Not all axons are myelinated: only those that must carry information at high speeds and over long distances. Myelinated axons in the human brain are as small as one-thousandth of a millimeter in diameter. Axons in the brain and spinal cord are myelinated by oligodendrocytes. Axons in the rest of the body that are myelinated, are wrapped by Schwann glial cells. (Fields, R. Douglas, PhD. The Other Brain. p 40-41. NY: Simon & Schuster, 2009.)