Non-Neuronal Cells: Glial Cells
Although the figure of 100 billion neurons in the brain is certainly impressive, within that same volume are at least ten times as many non-neuronal cells called glia. Glial cells fall into three major types — astrocytes, oligodendrocytes and Schwann cells, and microglia — each with a function, as the following sections explain.
There is also a type of glial cell called a satellite glial cell. These cells enclose the surface of neural cell bodies in various ganglia (concentrations of nerve cells) in the peripheral nervous system.
Astrocytes are glial cells that form much of the structure of the brain in which the neurons reside. Astrocytes regulate the brain environment and form the blood-brain barrier. In most of the body, the capillaries of the blood system are permeable to many substances so that oxygen, glucose, and amino acids move from the blood to the tissue, while carbon dioxide and other wastes go the other way.
In the brain, the astrocytes form an additional barrier that is much more selective, creating a more finely controlled environment for the complex operations taking place there. The astrocytes do this by lining the blood vessels and only allowing traffic of substances between the capillaries and the brain that they themselves control.
There are several downsides to the relative chemical isolation of the brain created by the astrocytes, however. One is that many drugs that could potentially help treat brain dysfunctions can’t pass this astrocyte barrier; therefore, treatments can’t be done by injecting these drugs in the bloodstream.
Another downside of brain isolation is that brain cancers are not readily attacked by the immune system because antibody cells also have a hard time getting from the blood to brain tumors. Most cancer treatment protocols effectively involve using toxic chemicals and radiation to wipe out the vast majority of dividing cancer cells, hoping the immune system can mop up the last few percent. But because the antibody mop-up operation in the brain is so inefficient, brain cancers typically have very poor prognoses.
Another type of glial cell with a similar function is the ependymal cell. These glial cells form a barrier at the inner surface of the cerebrospinal fluid-filled ventricles of the brain and the central canal of the spinal cord.
Oligodendrocytes and Schwann cells
The second class of glial cells are called oligodendrocytes and Schwann cells. The function of both of these cells is to do the myelin wrapping of axons for saltatory conduction, wherein the nodes of Ranvier with high voltage-gated sodium channels form “repeaters” allowing long distance spike propagation. Oligodendrocytes perform this function in the central nervous system, while Schwann cells perform the same function in the peripheral nervous system.
An area of intense research now concerns the fact that peripheral nerves regenerate their axons, while central neurons usually do not. You can, for example, gash your finger and cut the nerves so severely that all sensory and motor control functions are lost in the fingertip. But wait a month or two, and full function generally returns. The same injury in the spinal cord, however, paralyzes you for life. Some differences between oligodendrocytes and Schwann cells in response to injury are thought to be important for regeneration.
The last class of glial cells are microglial cells. These are scavenger cells that migrate through the brain when some area is injured and remove (scavenge) debris. This is just one aspect of the many housekeeping functions that different glial cells do, which include maintaining the proper ionic constituencies in the extracellular space, interacting with blood vessels during brain injuries, and providing the structural framework in which neurons initially grow during development.