determined by the development of myelin, a substance that wraps itself around nerve axons. By insulating the nerve cell axon, myelin increases conduction velocity. The development of myelin is a protracted process extending well into the postnatal period. The rate and extent of myelination is also affected by experience. Most myelinated pathways are laid down in the early years, but for some, as in the frontal cortex, myelination continues into the third decade of life. The unique wiring diagram that brain development produces in each individual brain guides thoughts, memories, feelings, and behaviors.

Synaptic Overproduction and Loss

Beginning 20 years ago, Huttenlocher (e.g., Huttenlocher, 1979; Huttenlocher and Dabholkar, 1997) first showed that there is a pattern to synaptogenesis in the human cerebral cortex characterized by the rapid proliferation and overproduction of synapses, followed by a phase of synapse elimination or pruning that eventually brings the overall number of synapses down to their adult levels. This process is most exuberant during the first few years of life, although it can extend well into adolescence. Within this developmental span, however, different brain regions with different functions appear to develop on different time courses (see Figure 8-1). Huttenlocher estimated that the peak of synaptic overproduction in the visual cortex occurs about midway through the first year of life, followed by a gradual retraction until the middle to the end of the preschool period, by which time the number of synapses has reached adult levels. In areas of the brain that subserve audition and language, a similar although somewhat later time course was observed. However, in the prefrontal cortex (the area of the brain where higher-level cognition takes place), a very different picture emerges. Here the peak of overproduction occurs at around one year of age, and it is not until middle to late adolescence that adult numbers of synapses are obtained.1

Scientists have pondered the purpose of synaptic overproduction and loss for a very long time. One of the earliest observations was made by

1  

Many of the human findings regarding synaptic overproduction and loss were based on measurements of the density of synapses, rather than on measurements of the actual number of synapses. Density measures reflect both how many synapses are present and how many other things (e.g., nerve cell bodies, dendrites and axons, glial cells, and blood vessels) are present in addition to synapses. The human brain adds lots of cells to the cerebral cortex postnatally (almost two-thirds of the mass of the cerebral cortex is added after birth), and this makes density estimates very difficult to interpret. Thus, evidence available to date does not enable determination of how ubiquitous synapse overproduction and loss are in brain development generally or in humans specifically.



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