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

As development continues, chemical markers divide the brain into progressively more regions, defining particular cortical areas, such as those for certain aspects of vision or language. A cluster of cells with a common function is often called a nucleus . Once all the brain areas are specified, they grow larger, maturing in sequence from the back of the brain to the front (see figure ). This process continues through childhood and into adolescence (see chapter 9 ).
    The main construction technique in the early stages of brain development is the production of new cells—billions and billions of them. Cells of the early nervous system divide repeatedly to make additional progenitor cells. These cells can even divide as they move, leaving trails of neurons behind them. Cell divisionalso produces various types of glial cell , which contribute to brain function in many ways. One type of glial cell helps guide the placement of neurons early in development by extending long fibers that act as trails for neurons to follow.
    The number of cell divisions and what type of cells they produce are tightly regulated by a combination of chemical signals, which vary across brain regions, and interactions with preexisting cells. The addition of new neurons is largely complete by about twenty weeks of gestational age (which is counted from the first day of the last menstrual period), or eighteen weeks after conception. A very small number of neurons continue to be generated even into adulthood, and new glial cells are generated throughout life.
    During this period, cells are also beginning to differentiate, taking on particular jobs in the brain. Cells differentiate in a series of steps, as their jobs are slowly made more specific by increasingly restrictive chemical signals.
    At a basic level, neurons have a lot in common (see figure ). They receive chemical signals called neurotransmitters that are released from other neurons. When neurotransmitter molecules bind to receptors on the dendrites of the neuron, electrical and chemical signals are generated that can spread—all the way to the cell body in the case of electrical signals. If enough electrical signals occur at the same time, the cell body can make an electrical impulse that is used to talk back to other neurons.
    This output signal, called an action potential or spike, is conveyed down the axon , a very long, thin fiber that reaches from the brain to the target, as far away as the toe in some cases. Each neuron has a single axon, which often branches to reach multiple targets. Neurotransmitter molecules are contained in specialized areas at the ends of the axonal branches and released by the arrival of a spike. When a neurotransmitter binds to receptors on another neuron’s dendrites, that target neuron may be electrically excited or inhibited, depending on the identity of the neurotransmitter. The point of connection between axon and dendrite is called a synapse . Final stages of differentiation often depend on neurons’ interactions at synapses.
    Glia also come in different flavors. Some glia wrap themselves around axons like the insulating plastic sheath on electrical wire, forming a layer called myelin to increase the speed of neural communication. Other glia line blood vessels to control which chemical signals are permitted to pass into and out of the brain. Still others form the brain’s defense system, engulfing and removing foreignmatter and debris from dying cells. Glia too become differentiated by exposure to chemical signals, generally a bit later than the neurons in the same areas.
    The first step in the wiring process occurs before birth, as these billions of neurons extend axons toward their targets. Fortunately, distances are much shorter in the fetus than they would be in an adult. It also helps that brain tissue is less crowded than it will eventually become, just as it’s easier to run electrical wires and plumbing in a house before the interior walls have been