White matter describes those tissues in the central nervous system that appear pinkish white to the naked eye due to the presence of fats within them, and appear white when preserved in formaldehyde. It is largely made up of two types of cells - glial cells and axons of neurons covered in myelin. Originally thought to be functionless filler tissue within the brain and the spinal cord, it is now understood that this tissue is involved with signal processing and understanding. White matter links the different areas of grey matter within the nervous system and passes information between the different areas of grey matter.
When white matter is damaged, the signals will find alternative routes to the different areas of the brain over time. This is particularly true of children. In addition, creation of white matter continues well into adulthood and peaks in late middle age. In a well functioning undamaged brain, white matter changes in response to the learning of a new skill, particularly new motor skills.
Several diseases affect white matter, particularly multiple sclerosis (which attacks myelin) and Alzheimer's disease. However, research shows that lost white matter may be capable of being replaced over time.