The nervous system controls all activities of the body and allows reactions to stimuli. It is composed of the brain, spinal cord, and nerves. The central nervous system includes the brain and spinal cord and controls everything in the body, while the outer nervous system connects it to the rest of the body through nerves and sense organs like the eyes, ears, skin, tongue, and nose. The brain is the main organ of the central nervous system and is divided into the cerebrum, cerebellum, and brain stem.