The brain and spinal cord make up the Central Nervous System (CNS) and are usually among the first body parts to develop in an embryo. In a significant stage of human development, the surface of an embryo typically folds and closes off to form a tube, which marks the beginning of central nervous system development. There is a structure called a neuropore that forms as a result, and which usually closes off after four weeks of pregnancy. Several main vesicles of the brain can then develop. The CNS continues to be modified throughout life, as nerve cells called neurons grow axons, synapses, and growth cones when stimulated.
Central nervous system development begins when embryonic tissue above a structure called the notochord becomes differentiated as neurological. A groove typically forms and then folds develop on each side. These normally close off in the middle and then sequentially on each side to form a neural tube. The neural canal continues to develop and the top, or rostral, part becomes parts of the brain, while the rest of the tract typically turns into the spinal cord.
In the neural crest folds, many cells become nerve structures called ganglia, as well as protective tissues that surround the brain and spinal cord. The vesicles that form the general framework for specific parts of the brain usually form afterward. These include the forebrain, midbrain, and hindbrain vesicles. The three layers of cells in the neural tube also develop to form the spinal cord; different cells and different structures in the cord form out of this process. By eight weeks of pregnancy and central nervous system development, the cord is usually formed along with entire vertebral canal.
Nerve cells typically live for the lifetime of a person. They can die off but instead of being replaced by other cells, the connections between neurons can change. Central nervous system development is often dependent on the growth of axons, which extend from the cells. The process is influenced by the appropriate amount of calcium in each cell, while synapses at the ends of mature axons usually form when neurotropin proteins are triggered.
Axons grow and connect to various areas of the brain. Some cells can act as guides along a particular path, while molecules on some cellular surfaces can repel the growth cones of axon structures. The chemical composition of particular areas can cause axons and nerves to be secured there. Proper central nervous system development is typically most crucial during the embryonic stage, because it can affect someone’s well being throughout life. The CNS is also affected by cellular processes that occur into adulthood, which often enable the brain to adapt to the environment.
