Brain tissue composed of myelinated axons, connecting different brain regions.White matter is one of the two main kinds of tissue in the brain and spinal cord, the other being grey matter. Where grey matter contains the cell bodies of neurons and is where most local computation happens, white matter consists mainly of the long cable-like extensions of neurons — axons — bundled together and wrapped in a fatty insulating sheath called myelin. That sheath is what gives the tissue its pale colour and its name.
Its role is communication. Axons in white matter run between distant regions, carrying electrical signals from one area to another, so white matter forms the brain's long-distance wiring. A tract connecting the frontal lobes to the back of the brain, or the two hemispheres to each other, is white matter. When these connections are damaged or poorly insulated, regions that should cooperate lose their coordination, which is why white-matter injury shows up as slowed processing, disrupted movement, or altered cognition rather than as the loss of one specific skill.
White matter continues to develop well into adulthood, and its organization can be mapped in living people, which has made it central to the study of how brain regions form networks.
A glial cell called an oligodendrocyte extends processes that wrap around a segment of an axon, laying down layers of a lipid-rich membrane. These myelin segments are spaced with small gaps between them. Because myelin is a poor conductor and an even better insulator, the electrical signal does not travel smoothly along the whole axon; instead it jumps from one gap to the next, a process called saltatory conduction. This makes transmission far faster than in an unmyelinated fibre of the same diameter.
A bundle of such myelinated axons running together forms a tract. Because water diffuses more easily along the fibres than across them, magnetic resonance imaging can detect the direction of these bundles, which is how white-matter pathways are traced non-invasively.
White-matter tracts are mapped in clinical and research settings using diffusion MRI, and damage to them is assessed in stroke, multiple sclerosis, and traumatic brain injury.
The myelin sheath is roughly 70 to 80 percent lipid, which is why white matter looks pale and why the brain's fatty insulation is sometimes compared to the plastic coating on a wire.