The nervous system is a highway along which the brain transmits and receives information about what is happening in and around the body. It consists of the brain, spinal cord, and nerve fibers that extend to all parts of the body, including the neck and arms, trunk, legs, skeletal muscles, and internal organs.
In order to control the movement of muscles, and the functions of internal organs, messages are transmitted through the spinal cord and nerve endings. The main working unit is the cells, they are called neurons, there are more than 100 billion of them.