The major structures of the nervous system form during the initial months after the fertilisation, but the neuronal connections (the “wiring”) of the nervous system continues to change briskly during the first years after birth in response to experience, and to a lesser extent throughout life. In the first weeks after fertilisation, the developing embryo forms a three-layered disk. One of the outer layers is called the ectoderm, and its center goes on to become most of the nervous system. A layer of cells in this area, called neural stem cells, roll into a tube called the neural tube. These cells then multiply and move to form most of the structures of the central nervous system. The cavity inside the neural tube develops into cavities inside the central nervous system called the ventricular system. Other cells of the ectoderm just next to the neural tube, called neural crest cells, move through other tissues to form most of the peripheral nervous system. Some axons grow out of the central nervous system to add to the nerves, as well.
Developing neurons sprout axons and dendrites, and form many synapses with other cells. After birth, experience changes how information flows through networks of neurons in the nervous system. Individual synapses, and groups of synapses connecting chains of neurons, that are used frequently become more efficient at transmitting information. Synapses and chains of neurons that are used infrequently become less efficient at transmitting information, or they are removed. These neural changes that occur from experience are called neuroplasticity. This happens the most early in development, but continues to a lesser extent throughout life, such as when learning new information or skills. Clinicians assess the progress of neurodevelopment by noting things called milestones. These may be categorized several ways, such as by the domains (categories) of gross motor, fine motor, cognitive, and social functions. A clinician compares a child’s acquisition of milestones to the average of many children that are the same age to determine if the nervous system is developing normally.