The outermost layer of the brain is the cortex or gray matter, which is made up of brain cells (neurons). The cortex completely covers the white matter, which is the greatest part of the brain and serves to connect different neurons. The neurons in the cortex send nerve fibers (axons) to and receive axons from other parts of the brain and spinal cord.
White matter is largely made of myelin and gets its name because it has a lot of fat in it and looks whitish. Although MRI pictures allow us to identify areas of damage in the white matter of the brain or spinal cord, they actually show us areas of increased water in the brain. MRI scans in this way identify areas of inflammation or scarring from previous damage. These areas of damage that are seen may be plaques that MS caused or may be the result of some other disease process. It has recently been clearly shown that damage to myelin that is not caused by inflammation will not show up in an ordinary MRI brain scan.
Axons are damaged by the inflammatory process, and some are lost permanetly. This loss of axons is thought to be important in the development of disability and disease progression.