Aside from neurons, many other cells play a vital role in the functioning of the nervous system. Collectively, these cells are known as glial cells or neuroglia (Figure 1). Historically, glial cells were primarily thought of as support cells for neurons. However, further research has indicated that they have a broader and more critical role in the nervous system. The principal glial cells that are important to know for the MCAT are the Schwann cells, oligodendrocytes, astrocytes, ependymal cells, and microglia.

In terms of Schwann cells and oligodendrocytes, both are involved in the formation of the myelin sheath around axons. However, their location differs. Schwann cells form the myelin sheath around axons in the peripheral nervous system, whereas the oligodendrocytes do the same in the central nervous system. In the disease multiple sclerosis, the oligodendrocytes are damaged as a result of inflammatory infiltrates. Loss of oligodendrocyte function leads to the destruction of the myelin sheath and irreversible axonal damage.

Another critical glial cell in the nervous system is the astrocyte. Neurons, like all cells, need access to a blood supply to receive nutrients and eliminate metabolic waste. Astrocytes, or astroglia, are specialized, star-shaped cells, found in the brain and the spinal cord. They have several functions, one of which is to link neurons to the blood supply of the brain.

Ependymal cells are cells found within the ventricles of the brain and are responsible for the production and regulation of cerebrospinal fluid or CSF. Lastly, microglia are the resident macrophages in the central nervous system, and they have an immune role. One interesting point to note is that microglia may become chronically active and play a destructive role in several neurodegenerative disorders, such as Alzheimer’s disease, Parkinson’ s disease, multiple sclerosis, and even some bacterial and parasitic infections.