The nervous system is the part of an animal 's body that coordinates its voluntary and involuntary actions and transmits signals to and from different parts of its body. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago.he size of the nervous system ranges from a few hundred cells in the simplest worms, to around 100 billion cells in humans. The central nervous system functions to send signals from one cell to others, or from one part of the body to others and to receive feedback.