In humans, the spinal cord extends from the base of the brain to the middle of the back and is about 18 inches long. It is composed of 31 paired spinal nerves that transmit information about internal and external conditions to the brain, and also distribute impulses that mediate reflex actions from the brain to other parts of the body. Essentially, all thoughts and actions depend on the proper functioning of the nervous system. Any disturbance in the nervous system can affect the entire body.