall believe, the needles are inserted at sires that activate the reticular formation of the brain stem, which is one of the gate-control areas of the nervous system. As a result, pain signals are blocked before they reach the brain, and no pain is experienced.