Glial cells are a category of non-neuronal (not voltage excitable) cells in the nervous system which perform various support roles. There are a number of glial cell types, all of which are necessary for the function of the nervous system as a whole. Unlike neurons, all glial cell types are replaced by precursors throughout the lifecycle.
The five main glial cell types are Schwann cells, oligodendrocytes, microglia, astrocytes, and ependymal cells. The first two cell types, Schwann cells and oligodendrocytes, both provide myelin to the axons of neurons. One Oligodendrocyte is responsible for the myelination of multiple axons in the central nervous system (the brain and spinal cord), whereas a Schwann cell only myelinates a single axon and is located in the peripheral nervous system (the body). Myelination significantly increases the speed at which a neural signal travels from neuron to neuron through the axon, which is important for efficient information processing and coordination of signals. Multiple Sclerosis is a disease in which myelination by oligodendrocytes are lost, and leads to symptoms including but not limited to confusion, visual problems, numbness, and mobility issues.
Microglia are the primary form of immune cell of the central nervous system, performing the important roles of destroying pathogens and responding to injuries. These cells make up to 10-15% of the total number of cells in the brain. As the first line of defense against infection, they are derived very early in neural development from the embryonic yolk sac. While these cells are very important for protecting the central nervous system, if they mistakenly attack the body’s own cells due to an autoimmune disease, they can cause major health issues.
Astrocytes have the most diverse set of roles of the glial cell types, performing metabolic, homeostatic, and protective roles. These roles include removing excess neurotransmitters, maintaining the blood-brain barrier, and promoting the formation of new synapses between neurons. As such, they are essential for many of the chemical signaling. As astrocytes have such diverse functions, their dysfunction can have equally diverse health consequences. Astrocyte dysfunction has been implicated in Huntington’s, Parkinson’s, and Alzheimer’s diseases.
Finally, Ependymal cells line the parts of the brain filled with cerebrospinal fluid, acting as a barrier which regulates what flows into and out of the rest of the brain. They are derived from radial glia, which are present during development. One of the most common molecules to be regulated by ependymal cells is glucose, which is the main source of energy for the brain. If too many ependymal cells develop, a condition called Ependymoma can arise, in which the passageways that carry cerebrospinal fluid are constricted.
While historically neurons have been the primary focus of research on the nervous system, the wide variety of glial cells present in the body perform equally important roles. Learning about their diverse functions is key to understanding many neurological diseases and the body’s response to brain injury.
Written by Owen Smith