Reflex

An action that occurs automatically and predictably in response to a particular stimulus, independent of the will of the individual. In the simplest reflex, a sensory nerve cell reacts to a stimulus, such as heat or pressure, and sends a signal along its nerve fibre to the central nervous system. There, another nerve cell becomes stimulated and causes a muscle to contract or a gland to increase its secretory activity. The passage of the nerve signal from original sensation to final action is called a reflex arc. Reflexes may be inborn or conditioned. Some inborn reflexes occur only in babies (see reflex, primitive). Inborn reflexes include those that control basic body functions, such as contraction of the bladder after it has filled beyond a certain point, and are managed by the autonomic nervous system. Conditioned reflexes are acquired through experience in a process called conditioning. Several simple reflexes, such as the knee-jerk, are tested in a physical examination. Changes in the reflexes may indicate damage to the nervous system. The examination of vital reflexes controlled by the brainstem is the basis for diagnosing brain death.