Astrocytes are by far the most numerous cell type in the brain. Each one has many fine branches that come into contact with hundreds of dendrites and up to 150,000 individual synapses. These processes are highly motile, and rapidly extend toward and envelop active synapses. Electron microscopic examination of brain tissue reveals that their fibers interact with large dendritic spines in response to neuronal activity, and that these fibers are less motile than those associated with small spines. Large spines tend to be more persistent than smaller ones, and so it seems that astrocytes help to
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