What Is Myelin? Myelin Sheath Structure and Function

What Is Myelin?

Introduction

Myelin is a specialized membrane that wraps around many axons in the nervous system. It acts as electrical insulation, helping nerve signals travel rapidly and reliably between the brain, spinal cord, peripheral nerves, muscles, and sensory organs.

Damage to the myelin sheath can slow or block nerve-signal conduction and contribute to neurological symptoms.

Myelin is a lipid-rich membrane that surrounds segments of certain axons. It reduces electrical current loss and helps action potentials travel rapidly between gaps called nodes of Ranvier. Oligodendrocytes produce myelin in the central nervous system, while Schwann cells produce it in the peripheral nervous system.

What Is Myelin?

Myelin is a multilayered extension of the cell membrane of specialized glial cells. These membrane layers wrap repeatedly around part of an axon and become tightly compacted, forming the myelin sheath.[1,2]

Myelin contains a high proportion of lipids, including cholesterol, phospholipids, and glycolipids, as well as proteins that help maintain its structure.

However, myelin is not simply body fat surrounding a nerve. It is a highly organized membrane that is produced, maintained, and remodeled by living glial cells.

Not every axon is myelinated. Some small axons conduct signals without compact myelin, while others are supported by glial cells without being individually wrapped in a myelin sheath.

What Is the Structure of the Myelin Sheath?

Myelin does not form one continuous covering along the entire axon. Instead, it is arranged in separate sections called internodes.

Small gaps between adjacent myelin segments are called nodes of Ranvier. These nodes contain a high concentration of voltage-gated sodium channels that regenerate the action potential as it travels along the axon.[3]

The main structural components are:

• Axon: The neuronal extension that carries electrical signals
• Myelin sheath: The multilayered membrane surrounding part of the axon
• Internode: The myelinated section between two nodes
• Node of Ranvier: The short gap where the action potential is regenerated
• Paranode: The region where the ends of the myelin sheath attach closely to the axon

These structures work together to support rapid and reliable nerve conduction.

Which Cells Produce Myelin?

Two types of glial cells form myelin: oligodendrocytes and Schwann cells.

Oligodendrocytes in the Central Nervous System

Oligodendrocytes produce myelin in the brain and spinal cord.

A single oligodendrocyte can extend several processes and form myelin segments around portions of multiple axons. Oligodendrocytes also help support the metabolic and structural health of the axons they contact.[1]

Schwann Cells in the Peripheral Nervous System

Schwann cells produce myelin around axons in peripheral nerves.

One myelinating Schwann cell normally forms one myelin segment around one portion of a single axon. A long peripheral axon therefore requires many Schwann cells.[2]

Some Schwann cells are nonmyelinating. They surround and support groups of small peripheral axons without forming compact myelin around each one.

CNS Myelin and PNS Myelin Compared

Feature	CNS Myelin	PNS Myelin
Location	Brain and spinal cord	Peripheral nerves
Myelin-producing cell	Oligodendrocyte	Schwann cell
Segments formed by one cell	Multiple segments on several axons	Usually one segment on one axon
Axon regeneration after injury	Generally limited	More favorable under suitable conditions
Associated disorders	Multiple sclerosis	Some forms of Guillain–Barré syndrome

Central and peripheral myelin perform similar insulating functions, but they differ in cellular origin, molecular composition, organization, and response to injury.

How Does Myelin Speed Up Nerve Signals?

An action potential is a rapid electrical change that travels along the axonal membrane.

In many unmyelinated axons, the action potential must be regenerated continuously along successive sections of the membrane. Myelin changes the electrical properties of an axon by:

• Increasing resistance across the axonal membrane
• Reducing electrical current leakage
• Decreasing membrane capacitance
• Allowing local current to spread farther beneath each myelin segment

When an action potential reaches a node of Ranvier, local current travels rapidly beneath the next internode and brings the following node to threshold. A new action potential is then generated at that node.[3]

This process is called saltatory conduction.

The signal is often described as “jumping” from one node to the next. More precisely, electrical current spreads beneath the myelin, while action potentials are actively regenerated at successive nodes.

