Increasing white matter during childhood and adolescence allows for greater integration of disparate neural circuitry, the hallmark of many maturational changes in brain function. Neurons integrate information from other neurons by summing excitatory and inhibitory input. If excitatory input exceeds a certain threshold, the receiving neuron fires and initiates a series of molecular changes that strengthens the synapses, or connections, from the input neurons, or as described by Donald Hebb in 1940, “cells that fire together wire together.” The summation of input depends on exquisite timing of signals coming from neurons that may be nearby or more distant. Myelin participates in the fine-tuning of this timing, which encodes the basis for thought, consciousness and meaning in the brain. The dynamic activity of myelination during adolescence reflects how much new wiring is occurring. Myelination also plays a central role in developmental changes in the brains ability to change in response to its environment by inhibiting axon sprouting and the creation of new synapses (Fields 2008).
