What's Gray Matter?
The details of what is behind the increase and decline in gray matter are still not completely clear. Gray matter is made up of the cell bodies of neurons, the nerve fibers that project from them, and support cells. One of the features of the brain's growth in early life is that there is an early blooming of synapses—the connections between brain cells or neurons—followed by pruning as the brain matures. Synapses are the relays over which neurons communicate with each other and are the basis of the working circuitry of the brain. Already more numerous than an adult's at birth, synapses multiply rapidly in the first months of life. A 2-year-old has about half again as many synapses as an adult. (For an idea of the complexity of the brain: a cube of brain matter, 1 millimeter on each side, can contain between 35 and 70 million neurons and an estimated 500 billion synapses.)
Scientists believe that the loss of synapses as a child matures is part of the process by which the brain becomes more efficient. Although genes play a role in the decline in synapses, animal research has shown that experience also shapes the decline. Synapses "exercised" by experience survive and are strengthened, while others are pruned away. Scientists are working to determine to what extent the changes in gray matter on brain scans during the teen years reflect growth and pruning of synapses.