Myelin is a fatty coating wrapped around many of the body’s nerve fibers, much like the insulation on an electrical wire. It lets nerve signals move far faster than they could along a bare fiber. But it is more than insulation, because the cells that build it also help keep the nerve fiber alive. And myelin keeps changing through life and responds to what you do, so it is not a fixed part of the brain.
What the Myelin Sheath Is Made Of
A nerve cell sends its signals down a long, thin extension called an axon, which is the nerve fiber. Helper cells wrap their own flattened membrane around that fiber many times, building a layered sheath. The wrapping covers the fiber in segments, with short bare gaps between them. And myelin is part of the helper cell itself, not a separate substance poured on top.
The membrane is unusually fatty. Dried myelin is 70% to 85% fat and only 15% to 30% protein.1PubMed Central. Myelin Fat Facts: An Overview of Lipids and Fatty Acid Metabolism Most cell membranes in the body are about half fat and half protein.2PubMed Central. Myelin Fat Facts: An Overview of Lipids and Fatty Acid Metabolism That high fat content lets the layers pack tightly into a compact wrapping.3PubMed Central. Myelin Fat Facts: An Overview of Lipids and Fatty Acid Metabolism It also makes myelin-rich areas look pale, which is why they are called white matter.
Two kinds of cells make myelin. In the brain and spinal cord, it comes from cells called oligodendrocytes.4Cambridge Core. Comparative biology of Schwann cells and oligodendrocytes In the nerves of the rest of the body, it comes from Schwann cells.5Cambridge Core. Comparative biology of Schwann cells and oligodendrocytes The difference shows in their reach: one Schwann cell wraps a single stretch of one nerve fiber, while one oligodendrocyte can wrap up to 60 different fibers.6Cambridge Core. Comparative biology of Schwann cells and oligodendrocytes
The two kinds of myelin use different main proteins, so a disease can hit one and spare the other.7Cambridge Core. Comparative biology of Schwann cells and oligodendrocytes Multiple sclerosis damages myelin in the brain and spinal cord only. Guillain-Barré syndrome and Charcot-Marie-Tooth disease affect the body’s nerves. That means the location of symptoms can hint at which system is involved.
Not every nerve fiber is wrapped. Many of the thinnest fibers, including a lot of those that carry dull, aching pain and some of the body’s automatic signals, work without any myelin. Wrapping pays off mostly on long fibers or ones that must act quickly, so the body leaves many short or slow-duty fibers bare. A person who loses myelin can therefore keep some functions, because the bare fibers are untouched by the loss.
How Myelin Speeds Up Nerve Signals
Nerve signals are tiny electrical pulses traveling along the axon. Myelin greatly speeds them up by making them jump from one bare gap to the next.8PubMed Central. Saltatory Conduction along Myelinated Axons Involves a Periaxonal Nanocircuit The sheath stops electrical charge from leaking out between the gaps, so the signal only needs to be renewed at the gaps themselves. A thin, fluid-filled space under the myelin also helps carry the signal from one gap to the next.9PubMed Central. Saltatory Conduction along Myelinated Axons Involves a Periaxonal Nanocircuit The result is that wrapped fibers can carry signals many times faster than bare ones of the same width.
You feel the difference in everyday moments, like yanking your hand off a hot pan or catching a falling glass, when a delay of even a fraction of a second changes the result. Coordinated movement and quick thinking also rely on signals arriving on time.
Timing matters as much as raw speed. Brain areas have to receive signals in sync to work together, and the thickness of the myelin helps set how long a signal takes to arrive. Uneven or slowed signals can disrupt that coordination even when no nerve cell has died.
Myelin doesn’t create the signal; the nerve cell does. Myelin changes how the signal travels, so a nerve can be alive yet work poorly once its myelin is lost.
Myelin also solved a space problem. Animals without it, such as squid, get speed by growing very thick nerve fibers.10PubMed Central. Do we have to reconsider the evolutionary emergence of myelin? Myelin let vertebrates get fast signals from thin fibers, saving room for a complex brain.11PubMed Central. Do we have to reconsider the evolutionary emergence of myelin?
