pain mechanism, the network that communicates unpleasant sensations and the perceptions of noxious stimuli throughout the body in association with physical disease and trauma involving tissue damage. The gate control theory of pain is an attempt to explain the role of the nervous system in the pain response. It states that pain signals that reach the nervous system excite a group of small neurons that form a “pain pool.” When the total activity of these neurons reaches a minimum level, a theoretic gate opens up and allows the pain signals to proceed to higher brain centers. The areas in which the gates operate are considered to be in the spinal cord dorsal horn and the brainstem. The pattern theory holds that the intensity of a stimulus evokes a specific pattern, which is interpreted by the brain as pain. This perception is the result of the intensity and frequency of stimulation of a nonspecific end organ. Some authorities believe that bradykinin and histamine, two chemical substances produced by the body, cause pain. Recently discovered pain killers produced naturally by the body are the enkephalins and the endorphins. Some studies indicate that the enkephalins are 10 times as potent as morphine in reducing pain. It is known that after histamine and some other naturally occurring chemical substances are released in the body, pain sensations travel along fast-conducting and slow-conducting nerve fibers. These pain-transmitting neuropathways communicate the pain sensation to the dorsal root ganglia of the spinal cord and synapse with certain neurons in the posterior horns of the gray matter. The pain sensation is then transmitted to the reticular formation and the thalamus by neurons that form the anterolateral spinothalamic tract. It is then conveyed to various areas of the brain, such as the cortex and the hypothalamus, by synapses at the thalamus. The immediate reaction to pain is transmitted over the reflex arc by sensory fibers in the dorsal horn of the spinal cord and by synapsing motor neurons in the anterior horn. This anatomical pattern of sensory and motor neurons allows the individual to move quickly at the touch of some harmful stimulus, such as extreme heat or cold. Nerve impulses alerting the individual to move away from such stimuli are simultaneously sent along efferent nerve fibers from the brain. Also called gate theory of pain, pain pathway.
