A characteristic of all living organisms is that they respond to their environments, both internal and external. In animals, the nervous system is intimately involved with those responses. The nervous system is a communication center that sends rapid messages in the form of electrical impulses. It receives information about the state of the external environment and about the internal environment within the animal's body. It processes or integrates this information and sends signals back that will initiate some response to the stimuli. In effect, the nervous system acts as a central switchboard and keeps all parts of the body functioning properly and in harmony with each other. Without the nervous system there would be chaos, and vertebrates as we know them could not exist. The nervous system exerts its fine-tuned control through feedback loops and antagonistic control systems. It has a complex architecture in the higher vertebrates, the result of evolutionary trends leading toward greater complexity, differentiation of parts, and sophistication. Despite the complexity and elaborate functionings, though, remember that the central and peripheral nervous systems are built "merely" of neurons; billions of them, to be sure, but each and every one of them operates on the same basic principles. This chapter focuses on the neuron, the functional unit of the system, and the central nervous system, the main processing center. The sensory systems are covered in the next chapter.