The nervous system is an intricately organized, interconnected system of nerve cells that relays messages to and from the brain and spinal cord of an organism in vertebrates. It receives sensory input, processes the input, then sends messages to the tissues and organs for an appropriate response. In vertebrates, the nervous system is made up of two parts: the central nervous system, consisting of the brain and spinal cord; and the peripheral system, consisting of nerves that carry signals to and from the central nervous system.