The system of cells and tissues in multicellular animals by which information is conveyed between sensory cells and organs and effectors (such as muscles and glands). It consists of the central nervous system (in vertebrates the brain and spinal cord; in invertebrates the nerve cord and ganglia) and the peripheral nervous system. Its function is to receive, transmit, and interpret information and then to formulate appropriate responses for the effector organs. It also serves to coordinate responses that require more than one physiological process. Nervous tissue consists of neurons, which convey the information in the form of impulses, and supporting tissue. During development neurons compete for survival (see competition sense 2), given the limited availability of growth factors such as neurotrophin. Superfluous neurons and synaptic connections are eliminated—a process called pruning—so that only neurons in the correct location and with the appropriate connections become established. In humans this process, which begins in the early embryo, continues into infancy and early childhood. See also growth cone; guidepost cell.

http://www.innerbody.com/image/nervov.html Basic interactive guide to the anatomy of the human nervous system, from Human Anatomy Online