The brain's supporting cells, called glia, exist in concert with the better- known neurons. In fact, they outnumber neurons 3 to 1. Glial cells are generally smaller than neurons, and they lack axons and dendrites. Supporting cells are not capable of electrical signaling, and therefore do not participate directly in information processing. However, glia in the central nervous system carry out supportive functions that help maintain neurons' signaling abilities. There are many types of glia; three of the most common are described below.

Astrocytes have many arms, called processes, that make them resemble stars. Astrocytes carry out many specific functions to regulate the neurons' biochemical environment in order to keep it stable and favorable for functioning, including maintaining the correct proportions of various ions (such as sodium, potassium, and calcium) in the area near the neuron.

Oligodendrocytes are responsible for making and applying the myelin sheaths that surround the axons of neurons. The myelin, a thick, white substance, insulates certain axons so that the action potential can travel faster over a longer distance (similar to the plastic insulation on a telephone wire). If you cut through an axon, you can see that the myelin lies in layers, like the rings in a tree trunk. In the peripheral nervous system, the cells that generate myelin sheaths are called Schwann cells.

Microglia, the third type of glial cells, are cellular repairmen. Whenever cells in the brain are damaged, microglia appear in large numbers. Because they tend to proliferate after damage and because they resemble macrophages (cells which repair damage in the body), it is presumed that microglia help repair damage after neuronal injury.