White matter is the commonly used term for the myelinated axons that provide connections between neurons, or grey matter. The main function of white matter in the brain is to regulate the electrical signals in axons. These signals are a form of communication, and work to pass along information that is translated into chemical signals between neurons. Without white matter, electrical signals would weaken as they traveled along the axon.
Understanding the structure of white matter is necessary to understand its function. Support cells in the brain, known as glial cells, produce the fatty tissue known as myelin, which wraps around axons like a sheath. Essentially, these sheaths seal the axon and prevent the ions that propagate electrical signals from diffusing out. The result is that these signals can travel more quickly over longer distances than they could without insulation.
Traveling electrical signals are known as action potentials. To maintain the ionic difference that allows action potentials to take place, there are gaps between myelinated cells, which allow ions to transfer in and out of the axon. This allows the charge in the axon to maintain its strength, even over long distances. The importance of white matter increases for signals that must travel great distances. Nerve cells that transmit sensory signals to the brain, or neurons that regulate breathing or heart rate, would be unable to perform their tasks without the white matter in the brain and spinal cord.
Inside the brain, white matter is found in many structures, but it is particularly concentrated in areas where many signals must be sent long distance. These areas include the thalamus and hypothalamus, which govern processes like blood pressure and other essential life support functions that do not require conscious attention to execute. The area beneath the cortex of the brain, known as the subcortex, also contains a large amount of white matter.
Within the subcortex, myelinated axons pass signals between the two hemispheres of the brain, and between different areas in the same hemisphere. These connections are necessary for proper functioning. In diseases and conditions like Alzheimer's, the myelin cells die off, and the axons are no longer able to send signals. When axons cannot propagate their electrical information, they atrophy, and communication between regions suffers. The loss of white matter in Alzheimer's is believed to be directly responsible for the problems with memory and functioning that result.