- H. Kettenmann
- A. Verkhratsky
In the human brain glial cells are as abundant as neurons. The relative number of glial cells has increased with increasing complexity of brains during evolution. In vertebrates three types of glial cells can be distinguished in the central nervous system, namely astrocytes, oligodendrocytes and microglia. In the peripheral nervous system the Schwann cell is the only glial cell type. Astrocytes are a heterogeneous cell population, are most abundant and fulfill different tasks such as providing guiding structures during development, controlling homeostasis of the extracellular space, providing energy substrate for neurons, controlling blood flow and modulating synaptic transmission. Oligodendrocytes in the central and Schwann cells in the peripheral nervous system form myelin and thereby enable a high conduction velocity within the axons. Microglial cells are the immune competent cells of the brain and are activated during any pathologic process. The activated microglial cells can release many factors which influence the pathologic process. Taken together brain function is only possible by a concerted action of neurons and glial cells.