The nervous system of vertebrates is a communication and control device. The functioning of the vertebrate body is controlled by the nervous system as it receives information, integrates it, and signals some response. The central nervous system is the information-processing center and the command post for sending out responsive orders. But sensory systems provide the information to the brain in the first place. Without sensory systems, the central nervous system would receive no data upon which to act. Sensory neurons monitor both the internal and external environments of the organism and relay this information to the brain. Sensory neurons differ in the stimuli they respond to and in their degrees of complexity, but all but one type work in the same basic way. When they are stimulated, they depolarize and send a nerve impulse to the central nervous system. The brain builds a complete picture of the internal and external environments by monitoring the pattern of which sensory neurons are active and how frequently they fire.