Like the circulatory system, the nervous system has branches all over the body. Its nerves divide many times. The thickest nerves look like pieces of cream-coloured rope, and the thinnest are narrower than a hair. Nerves are made of bundles of nerve cells. These cells are specially designed to carry tiny electrical messages. Some nerve cells are called sensory cells. They carry signals from the sense organs, such as the eyes and ears, to the brain, where they are analysed. Other nerve cells are called motor cells. They carry nerve signals from the barin and spinal cord to the muscles, telling them when to contract and when to relax. Many nerves contain both sensory and motor nerve cells.