Myelin Also Feeds and Protects Nerves
Myelin-making cells in the brain pass a fuel molecule into the nerve fibers they wrap.12PubMed Central. Oligodendroglia metabolically support axons and contribute to neurodegeneration This supply is critical for keeping nerve fibers working and nerve cells alive.13PubMed Central. Oligodendroglia metabolically support axons and contribute to neurodegeneration So losing these helper cells can starve a fiber even if the fiber itself was healthy.
When myelin is stripped away, a fiber loses both its insulation and the fuel supply from its helper cell. A fiber left bare often needs more energy to keep sending signals, which strains one that is already short of supplies. Over months and years, fibers that stay bare can slowly break down. This loss of fibers is a main reason disability can build up after repeated attacks, even when inflammation seems quiet.
Energy is part of myelin’s story in another way. In thin fibers of the brain’s outer layer, myelin roughly halves the energy cost of sending a signal while barely changing its speed.14PubMed Central. Cortical gray matter myelin cuts energy cost of spike propagation without increasing conduction velocity But keeping myelin-making cells alive costs energy too, so across the brain’s white matter myelin does not always save energy overall.15PubMed Central. The energetics of CNS white matter
What Happens When Myelin Is Damaged
Multiple sclerosis (MS) is a long-term condition in which the immune system attacks the brain and spinal cord.16PubMed. The immune pathogenesis of multiple sclerosis It damages myelin and the cells that make it, causes inflammation, and over time loses nerve fibers.17PubMed Central. Multiple sclerosis: molecular pathogenesis and therapeutic intervention Symptoms depend on which area is hit, such as vision, balance, numbness, or weakness.
Guillain-Barré syndrome is an immune attack on the myelin of the body’s nerves, often following an infection.18Sketchy Medical. Guillain-Barré & Charcot-Marie-Tooth The immune system mistakes parts of the nerves for the germ it was fighting.19Sketchy Medical. Guillain-Barré & Charcot-Marie-Tooth Weakness can spread quickly, and in severe cases the breathing muscles are affected.20PubMed. Guillain-Barré syndrome in patients with Charcot-Marie-Tooth type 1A disease, probably a non-random association Most people recover well, though some improve slowly.21PubMed. Guillain-Barré syndrome in patients with Charcot-Marie-Tooth type 1A disease, probably a non-random association
Charcot-Marie-Tooth disease is an inherited condition in which faulty proteins weaken nerve myelin or fibers.22Sketchy Medical. Guillain-Barré & Charcot-Marie-Tooth It is usually passed down from one parent.23Sketchy Medical. Guillain-Barré & Charcot-Marie-Tooth One inherited form can cause sudden but fully reversible episodes of weakness or trouble speaking, often triggered by fever or high altitude.24PubMed. Charcot-Marie-Tooth Disease 1X Simulating Paraparetic Guillain-Barre Syndrome
Doctors check for myelin damage in a few ways. A nerve conduction test places small sensors on the skin over an arm or leg nerve, delivers a mild electrical pulse, and measures how fast the signal travels. Slowed signals suggest the myelin on those nerves is damaged, while a weaker signal with normal speed points more toward loss of the fibers themselves. In suspected MS, an MRI scan of the brain and spinal cord can show patches of damage, which doctors weigh together with symptoms and other tests.
Some symptoms call for prompt attention: numbness or tingling that spreads over days, sudden blurred or painful vision in one eye, or balance problems that come and go. Weakness climbing from the feet upward needs urgent care.
Can Myelin Repair Itself
Sometimes it can. Reserve cells in the brain can mature and wrap new myelin around damaged fibers.25Medicine Bulletin. Oligodendrocyte Precursor Cells in Demyelination Repair: Mechanisms, Crosstalk, and Therapeutic Frontiers In long-standing MS patches, though, this repair often fails, leaving scarred areas.26Medicine Bulletin. Oligodendrocyte Precursor Cells in Demyelination Repair: Mechanisms, Crosstalk, and Therapeutic Frontiers The reserve cells reach the damaged spot but fail to mature into myelin-makers.27PubMed. Overcoming remyelination failure in multiple sclerosis and other myelin disorders Leftover broken myelin itself blocks them from maturing.28PubMed Central. Myelin impairs CNS remyelination by inhibiting oligodendrocyte precursor cell differentiation
This also helps explain why symptoms can improve after an attack. Inflammation settles and swelling goes down, some myelin regrows, and nerves can adapt to signal around the damage.
