White matter is a highly organized network of insulated nerve fibers, supportive cells, and blood vessels that fills nearly half the human brain. Its defining feature is myelin, a fatty coating wrapped around nerve fibers that gives the tissue its pale, white appearance. Understanding what’s inside white matter helps explain how the brain communicates with itself and why damage to this tissue can have such wide-ranging effects.
Myelinated Axons: The Core Component
The most important structures in white matter are myelinated axons. Axons are long, cable-like extensions of nerve cells that carry electrical signals from one brain region to another. The nerve cell bodies themselves sit in gray matter, but their axons stretch through white matter like bundled wiring, connecting distant parts of the brain and spinal cord.
Each axon is wrapped in segments of myelin, a fatty insulating layer that dramatically speeds up signal transmission. 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. That difference is what allows you to react quickly, coordinate movement, and process complex thoughts without noticeable delay. The myelin doesn’t cover the axon continuously. Small gaps between segments force the electrical signal to “jump” from one gap to the next, which is the mechanism behind that speed boost.
What Myelin Is Made Of
Myelin itself is unusual compared to most biological membranes. It’s 70% to 85% fat by weight, with only 15% to 30% protein. That high fat content is what makes white matter look white. The three main types of fat in myelin are cholesterol, phospholipids, and glycolipids, present in a roughly 40:40:20 ratio. This is quite different from typical cell membranes, which contain much more phospholipid and less cholesterol.
None of the individual fats in myelin are unique to it, but the combination and proportions are distinctive. The three most abundant specific lipids are cholesterol, a glycolipid called galactosylceramide, and a type of phospholipid called plasmalogen. This lipid-rich composition is what makes myelin such an effective electrical insulator.
The Three Types of Glial Cells
White matter isn’t just axons and myelin. It contains several types of supportive cells, collectively called glia, that maintain the tissue and keep it functioning.
Oligodendrocytes
These are the most numerous cells in white matter and the ones responsible for producing myelin. Each oligodendrocyte extends multiple arm-like processes that wrap around nearby axons, forming the myelin sheath. A single oligodendrocyte can myelinate many passing axons at once. When viewed under a microscope, they appear in organized rows of 5 to 10 cells, lined up parallel to the axons they insulate. Beyond making myelin, oligodendrocytes also provide metabolic support to axons, feeding them energy and nutrients they need to keep firing.
Astrocytes
Star-shaped cells called fibrous astrocytes are scattered between the rows of oligodendrocytes in white matter. They make contact with both the gaps in the myelin sheath and nearby blood vessels, essentially bridging the connection between the brain’s wiring and its blood supply. Astrocytes help maintain the blood-brain barrier, support the survival of nearby cells, and even help sustain myelin production. They form direct cell-to-cell connections with oligodendrocytes through gap junctions, and research suggests that nutrients flow preferentially from astrocytes into oligodendrocytes, meaning astrocytes act as a metabolic lifeline for the cells that make myelin.
Microglia
Microglia are the brain’s immune cells and make up about 10% of all glial cells in nervous tissue. Unlike oligodendrocytes and astrocytes, which develop from brain stem cells, microglia originate from immune cell precursors outside the nervous system and migrate into the brain during early embryonic development. In white matter, they’re positioned at regular intervals along axon bundles, though much less frequently than oligodendrocytes. Under normal conditions, they act as surveillance cells, scanning for damage, dying cells, or pathogens. They also support the development of new oligodendrocytes by providing growth signals to immature precursor cells.
Where White Matter Sits in the Nervous System
White matter and gray matter are arranged differently depending on where you look. In the brain, gray matter forms the outer surface (the cortex), while white matter fills the interior, bundling connections between cortical regions. In the spinal cord, the arrangement flips: gray matter sits in the center, shaped like a butterfly, and white matter surrounds it on the outside. This difference reflects each structure’s primary job. The brain’s cortex processes information at the surface, with white matter relaying signals underneath. The spinal cord’s outer white matter carries long-distance signals up and down the body, while the central gray matter handles local processing.
How White Matter Changes Over a Lifetime
White matter isn’t a static structure. Its volume increases steadily through infancy, childhood, and adolescence as more axons become myelinated and existing myelin thickens. This process continues well into adulthood, with white matter reaching its peak volume during a person’s 30s. That extended timeline means the brain’s internal wiring is still being refined long after other aspects of brain development have finished, which partly explains why judgment, planning, and impulse control continue maturing into early adulthood.
After that peak, white matter gradually declines with age. Brain scans of older adults often show bright spots called white matter hyperintensities. These were once dismissed as a normal part of aging, but that’s not entirely accurate. While they do become more common with age, their prevalence varies widely between individuals. Early changes may involve shifts in fluid content that are potentially reversible, but more advanced changes reflect actual loss of myelin and axon damage, which is harder to undo. These spots are associated with vascular problems and are commonly seen in people with stroke or dementia.
Blood Supply and Vulnerability
Deep white matter gets its blood from tiny perforating arteries that branch off larger vessels and dive into the brain’s interior. These small arteries are especially vulnerable because they supply “watershed” zones, areas at the far edges of blood supply territories where pressure is lowest. When blood flow drops, either from aging, high blood pressure, or narrowing of the carotid arteries in the neck, these deep white matter regions are often the first to suffer. Reduced flow through perforating arteries is linked to a higher burden of white matter damage visible on brain scans, which helps explain why cardiovascular health and brain health are so closely connected.