Neuroplasticity
Until the end of the twentieth-century, prevailing theories held that, though young children generated new brain cells, the adult human brain was hardwired after early formative years. In other words, upon reaching adulthood, we would develop all the brain cells we would get, and those cells were genetically coded to do certain tasks.
Scientists also knew connections (synapses) between brain cells (neurons) could change and that learning resulted from changing these connections (sprouting dendrites) or strengthening them. The accepted theory of "cells that fire together wire together," drove these concepts. In other words, the neurons that fired together in a chain changed in such a way that made it more likely that firing one would fire the other. Thus, "wired together.”
And this was the extent of what scientists thought the brain could do. Now we know differently. "Neuroplasticity" is well beyond this retail variety of synaptic connection or strengthening. We’re talking about wholesale changes previously deemed impossible. The brain is capable of reorganizing itself. Go to tabs on cortical expansion, reorganization, and neurogenesis for more.