Approximately 9 months after conception, neurons in the human nervous system lose their ability to divide. Unlike most other cells in the body, neurons that die are not replaced. (Outside the central nervous system, however, an axon that is severed may be regenerated if the cell body remains intact). Despite the fact that no new neurons are formed after birth, the brain continues to grow from about 350 grams in a newborn infant to about 1,400 grams at puberty. This quadrupling in weight is partly due to an increase in the size of individual cells. In addition the axons of many cells gradually grow a multilayered jacket of insulation made up of fatty cells called myelin. Since the transmission of neural impulses is as much as 20 times faster in myelinated fibers, the process of myelination is essential to the maturation of the nervous system. About half the cells in the nervous system eventually acquire a myelin sheath.
Probably the most important factor in the brain’s growth in size during childhood is the proliferation of a second kind of brain cell the glial cell.
Unlike neurons, glial cells continue to divide; by adulthood they are 10 times as numerous as neurons. When a neuron dies a glial cell grows to fill the gap.
Glial cell were once thought to serve primarily to hold the neurons togther (“glial” comes from a Greek word meaning ”sticky oils”). Now it is known that they have several other functions. They are responsible for the myelination of axons in the brain; they direct the growth of neuronal pathways or interconnections; and they play a general role in nervous system metabolism.
We have said that no new neurons are formed after fetal development. But parts of them are continuously being replaced as they wear out or are used up. In a neuron all-replacement parts are manufactured in the cell body and must be transported from there to wherever they are needed. A slow-moving system known as axonal transport carries the new cellular components down the axon to their destination. Cells in the brain are no more than a few centimeters in length, but in the peripheral nervous system-where an axon may extend more than a meter from its cell body-it may take weeds for the replacement parts to reach their intended site.
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