Welcome to Your Child's Brain: How the Mind Grows From Conception to College Read Online Free PDF Page B

autism.
    Similar studies have yielded comparable results. One found that children whose mothers experienced severe stress from a major ice storm while pregnant had lower intelligence quotient (IQ) scores and language ability at age five. The risk of schizophrenia is higher in children whose mothers were in the first trimester of pregnancy when a close relative died or was diagnosed with a serious illness. Children whose mothers experienced an earthquake during pregnancy were more likely to be diagnosed with depression or to be born with a cleft palate. It’s not yet clear whether moderate stress, such as dealing with an annoying boss, might cause similar problems, but as the research is ongoing, it’s best to keep it simple: it’s probably a good idea to take time to relax and be kind to yourself during pregnancy as much as you can.
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    Once axons are close to their destination, they begin to make contacts with nearby cells, initiating the chemical conversation that leads to the formation of synapses. This process begins in the spinal cord five weeks after fertilization, and it is not complete until years after birth in some brain areas. Axons initially form a lot of extra synapses with targets that are only roughly appropriate. Only some of these synapses survive in the long term. Synapses that are more successful at activating their target cells are more likely to be retained. This competition for synaptic survival provides a way of fine-tuning the brain’s function to match each child’s individual circumstances, whether that means adapting the responses of vision neurons to the distance between each child’s eyes or tuning the auditory cortex to respond most easily to the sounds of each child’s native language. To a lesser extent, this process will continue throughout life, as a mechanism of learning and memory (see chapter 21 ).
    The process of eliminating unnecessary components is a major theme of early development. The adult brain contains about 100 billion neurons and many more glia. However, the young brain produces even more cells than that and then reduces their number through planned cell death. In some brain regions, planned death kills as many as four out of every five cells born. These events are called regressive by neuroscientists, and they are essential for normal development.
    Why does the nervous system take such a wasteful approach? It seems to be a way of matching the size of the incoming population of axons to the number of neurons in the target region. Cell death occurs after the axons have reached their target and formed synapses. The target neurons produce a protein, necessary for cell survival, which is taken up at synapses and transported back along the axon to the cell body of the input neuron. Cells that have failed to form enough connections with the target do not get enough of the survival substance, so they die. This type of cell death is an active process, resulting from a biochemical death pathway within the cell. The best-known survival protein (or neurotrophin ) is nerve growth factor, which controls the survival of neurons involved in the sense of touch and the fight-or-flight reflex in the peripheral nervous system. Other factors also influence cell survival, including incoming synaptic activity and sex hormones, which control cell death in brain regions that differ between males and females.

    Even after all the cellular elements of the brain are in place, much construction work remains to be done. Newborn neurons look very simple compared with mature neurons. Toward the end of gestation and especially in the first two years of life, dendrites form additional branches, becoming more and more complex to accommodate the many new synapses that are added during this period. Synapse elimination begins in the first year of life and continues through early adolescence, forming one of the basic mechanisms by which experience helps to shape the brain (see chapter 5 ).
    The final step in