I've been interested in glial cells-which make up more than half the cells of the brain-since 1 was a research assistant at the National Institutes of Health. Of the three types of glial cells, microglia are my favorite. There is so little known about them, which is crazy because they make up 10 percent of our brains. For years, scientists essentially ignored microglia. One of the things they do is prune synapses. There are trillions of synapses in the brain, but initially you start with way more. Synapses that are firing a lot get strengthened, and those that are not firing very much get eliminated. Microglia are like the brain's Pac Men- regulated garbage collectors that eat synapses that no longer function.
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