Oligodendrocytes are the small cells of the central nervous system, which includes the brain and spinal cord. The cells carry out the important job of forming a fatty covering, called a myelin sheath, around neurons. The myelin sheath insulates nerve cell axons, the cell outgrowths that conduct electrical signals. According to Mayo Clinic, the loss of myelin sheath around the nerve cells results in diseases such as multiple sclerosis.
Astrocytes, the most numerous glial cells, owe their name to their roughly star-shaped cell bodies. The long processes of astrocytes have structures called end feet. These end feet help in forming the blood-brain barrier. The barrier prevents toxic substances in the blood from entering the brain.
Schwann cells help in creating the myelin sheath around the axons of the peripheral nervous system, which constitutes the nerves outside the brain and spinal cord.
In the central nervous system, microglia serve as the major antigen-presenting cells --- the specialized white blood cells that help fight against foreign substances that enter the body. Microglia usually remain in the resting state, but they are thought to become activated in a number of diseases including multiple sclerosis.