The brain is composed of two major cell types - neurons and glial cells. While neurons have been extensively studied, research on glia cells has picked up only in the last decades. There are three types of glia cells in the central nervous system: astrocytes, oligodendrocytes and microglia cells. In the peripheral nervous system the glia cells are called Schwann cells. Astrocytes are a very heterogeneous population of cells which interact with neurons and blood vessels. These cells detect neuronal activity and can modulate neuronal networks. Oligodendrocytes in the central and Schwann cells in the peripheral nervous system form myelin and therefore are prerequisites for the high conduction velocity of axons in vertebrates. Microglia cells are the immune cells of the central nervous system and respond by a process called activation to any change in the environment. They are therefore considered as pathological sensors of the brain. They migrate to the site of injury, can proliferate and phagocytose and interact with the peripheral immune system by antigen presentation. Today, we view the brain as an organ which fulfils its function by the interaction of all these cell types. This is also particularly relevant for brain diseases.
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