Researchers are testing drugs that remove the brakes on these reserve cells, such as opicinumab.29PubMed Central. Promoting remyelination in multiple sclerosis No treatment that reliably rebuilds myelin is in everyday use yet.
Current MS medicines don’t count as repair. Most calm the immune system so it attacks less, cutting new episodes and new damage rather than rebuilding lost myelin.
How Myelin Changes From Birth to Old Age
Myelin building starts around birth and is fastest in the first 2 years.30ZERO TO THREE. When is the brain fully developed? It continues perhaps as late as age 30.31ZERO TO THREE. When is the brain fully developed? Brain scans show a second growth spurt in adolescence and adulthood, then decline in late life.32PubMed Central. The shifting landscape of brain-wide myelination across the human lifespan
The front parts of the brain used for planning, weighing risks, and controlling impulses are among the last to be wrapped. That wrapping carries on into the twenties, so these circuits are still becoming faster and better coordinated during the teen years. It is one piece of why judgment and self-control keep maturing into early adulthood, though not the only one.
Myelin was once thought to be fixed after childhood.33PubMed Central. Activity-dependent myelination: A glial mechanism of oscillatory self-organization in large-scale brain networks Yet learning and activity can change how fibers are wrapped, even in adulthood and old age.34PubMed Central. A new mechanism of nervous system plasticity: activity-dependent myelination
In older animals, myelin both breaks down and is renewed less efficiently, which is linked to memory problems.35PubMed Central. Myelin degeneration and diminished myelin renewal contribute to age-related deficits in memory Myelin breakdown in age tends to follow the order in which it was built, in reverse, and may contribute to memory loss and Alzheimer’s.36PubMed. Age-related myelin breakdown: a developmental model of cognitive decline and Alzheimer’s disease
No habit is a proven way to rebuild myelin, but some are well supported for brain health. Staying mentally active and learning new skills challenges brain circuits, and since activity can reshape wrapping, this may help keep them in working order. Physical activity is generally good for the brain, even though its exact effect on myelin is still being studied. Keeping blood pressure under control protects the small blood vessels that feed white matter, which is vulnerable when blood flow is poor.
Diet, Toxins, and Myelin Health
Lead exposure is linked to changes in brain wiring and delays in myelin build-up.37PubMed Central. Lead exposure is associated with functional and microstructural changes in the healthy human brain In rats, long-term lead in drinking water disrupts myelin’s tidy layers.38PubMed. Chronic lead intoxication affects the myelin membrane status in the central nervous system of adult rats Nutrition matters too: a severe inherited B12 processing problem altered the fats in one person’s myelin.39PubMed. Fatty acid composition of myelin isolated from the brain of a patient with cellular deficiency of co-enzyme forms of vitamin B12 Severe B12 deficiency can also cause numbness and needs medical treatment.
Lead is most often a worry in older homes, where aging pipes or old paint can be sources, so testing the water and checking paint is sensible if you live in one. Young children are most at risk because their brains are still building myelin. If a test finds lead, the usual steps are to use a safe water source and to ask the local water supplier or health authority how to fix the source.
On the helpful side, in lab dishes a mix of omega-3 DHA, B12, folate, iron and other fats helped myelin-making cells grow and wrap.40PubMed. A blend containing docosahexaenoic acid, arachidonic acid, vitamin B12, vitamin B9, iron and sphingomyelin promotes myelination in an in vitro model That does not prove supplements build myelin in people. A balanced diet with enough B12, iron and oily fish is still a reasonable goal, especially for children and during pregnancy. Vegans and older adults in particular should check their B12 intake.