Supportive cells

Although neurons are the fundamental cell of the nervous system and are responsible for transmitting a signal, they make up only 10% of nervous tissue. The remaining 90% is made of several cell types collec­tively called neuroglia, or glia, that support, protect, and nourish neighboring neurons both in the CNS and PNS.

Ependymal cells

The brain has four fluid-filled cavities within its borders called Cerebroventricles; the spinal cord has a central canal. These cavities are lined with ependymal cells that range in shape from squamous to columnar and may be ciliated. These cells generally form a per­meable barrier between the cerebrospinal fluid and tissue. The exception is ependymal cells covering the choroid plexus (capillary tuft in each Cerebroventricle responsible for forming cerebrospinal fluid). These ependymal cells form tight junctions creating a barrier between the blood and brain. The cilia located on ependymal cells help "circulate" or move the cerebro­spinal fluid within the ventricular system.

Astrocytes

Astrocytes are star-shaped and are the most abundant glial cells. They have radiating processes that expand at the end and form end feet that wrap around capil­laries. The astrocytes secrete some chemical that causes the endothelial cells lining the brain capillaries to form tight junctions. These tight junctions form the blood­brain barrier that greatly restricts the movement of compounds into the brain. Astrocytes also help guide the migration and connections of new neurons, control the chemical environment around neurons, and help inactivate neurotransmitters that are released into the synapse. Since astrocytes are also connected via gap junctions, they also signal one another, although the function is unclear.

Oligodendrocytes

Oligodendrocytes have fewer processes than astro­cytes and smaller cell bodies. As already discussed, these cells are responsible for producing the myelin sheath found around axons in the CNS. This sheath is formed by the process of the oligodendrocyte that wraps around an axon forming concentric layers or circles, much like wrapping layers of gauze around a cut finger.

Microglia

These are small ovoid cells that have narrow cytoplas­mic processes with many branches. These cells origi­nate from mesodermal stem cells related to those that produce monocytes and macrophages. They can migrate within the CNS where they phagocytize microorganisms and dead neurons. Although cells in the peripheral immune system cannot enter the CNS, the microglia seem to partially fill this role.

Satellite cells and Schwann cells

Also called amphicytes, satellite cells surround neuro­nal cell bodies in peripheral ganglia. Their function is unknown. As previously discussed, Schwann cells form the myelin sheath around peripheral axons. Therefore, they are analogous to oligodendrocytes.