white-matter tract

A white-matter tract is a bundle of nerve fibers that carries signals from one part of the brain to a distant part. Each fiber is the long output cable of a neuron, called an axon, and these axons run side by side in their thousands or millions like the wires inside a thick electrical cable. The tract's job is simple to state but vital: it links faraway brain regions so they can work together as a network rather than as isolated islands.

The fibers in a tract are coated in a pale, fatty wrapping called myelin, which insulates each axon and lets its signals travel far faster, much as plastic insulation around a copper wire keeps the current clean and quick. That fatty coating is why these bundles look whitish to the eye, and why the deep wiring of the brain is called white matter, in contrast to the grayer cell bodies on the surface. So a brain has two broad ingredients: gray matter, where neurons do their computing, and white-matter tracts, the long-distance cabling that connects it all.

Tracts come in three broad styles by where they run. Some, like the corpus callosum, cross between the left and right halves of the brain; some run forward and back within one half, linking front regions to back ones; and some run up and down, connecting the cortex to the spinal cord and the body. Researchers can now trace these bundles in a living person with a special kind of MRI scan, mapping the brain's wiring without ever opening the skull, and damage to a tract can quietly disconnect two healthy regions and disrupt skills like speech or movement.