White matter

White matter is the brain's wiring. It consists of long axons — the projections of neurons — wrapped in a fatty insulating sheath called myelin. The myelin is what gives it its pale, off-white appearance.

While gray matter does the computing, white matter carries the signals: between brain regions within a hemisphere, between the two hemispheres (via the corpus callosum), and between the brain and the spinal cord.

Why it matters

Healthy white matter is essential for:

• Speed — myelin lets electrical signals jump from gap to gap along axons, making transmission roughly 100 times faster than along unmyelinated fibres
• Coordination — complex behaviours require many brain regions to communicate in milliseconds; this depends on intact white matter
• Memory and cognition — white matter integrity declines with age and correlates with processing speed and executive function

When it goes wrong

White matter is vulnerable to:

• Small-vessel disease, which produces white matter hyperintensities
• Demyelinating conditions like multiple sclerosis
• Stroke, when blood flow to white matter tracts is interrupted
• Diffuse axonal injury in traumatic brain injury

How it's measured

Two common imaging approaches:

• Volumetric analysis of T1-weighted MRI — total white matter volume, regional breakdowns
• Diffusion tensor imaging — measures the directionality of water diffusion along white matter tracts, sensitive to microstructural integrity

Lesions and hyperintensities are best seen on FLAIR sequences.