Glial cells are non-neuronal cells, with a broad range of duties in the Nervous System. They not only keep the neurons in place, but also insulate them from one another (e.g. by forming the myelin sheath). They supply the neurons with oxygen and other nutrients needed. And they take over the role of the housekeepers in the nervous system by removing pathogens and dead neurons.
There are two major types of glial cells:
Microglia act as Macrophages in the brain and spinal cord – they are the immune defense of the nervous system. They clean the CNS from dead or damaged neurons, plagues and infectious agents.
Only few antibodies are small enough to cross the blood brain barrier, therefore microglia are highly sensitive to foreign bodies, which they remove by swallowing them, and act as antigen-presenting cells activating T-cells. Approximately 15% of the total cell number in the central nervous system are microglia.
Macroglia is more a collective term for several subtypes than a cell type itself, including Astrocytes, Oligodendrocytes, Schwann cells and others.
The most abundant cells in the human brain are Astrocytes, also known as astroglia. They have a very characteristic star-shape. Not long ago, they were considered as gap-fillers, but now it was discovered, that they also play highly important roles in the nervous system. The most important one is that they maintain homeostasis in the brain i.e. they keep constant the concentration of or ions and molecules as neurotransmitters and glucose in the extracellular space. This is essential to enable proper neuronal network functionality. They also provide the neurons with lactate.
Recently it has been clearly demonstrated that they modulate synaptic transmission. Oligodentrocytes are coating the axons, providing them with the so called myelin-sheath. This reduces ion leakage and decrease the capacitance of the cell membrane. Myelin also increases impulse speed – so it serves as isolator around the wire (=axon). In the peripheral nervous system, the Schwann-cells have the same function.