White matter is located at the base of the brain between the cerebellum and the brainstem. It is made of long nerve fibers called axons. The axons are insulated by myelin, sheaths of fat that give the region the white color for which it is named. The myelin helps speed up the transmission of signals along the axons. Those signals send sensory information from the body to the cerebral cortex, helping to control autonomic functions of the body. Autonomic functions are those bodily functions such as breathing, blinking, and sweating that are carried out without the brain consciously thinking about them.
Part of the central nervous system, white matter includes the thalamus and hypothalamus which are located in the cerebellum. The hypothalamus is located below the thalamus. Both are neural networks which have several functions, but the most important seem to be that the thalamus relays messages to the cerebral cortex, and the hypothalamus plays a large role in regulating the body’s metabolic system.
White matter differs from gray matter in several important aspects. Although gray matter also contains axons, they are not myelinated. It also contains nerve cells and dendrites, which branch out from the nerve cells. Gray matter serves as the message processing centers of the brain and is in charge of the brain’s conscious thought. While gray matter is located on the outside of the brain and the inside of the spinal cord, white matter follows the opposite pattern. It is located within the brain and outside of the spinal cord.
In humans, distribution of white and gray matter varies between the sexes. Men have larger brains on average than women, but women have proportionally more gray matter while men have higher volumes of white matter. Women have more of the matter connecting the two hemispheres of the brain, but men have higher concentrations within each hemisphere. These differences seem to correlate to tendencies in men to do better on spatial tasks than women. Women, in contrast, tend to outperform men on verbal tasks.
Aging also affects the brain’s white matter. As the brain ages, lesions begin to appear on these areas, which begin to limit the brain’s cognitive function, presumably by impairing the ability of the brain to send and receive messages. The more lesions a person has, the more his cognitive ability is impaired. While lesions are a normal part of aging, they can be exacerbated by other health problems, notably high blood pressure.