evidence that synapses that are used are retained and those not used are eliminated, there has been a frenzy of concern expressed as “use it or lose it” in the first years of life. It turns out, however, that synapse elimination is a normal part of development. In comparison to the brain's wiring, far less attention has been paid to the neurochemistry of early brain development, which is essential to the brain's capacity to learn from experience and is likely to play an important role in the regulation of behavior.

Development of the Brain's Wiring Diagram

Brain development proceeds in overlapping phases: making the brain cells (neurulation and neurogenesis), getting the cells to where they need to be (migration), growing axons and dendrites, which are structures needed to link with other nerve cells (neuronal differentiation and pathfinding), developing synapses or points of communication with other cells (synaptogenesis), refining those synapses (maturation and pruning), and, finally, forming the supportive tissue that surrounds the nerve cells and makes for efficient communication among them (gliagenesis or myelination).

The brain and spinal cord arise from a set of cells on the back (dorsal part) of the developing embryo called the neural plate. Two rows of rapidly dividing cells arise from the plate on each side along its length and fold over centrally into the neural tube. The anterior or head end of the neural tube forms a set of swollen enlargements that give rise to the various parts of the brain—the forebrain containing the cerebral hemispheres, the midbrain containing important pathways to and from the forebrain, and the hindbrain containing the brainstem and cerebellum. The remainder of the neural tube becomes the spinal cord, peripheral nerves, and certain endocrine, or hormone, glands in the body. Under the control of regulatory genes, the brain cells migrate to where they belong in accord with the functions they will ultimately serve. These genes provide developmental directions to particular groups of cells, which tell them what to do and where to go in the embryonic brain.

Within the neural tube, the innermost cells divide repeatedly, giving rise first to the cells that primarily become nerve cells, or neurons, and later giving rise to both neurons and the supportive tissue components called glia. Once the nerve cells are formed and finish migrating, they rapidly extend axons and dendrites and begin to form connections with each other, called synapses, often over relatively long distances. These connections allow nerve cells to communicate with each other. This process starts prenatally and continues well into the childhood years. There is evidence in many parts of the nervous system that the stability and strength of these synapses are largely determined by the activity, that is, the firing, of these connections. The speed with which neurons conduct nerve impulses is



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