Neuroglia are not directly involved in information processing but they are important for normal functioning of the nervous system. There are various types of neuroglial cell that are recognised and the three main types are astrocytes oligodendrocytes (oligodendroglia) and microglia.
Astrocytes possess numerous processes. Some of these form so-called perivascular endfeet upon the walls of blood capillaries. Astrocytes are most probably involved in the exchange of chemicals between the circulatory system and the nervous tissue. It has been suggested that they may constitute the blood-brain barrier, which restricts the access of circulating chemicals to the brain and spinal cord.
Oligodendrocytes possess few processes. Their main role is the production of the myelin sheath that surrounds many axons in the central nervous system (CNS). Schwann cells perform this function in the peripheral nervous system. Myelination increases the rate of conduction of axons. The myelin sheath comprises numerous concentric layers of oligodendrocytes or Schwann cell membrane. Eeach glial cell produces the myelin sheath over over only a short segment of axon, usually up to about 1mm. A long axon is, therefore, enveloped by the membranes of many glial cells. Adjacent segments of myelin, derived from different glial cells, are seperated by a small gap, the node of Ranvier. Ionic fluxes across the axonal membrane, which mediate generation of the action potential, occur only at the nodes of Ranvier where the axon is exposed. Between nodes, where the axon is insulated by myelin, depolarisation spreads by passive means. This means of propagation is known as saltatory conduction (from the latin salture, to jump), since the action potential may be thought of as jumping from node to node. It is considerably faster than conduction in unmyelinated axons.
Microglia are small cells with few processes. Microglia increase in number at sites of damage in the CNS and have a phagocytic role, similar to macrophages elsewhere.
Neuronal and glial disorders
Once development has been achieved, individual neurones no longer replicate but undergo continual repair to maintain their integrity. They are therefore prone to neurodegenerative diseases, which in childhood and youth are often genetically determined and in the elderly are sporadic. Glia, unlike neurones, are capable of replication and they are therefore susceptible to neoplasia. Consequently most brain tumours are glomas and not tumours of neurones themselves. According to the cell of origin, they may become either oligodendrogliomas or astrocytomas. Microglia form part of the immune system and may proliferate to become cerebral lymphomas.
The myelin sheath of neuronal axons can be the seat of inflammatory diseases. In Europe and North America, the most common immune disorder of the CNS is multiple sclerosis, which leads to episodes of demyelination and remyelination of axons corresponding to relapses and remission of neurological symptoms and signs. As the disorder is mainly of axons, not cell bodies, magnetic resonance brain imaging can detect abnormal signals from the demyelinating foci in the cerebral white matter.