pain

n. An aversive experience usually associated with stimulation of free sensory nerve endings, nerve damage, or high-intensity sensory stimulation such as loud sounds or very bright flashes of light. It is associated with an increase in heart and respiration rate, narrowed attentional focus, pupil dilation, and facial grimacing. Pain is mediated by the brain so that the same stimulation may sometimes produce pain and at other times not. There are several types of pain specific receptors located throughout the body but concentrated in the skin, which respond to physical deformation, tension in the muscles or tendons, heat or cold, and chemical damage.