At birth, an infant's brain is packed with roughly 100 billion neurons-some 15% more than it will have as an adult. As we learn and grow, our experiences strengthen the circuits that prove most relevant while the others weaken and fade."One extreme view of this would be that you start out wired up for every possible contingency," says Jeff Lichtman, a neuroscientist at Harvard University in Cambridge, MA. Over time, a large percentage of those wires are permanently disconnected, says Lichtman."What you're left with is a narrower nervous system," he explains. "But it's tuned exactly to the world you found yourself in."The process of elimination is key to forming a healthy, adaptive brain. Researchers have documented waves of neuronal cell death and the dramatic reduction of neurons' connecting axon fibers early in neural development. But synapses, the fixed points where one cell's axon exchanges signals with another cell, continue to be selectively removed at least through adolescence in humans, refining a coarse neural map into mature circuits.Nerve cells in the cerebellum called purkinje cells (blue) are among the brain cells that undergo synaptic pruning as we age. Researchers are starting to recognize how pruning gone awry in children and teenagers could lay the foundation for neurological disorders. Image credit: Science Source/
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