The nervous system derives its name from nerves, which are cylindrical bundles of fibers (the axons of neurons), that emanate from the brain and spinal cord, and branch repeatedly to innervate every part of the body.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.