The nervous system is the body's information gatherer, storage center and control system. Its overall function is to collect information about the external conditions in relation to the body's external state, to analyze this information, and to initiate appropriate responses to satisfy certain needs. The most powerful of these needs is survival. The nerves do not form one single system, but several which are interrelated. Some of these are physically separate, others are different in function only. The brain and spinal cord make up the central nervous system. The peripheral nervous system is responsible for the body functions which are not under conscious control - like the heartbeat or the digestive system. The smooth operation of the peripheral nervous system is achieved by dividing it into sympathetic and parasympathetic systems. These are opposing actions and check on each other to provide a balance. The nervous system uses electrical impulses, which travel along the length of the cells. The cell processes information from the sensory nerves and initiates an action within milliseconds. These impulses travel at up to 250 miles per hour, while other systems such as the endocrines may take many hours to respond with hormones.