The white matter of the brain contains nerve fibers (axons), interstitial space and supporting cells. Most of these nerve fibers--specifically, axons--are covered with myelin, a type of fat that insulates nerve fibers and increases the speed of neural transmission. White matter makes up 60 percent of the total brain volume and is responsible for communication between various grey regions in the brain and the rest of the body. Diseases affecting the brain's white matter are mostly demyelinating diseases such multiple sclerosis, progressive multifocal leukoencephalopathy and HIV encephalitis.
Multiple Sclerosis
Multiple sclerosis is a primary demyelinating disease affecting the white matter of the brain. About 95 percent of all lesions associated with multiple sclerosis occur in the white matter of the brain. Myelin is produced and maintained by a type of neural cell known as oligodendrocytes and is responsible for increasing the speed of signal transmission. According to the National Multiple Sclerosis Society, MS is the result of the destruction of the myelin sheath because of a defect in the formation or maintenance of oligodendrocytes. The destruction of the myelin sheath results in neurotransmission distortion or interruption, so people with MS often have involuntary contractions of muscles, paralysis, loss of vision and involuntary movements.
Progressive Multifocal Leukoencephalopathy
Progressive multifocal leukoencephalopathy is caused by a virus known as Polyomavirus JC. Polyomavirus JC attacks and destroys the myelin sheath that covers the axons in the white matter, says Empowher.com. White matter lesions characteristic of progressive multifocal leukoencephalopathy are initially patchy and round, and eventually becomes confluent and large. The polyomavirus JC is inactive in healthy people but causes diseases in immunosuppressed people, such as organ transplant recipients or AIDS patients. The gradual destruction of the myelin sheats impairs the transmission of nerve impulses and results in symptoms similar to multiple sclerosis.
HIV Encephalitis
Acute HIV encephalitis is characterized by diffuse myeline damage, according to Neuropathology.neoucom.edu. It occurs in 30 percent of AIDS patients and is caused by the direct invasion of neurons by HIV Type 1 virus, a neurotropic retrovirus. The demyelination of white matter results in headaches, memory loss, movement disorders and other sensory disorders.
