Neurons (the cells of the cortex that are involved in processing information) are formed before birth during the sixth to eighteenth weeks after conception. In the cerebral cortex, neurons find their way to the correct position by moving along the long fibers of radial glia cells, which are like ropes extending from the inner to the outer surface of the brain. The length that neurons must travel is especially long for those that will end up in the frontal lobes, and this may increase the likelihood that they will end up in the incorrect position and disrupt information processing. Schahram Akbarian and his colleagues suggested in a 1993 paper that such errors might contribute to schizophrenia.
Once neurons have traveled to their final positions, they begin to differentiate or take on their mature characteristics. One aspect of differentiation is the growth and branching of dendrites. The den-drites of a neuron are like antennae that pick up signals from many other neurons and, if the circumstances are right, pass the signal down the axon and on to other neurons. The pattern of branching of dendrites is important because it affects the amount and type of signals the neuron receives. During development one change that occurs is an increase in size and complexity of neurons' dendritic trees. For example, by adulthood the length of the dendrites of neurons in the frontal cortex can increase to more than thirty times their length at birth. A second aspect of differentiation that occurs in most neurons is myelination. Myelin is a fatty sheath that forms around neurons and helps them transmit signals more quickly. Myelin begins to form around neurons before birth and continues to do so even into adulthood in some areas of the cortex.
The points of communication between neurons are called synapses, and these begin to form in the brain in the early weeks of gestation. The generation of synapses occurs at different times in different cortical areas. For example, the maximum density of synapses is reached at about four months in the visual cortex but not until about twenty-four months after birth in the prefrontal cortex. This pattern parallels behavioral development, where functions of the visual cortex (such as 3-D vision) develop earlier than some functions of the prefrontal cortex (such as planning for the future).
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