What is white matter

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 (Figures 1 and 2).

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.

Image White Matter

Figure 1. MRI of the brain. A transverse section shows "T2" bright spots in the white matter (labeled "W") of the brain. Note the cortex ("C"), the slightly lighter gray area in the image, which is the layer over the white matter. The bright T2 areas are often referred to by physicians as "hyper-intense areas T2 signal" and at other times are simply called plaques.

Figure 1. MRI of the brain. A transverse section shows "T2" bright spots in the white matter (labeled "W") of the brain. Note the cortex ("C"), the slightly lighter gray area in the image, which is the layer over the white matter. The bright T2 areas are often referred to by physicians as "hyper-intense areas T2 signal" and at other times are simply called plaques.

Mri Brain Bright Spots

Figure 2. MRI of the same brain shown in Figure 1. A transverse section shows a smaller bright area labeled "Gd+" in a different type of MRI. This is a "T1 weighted" image with an "enhancing" lesion that appeared after gadolinium was injected intravenously. The areas that were bright in Figure 1 are not bright in this image. The enhancing lesion is evidence of active inflammation present at the time of the MRI scan. Note that the enhancing lesion is a small portion of one of the T2 lesions shown in Figure 1. This means that only part of the bright area in Figure 1 is "active" and is added onto a pre-existing plaque.

Figure 2. MRI of the same brain shown in Figure 1. A transverse section shows a smaller bright area labeled "Gd+" in a different type of MRI. This is a "T1 weighted" image with an "enhancing" lesion that appeared after gadolinium was injected intravenously. The areas that were bright in Figure 1 are not bright in this image. The enhancing lesion is evidence of active inflammation present at the time of the MRI scan. Note that the enhancing lesion is a small portion of one of the T2 lesions shown in Figure 1. This means that only part of the bright area in Figure 1 is "active" and is added onto a pre-existing plaque.

Myelin lipoproteinaceous material composed of alternating layers of lipid and protein of the myelin sheath.

Nerve fibers (axons) coming from neurons and going to other neurons are surrounded by myelin. The axons and the myelin that cover them make up the white matter. Myelin insulates the nerve fibers just like insulation on copper wires. This insulation is important because it speeds up the communication between different areas of the brain. Myelin allows a signal to travel the length of a football field in one second. The plaques of inflammation are areas of damage to myelin and therefore interfere with the function of axons to send messages. Although symptoms of MS will depend on the location of the plaque, many plaques in the brain will not cause any obvious neurologic symptoms (Figure 3).

Figure 3. An oligodendrocyte is labeled as "O" in the upper left hand of the drawing. Three of many arm-like extensions of the cell are shown in this drawing. Each of these extensions wrap around separate areas of individual axons, forming myelin sheaths. The myelin sheath is cut away (labeled as N) showing a bulging bit of axon between two myelin sheaths. (After Bunge, 1968).

Figure 3. An oligodendrocyte is labeled as "O" in the upper left hand of the drawing. Three of many arm-like extensions of the cell are shown in this drawing. Each of these extensions wrap around separate areas of individual axons, forming myelin sheaths. The myelin sheath is cut away (labeled as N) showing a bulging bit of axon between two myelin sheaths. (After Bunge, 1968).

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