Subcortical white matter, often simply called white matter, is a region inside the brain that has a high concentration of nerve fibers. These fibers are coated with a protein called myelin, which assists in transmission of electrical impulses down the fibers. The nerve fibers are not interrupted by cell bodies very often, so electrical signals move through them very quickly compared to other nerve cells. Many involuntary functions are regulated in this region of the brain, including reflexes and balance. About 60% of the brain is composed of some type of white matter, which makes thousands of miles or kilometers of neural connections over the course of a human lifetime.
While popular culture is more familiar with gray matter, the outer part of the brain that controls consciousness and cognition, subcortical white matter controls most functions that unconsciously keep the human body running. Injuries to the white matter can have disastrous effects on body functions. White matter disease can come from a variety of causes, but many of the most commonly known diseases arise from problems with the myelin sheaths on the nerve fibers within the tissue. Multiple sclerosis (MS) results from degradation of myelin, and encephalitis results both from myelin degradation and autoimmune inflammation.
Injuries to subcortical white matter do not always cause life-threatening problems, and in some cases they may be treatable or even reversible. The amount of treatment and the prognosis for any of these varies depending on the injury or disease, as well as the unique state of a patient. In general, white matter injuries are at least somewhat treatable, but the majority also cause permanent damage and are not fully reversible. Treatment for most diseases usually involves attempting to slow their progression, rather than trying to repair the damage they cause. White matter disease, although it almost always eventually affects quality of life, is not necessarily lethal.
Most injuries to subcortical white matter appear as lesions, which are significant changes in brain structure. These can look like tears, plaques, or a variety of abnormal structures. In children, the subcortical white matter can sometimes repair itself if given enough time, but in adults the chances of this occurring are much less. Any structural change to the brain, even partially healed, can increase the chances of a patient developing depression or other medical conditions. Maintaining the integrity of the white matter is essential for healthy brain and body function.