The nervous system coordinates behavior and helps to maintain the internal stability of animals. It may be as simple as the nerve net of Cnidarians or as complex as the centralized system of mammals. In all nervous systems the functional unit is the nerve cell or neuron, a cell specialized to transmit and receive a stimulus.

In humans, the brain and spinal cord are formed early in embryonic development. At the beginning of the third week of gestation, the embryo has already formed a neural plate on the dorsal surface that eventually folds together to form a hollow tube from which the brain and spinal cord develop. During this time the 100 billion neurons found in the brain are produced--the sum total of all the neurons that the brain will ever contain in an individual's lifetime. The brain is one of the largest organs in the body...