The brain consists of two major types of cells, neurons and glial cells. Glial cells are as numerous as neurons. While neurons have been studied for more than a century, extensive glial research has only developed over the last decades. For the past few years it has become evident that the functioning of the brain can be understood only if the interaction of all cell types, i.e. neurons and glial cells, is understood. For the last two decades glial cell research has considerably increased, and there are a number of reasons for this development; one aspect is that studies in molecular biology have revealed gene products in the brain which were often expressed by glial cells. Thus, the function of these molecules was related to the biology of glial cells. In addition, animal models for all major neurological and psychiatric diseases have been developed over the past years and these models enabled us to better understand the cellular responses in different pathological stages. It could be shown that there is not a single pathological process in the brain which occurs without participation of glial cells, specifically microglia and astrocytes. Another aspect is the evidence of glial excitability. This form of communication is much slower than the neuronal response, however it has become evident that glial cells, and astrocytes in particular, modulate neuronal activity and thus brain function. We can assume that the combination of the activities of glial cells and neurons is crucial for all brain functions, such as thinking, emotions and other functions which define human nature. There is a lot of speculation, and only future experiments will deliver final responses.
We can distinguish three types of glial cells in the central nervous system: astrocytes, oligodendrocytes and microglial cells. In the peripheral nervous system Schwann cells can be found as well. Astrocytes are a rather heterogeneous cell population which interact with neurons and blood vessels. These cells detect neuronal activity and modulate neuronal networks. Oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system produce myelin and are thus responsible for the high speed information processing in the axons of vertebrates. Microglial cells are the immune cells of the central nervous system and react with a so-called activation to all changes in the nervous system. They are therefore also called the pathological sensors of the brain. They migrate to the site of damage, they can proliferate and become phagocytes and they interact with the peripheral immune system by antigen presentation.
Today we conceive the brain as an organ which can achieve its function due to the interaction of all these cell types only. This is especially the case in pathological states.