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

put up. Only the earliest-arriving axons must find their way by themselves, navigating via chemical signals or by finding particular guidepost cells.
    Later axons extend along the pathways laid down by these early pioneer axons, just as you might guide a new wire through a bundle of previously installed wires, except that the new axon is actually being created as it progresses. A bundle of axons in the brain is called a nerve . A region at the tip of the elongating axon called the growth cone samples the environment within the brain in different directions by extending and retracting small protrusions, making it look as though the growth cone is sniffing out the correct path. Depending on their identity, these chemicals may either attract or repel the growth cone. Some can even cause it to abruptly change its responsiveness to other molecular cues, a form of sophisticated navigational logic.
    Once an axon has found its approximate destination in the brain, it must pinpoint its target cells from among millions of candidates. This process starts with molecular cues that tell the axon to slow down and start exploring an area whose boundaries may be marked with a repellent signal to prevent the axon from exiting. Some brain areas help the axon to navigate by providing a local map, in which the concentration of a chemical signal (or several) descends steadily across the area. Other areas use a large number of related proteins to mark local position so axons can find their way to the right neurons. Proteins are the universal building blocks made by cells for a wide variety of functions. In this case, the function is to say to an axon, “You are here.”
----
    PRACTICAL TIP: LESS STRESS, FEWER PROBLEMS

    Next time you’re stressing about your future child, ask yourself whether this stress is really necessary. Neuroscientists are able to discover what stress does by studying its effects on laboratory animals. Maternal stress increases the risk of a variety of problems, including cleft palate, depression-like behavior, a touchy stress-response system in adulthood (see chapter 26 ), and attention deficits and distractibility (see chapter 28 ). Stress hormones released by the mother animal act on the fetus directly and also reduce the placenta’s ability to protect the fetus from these hormones in the future. Because it would be highly unethical to stress pregnant women deliberately, most research in people has relied on looking for correlations, which is less reliable than experimental results (see Did you know? Epidemiology is hard to interpret ). Some recent studies have examined children born after their mothers experienced natural disasters during pregnancy. This type of study comes as close as is ethically possible to randomly placing women into stressed and unstressed groups.
    One group of researchers identified all tropical storms or hurricanes that hit Louisiana between 1980 and 1995 and then determined how many autistic children in the records of the state health system had been in the womb when their mother’s home was hit by one of these storms. The risk of autism was significantly higher for children whose mothers had been stressed during pregnancy—though most cases of autism probably result from other causes (see chapter 27 ).
    By scientific standards, this evidence is far from ironclad, but there are two reasons to believe it’s not mere chance. First, the incidence of autism was higher only for those children whose mothers were in the fifth, sixth, or ninth month of pregnancy at the time of the hurricane, suggesting that there is a period when the effects of stress on development are long-lasting (see chapter 5 ). Second, children whose mothers were exposed to more severe storms had a higher risk of autism than children whose mothers were exposed to less severe storms. This research will need to be replicated before we can consider it definitive, but it does suggest that prenatal stress may increase the chances of