The myelin sheath insulates the axon. It’s a layered wrapping of fat and protein that coats nerve fibers, preventing electrical signals from leaking out and dramatically speeding up how fast those signals travel. Without it, your nervous system would operate at a fraction of its normal speed.
What the Myelin Sheath Is Made Of
Myelin is unusually fatty compared to other biological membranes. It’s 70% to 85% lipid by weight, with only 15% to 30% protein. The three most abundant lipids are cholesterol, a type of phospholipid called plasmalogen, and a glycolipid called galactosylceramide. These lipids sit in a roughly 40:40:20 ratio of cholesterol, phospholipids, and glycolipids, which is quite different from most cell membranes in the body (which run closer to 25:65:10). That high fat content is what makes myelin such an effective electrical insulator, similar to the rubber coating around a copper wire.
This fatty composition is also what gives the brain’s white matter its color. Regions packed with myelinated nerve fibers appear whitish, while the unmyelinated cell bodies in gray matter look darker.
How Myelin Wraps Around the Axon
The myelin sheath isn’t a single continuous sleeve. It’s a series of individual segments, each called an internode, lined up along the axon with tiny gaps between them. Think of boxcars on a train: each car is a section of myelin, and the space between cars is a gap called a node of Ranvier. This segmented design is critical to how insulated nerves transmit signals.
Each segment of myelin is created by a specialized support cell that physically wraps itself around the axon in tight, concentric layers. The more wraps, the thicker the insulation.
Two Different Cells Build Myelin
The cells responsible for making myelin differ depending on where in the body the nerve fiber sits. In the brain and spinal cord (the central nervous system), cells called oligodendrocytes do the job. A single oligodendrocyte can myelinate up to 60 different axons at once, extending a separate arm of insulation to each one.
Outside the brain and spinal cord (the peripheral nervous system), Schwann cells handle myelination instead. Each Schwann cell wraps just one segment of one axon. Schwann cells also cover the gaps at the nodes of Ranvier with small extensions, while in the central nervous system those gaps are left exposed. The molecular makeup of central and peripheral myelin also differs: they rely on different primary proteins, and the layering structure has subtle variations in spacing and composition.
Why Insulation Makes Signals Faster
Myelin’s main job is speed. In an unmyelinated nerve fiber, an electrical signal has to regenerate itself continuously at every point along the membrane, like a slow-burning fuse. Myelinated axons work differently. The insulation forces the electrical signal to jump from one node of Ranvier to the next, skipping over the insulated segments entirely. This jumping pattern is called saltatory conduction.
At each node, the signal gets refreshed as charged particles rush into the axon, boosting the voltage before it races through the next insulated stretch. The myelin between nodes acts as a high-resistance barrier that prevents electrical charge from leaking out through the membrane, so the signal travels quickly through the interior of the axon to the next gap.
The speed difference is enormous. Unmyelinated axons conduct signals at roughly 0.5 to 10 meters per second. Myelinated axons can reach up to 150 meters per second, over 300 miles per hour. That gap is the difference between a sluggish reaction and the near-instant reflexes your body depends on.
When Myelination Happens
Myelin doesn’t appear all at once. In human development, the earliest myelination begins in the spinal cord around 12 to 14 weeks of pregnancy. The brain’s white matter starts getting myelinated in roughly the tenth fetal month. From there, the process follows a predictable hierarchy: regions responsible for basic functions like movement and sensation get insulated first, while higher-order pathways myelinate later.
Some brain connections finish myelinating by age 20, but the process is far from over at that point. Pathways connecting the frontal and temporal lobes, which handle complex decision-making, language, and emotional regulation, continue maturing well into the third decade of life. Some tracts don’t reach peak insulation until after age 40. This extended timeline helps explain why judgment and impulse control keep developing through early adulthood.
What Happens When Myelin Breaks Down
Because myelin is so essential to nerve function, damage to it causes serious neurological problems. The most well-known example is multiple sclerosis (MS), in which the immune system mistakenly attacks myelin in the brain and spinal cord. The attack strips insulation from axons and leaves behind distinctive patches of scar tissue called plaques or lesions. Over time, MS can also damage the axons themselves and the nerve cell bodies.
When myelin is lost, signals slow down, arrive out of sync, or fail to reach their destination entirely. Depending on where the damage occurs, this can cause muscle weakness, numbness, vision problems, difficulty with coordination, or cognitive changes. The specific symptoms reflect which nerve pathways have lost their insulation, since the location of the damage matters as much as the extent of it.