The development of the nervous system is directed by genetic information, nutrition, and cellular and molecular signals within the embryo. The brain and spinal cord begin formation early in gestation, though taking years to mature. In humans, the factors affecting children's neural development after birth include sense organs, environment, and the extent to which the brain and nervous system are challenged to grow. The development of the nervous system in adults depends on plasticity, the ability of the brain to adapt, learn new things, and perform new tasks.
In embryos, development is directed by genes called growth factors, which tell the cells when and where to grow. A layer of cells separate from the proto-tissue called the ectoderm. These become neural crest cells that differentiate into the spinal cord and peripheral nerves, and the neural tube, from which the brain is formed. Neural migration follows as neurons arrange according to chemical signals in the locations they will occupy permanently. Growing axons, the projections of nerve cell bodies, have a special tip called the growth cone that searches for these chemical cues.
During this early period, factors affecting development include the genetic “switches” that direct tissue growth in the embryo and, in mammals including humans, the mother's diet. Many chemicals, called teratagens, can impair formation of the nervous system. Alcohol, tobacco, some pesticides, viruses, and even overdoses of lipid-soluble vitamins can cause birth defects or embryonic or fetal death. The development of the nervous system is most vulnerable to these substances during the early weeks of pregnancy.
After birth, an important process called myelination accelerates for several years before being completed in adolescence. Myelin is a protective sheath around nerves that helps electrical communication. Cognitive and sensorimotor function depend on specific pathways insulated by myelin. Since electrical signals travel slowly and incompletely in uncovered nerves, myelination is a crucial factor in the development of the nervous system.
An important factor in neural development is the formation of neural networks linking millions of cells across the brain. Neurons function through feedback loops modified by experience. Many regions of the cerebral cortex are incompletely “wired” or networked at birth and develop later. A classic example of this occurs in the visual cortex, which is activated by light and can develop only if a young child has functional eyes. If eyesight is impaired during this critical stage, the brain may remain unable to process vision later in life.
Development of the nervous system in humans is strongly linked to activity after birth. Some regions of the brain connected during gestation may be pruned back during childhood, and new ones may become associated due to experiences like learning to speak, walk, or write. In cases of brain injury, active physical and cognitive therapy can also partly restore lost functions of the central nervous system. Due to neuroplasticity, one part of the brain may take over the function of another damaged part.