Answer: White matter is something that's completely normal, and something that we're supposed to see on an MRI. Different areas of the brain and spinal cord appear either gray or white.

The gray area is called the "gray matter," and comprises groups of nerve cell bodies. Gray matter forms the surface layer of the brain and an area deep inside the brain. If you look at the spinal cord in cross section, you'll see the gray matter appearing in the center as a large "H" or "butterfly."

The white matter consists mainly of nerve fibers that extend from those nerve cell bodies that are part of the gray matter. These nerve fibers carry nerve impulses to and from the brain and spinal cord.

These nerve fibers are covered by a white, fatty insulating material called "myelin." It's the myelin sheathing that surrounds nerve fibers and gives the tissue its white color. White matter forms a layer between the two areas of gray matter in the brain, and it also encloses that "H-shaped" gray matter column in the spinal cord. While not all the nerves in your body are insulated by a myelin sheath, those that do have it carry nerve impulses much faster.

If there's a destruction of the myelin, there's a loss of white matter. One such disease where this occurs is multiple sclerosis.

An organization working to accelerate the pace of research toward an eventual cure for these and other white matter diseases is the Myelin Project (www.myelin.org).