White matter is made up of millions of nerve fibers (axons) bundled together and wrapped in a fatty insulating layer called myelin. Unlike gray matter, which contains the main bodies of nerve cells where processing happens, white matter is purely connective. It contains no nerve cell bodies, no branching structures that receive signals, and no synapses. It’s the brain’s wiring system, and it makes up more than 40% of total brain volume.
Axons: The Core Wiring
The foundation of white matter is axons, the long, cable-like extensions that nerve cells send out to communicate with other nerve cells. A single axon can stretch from one region of the brain to another, or in the case of the spinal cord, run the entire length of your back. These fibers are bundled into tracts, functioning like highways that carry electrical signals between distant parts of the nervous system.
Axons in white matter don’t work alone. They’re supported by three types of glial cells, which collectively outnumber nerve cells in the brain. Each type plays a distinct role in keeping those signal-carrying fibers healthy and functional.
Myelin: The Insulating Layer
The “white” in white matter comes from myelin, a fatty coating that wraps tightly around axons in spiraling layers. Myelin is produced by cells called oligodendrocytes, which are the dominant glial cell type in white matter. A single oligodendrocyte can extend processes to wrap multiple axons at once, and the density of these cells varies by region depending on how many axons need insulation. In the optic nerve, for example, 100% of axons are myelinated, while other tracts like the corpus callosum (the bridge between brain hemispheres) have a lower percentage.
Chemically, myelin is unusual for biological tissue. Its dry mass is 70 to 85% lipid (fat) and only 15 to 30% protein. That high fat content is what gives it insulating properties and its distinctive pale appearance. In living tissue, myelin also contains about 40% water.
The insulation myelin provides is dramatic. Unmyelinated nerve fibers conduct signals at roughly 0.5 to 10 meters per second. Myelinated fibers can reach speeds up to 150 meters per second, a boost of at least 50-fold for fibers of similar diameter. This speed difference is the reason you can react to a hot stove in milliseconds rather than waiting for a slow signal to crawl up your arm.
How Signals Jump Along Myelin
Myelin doesn’t coat an axon in one continuous sleeve. It’s interrupted at regular intervals by tiny gaps about one micrometer long, called nodes of Ranvier. These gaps are packed with channels that let sodium ions rush into the axon, regenerating the electrical signal at each stop. Between the nodes, the myelin sheath forces the signal to travel rapidly through the interior of the axon until it reaches the next gap. This “jumping” pattern of conduction is what makes myelinated signaling so fast.
At each node in the brain, finger-like extensions from astrocytes (another type of glial cell) make direct contact with the exposed axon, helping maintain the chemical environment the signal needs to regenerate properly.
The Three Glial Cell Types
Beyond oligodendrocytes, two other glial cell populations live in white matter and keep it functioning.
Astrocytes are the most abundant glial cells in the nervous system overall. The variety found in white matter, called fibrous astrocytes, have especially long, slender extensions. They help maintain the blood-brain barrier, support nerve cell survival, and regulate the chemical environment around axons. Their contact with both blood vessels and nodes of Ranvier puts them in a position to shuttle nutrients from the bloodstream to active nerve fibers.
Microglia make up about 10% of all glial cells in the nervous system. They function as the brain’s resident immune cells, constantly surveying their surroundings for signs of damage or infection. When something goes wrong in white matter, such as loss of myelin or injury to axons, microglia are among the first responders.
Oligodendrocytes also have a precursor population scattered throughout white matter regions like the optic nerve, corpus callosum, and cerebellum. These precursor cells account for 5 to 8% of all cells in the central nervous system and serve as a reserve that can produce new oligodendrocytes, giving the brain some capacity to repair damaged myelin.
Where White Matter Sits in the Nervous System
White matter’s physical arrangement flips depending on where you look. In the brain, gray matter forms the outer layer (the cortex) while white matter fills the interior. In the spinal cord, the arrangement reverses: white matter sits on the outside, surrounding a core of gray matter. This difference reflects function. In the spinal cord, the outer white matter tracts carry signals up and down between the brain and body, while the inner gray matter handles local processing and reflex circuits.
How White Matter Develops Over a Lifetime
White matter isn’t fully formed at birth. It grows and matures on a gradual timeline that stretches well into adulthood. The most dramatic changes happen in the first five years of life, with especially rapid increases in volume and insulation quality during the first year. After age two, the rate of change stabilizes, and white matter continues to develop slowly through childhood and adolescence. Total brain volume typically peaks around age 10 to 12, but white matter specifically continues maturing after that point before eventually declining with aging.
This extended development window is one reason children’s brains are more vulnerable to certain injuries. It also explains why cognitive skills that depend on fast communication between brain regions, like complex reasoning and impulse control, take years to fully mature.
What Happens When White Matter Is Damaged
Because white matter is the communication infrastructure of the brain, damage to it disrupts the speed and reliability of signals between regions. The most well-known condition affecting white matter is multiple sclerosis, where the immune system attacks myelin and leaves nerve fibers exposed and slow.
A more common form of white matter change shows up on brain scans of older adults as bright spots called white matter hyperintensities. These are linked to small blood vessel disease and become increasingly common with age. They’re associated with a twofold increase in dementia risk and a threefold increase in stroke risk. White matter hyperintensities also affect physical function, contributing to balance problems and abnormal gait, and they raise the risk of late-onset depression.
White matter’s water content, about 71% in healthy tissue compared to 83% in gray matter, is one reason these changes are detectable on imaging. Even subtle shifts in water content from damage or disease alter the signal on a brain scan, making white matter a sensitive indicator of vascular health in the aging brain.