Myelin does not create the nerve impulse. It allows action potentials to travel more rapidly, efficiently, and reliably along many axons.

What Determines Nerve Conduction Speed?

Myelin is an important influence on conduction speed, but it is not the only factor.

Signal speed also depends on:

• Axon diameter
• Myelin thickness
• Internode length
• Node structure
• Ion-channel distribution
• Temperature
• Axonal health

This means that not all myelinated axons conduct at the same speed.

Why Is Myelin Important?

The nervous system depends not only on whether a signal arrives but also on when it arrives. Precise timing allows signals from different neurons and brain regions to work together.

Myelin contributes to:

• Rapid communication over long distances
• Reliable sensory processing
• Coordinated muscle activity
• Precise timing within neural circuits
• Communication between different brain regions
• Efficient organization of neural pathways
• Metabolic and structural support of axons

Myelin also allows relatively narrow axons to conduct signals rapidly. Without myelin, many axons would need to be much wider to achieve similar conduction speeds.

Where Is Myelin Found?

Myelin surrounds selected axons throughout the central and peripheral nervous systems.

In the brain, large bundles of myelinated axons contribute to the lighter appearance of white matter. Examples of major white-matter pathways include the corpus callosum and internal capsule.

However, white matter is not composed only of myelin. It also contains:

• Axons
• Glial cells
• Blood vessels
• Extracellular material
• Some unmyelinated fibers

Gray matter contains many neuronal cell bodies, dendrites, and synapses, but it can also contain myelinated axons. Myelin is therefore more concentrated in white matter but is not limited to it.

When Does Myelination Occur?

Myelination begins before birth in some neural pathways and continues rapidly during infancy and childhood.

Different parts of the nervous system become myelinated at different times. Many sensory and motor pathways develop myelin relatively early, while the maturation of some pathways associated with higher cognitive functions continues later.

White-matter development remains detectable through adolescence and early adulthood. However, changes observed with brain imaging do not necessarily represent myelin formation alone.

The development of myelin helps refine the speed, timing, and coordination of communication among neural circuits. A separate article will examine how myelination changes throughout brain development.

Can Myelin Change During Adulthood?

Myelin was once viewed mainly as fixed electrical insulation. Research now indicates that some myelin remains capable of change.

Neural activity may influence:

• Oligodendrocyte development
• The formation of new myelin segments
• Which axons become myelinated
• The thickness or length of certain myelin segments
• The timing of communication within neural circuits

This is sometimes called adaptive myelination or myelin plasticity. Experimental studies suggest that oligodendrocytes and changes in myelination contribute to some forms of motor learning.[4]

This does not mean that every learning experience creates new myelin or that memories are stored in myelin. Learning involves coordinated changes in synapses, neurons, glial cells, and neural circuits.

What Happens When Myelin Is Damaged?

Damage or loss of myelin is called demyelination.

When myelin is disrupted, electrical current can leak across the axonal membrane. Nerve signals may then:

• Travel more slowly
• Become poorly synchronized
• Require more energy to conduct
• Fail to reach the next node
• Stop propagating along the axon

Symptoms depend on the location and severity of the damage. They may include weakness, altered sensation, visual problems, poor coordination, or cognitive changes.

Demyelination does not necessarily mean that the axon has already been destroyed. However, persistent inflammation and myelin damage can eventually contribute to secondary axonal injury.

Can Damaged Myelin Be Repaired?

The formation of new myelin after damage is called remyelination.

In the central nervous system, oligodendrocyte precursor cells can develop into new oligodendrocytes and form replacement myelin. Remyelination may restore conduction and help protect the underlying axon, but it can become incomplete after repeated or chronic injury.[1]

In peripheral nerves, Schwann cells help clear damaged tissue, support surviving axons, and form new myelin. Peripheral axons have a greater capacity for regeneration than central axons when the neuronal cell body survives and the Schwann-cell pathways remain sufficiently intact.

Recovery still depends on the location, type, and severity of the injury.

Diseases Associated With Myelin Damage

Multiple Sclerosis

Multiple sclerosis is an immune-mediated disease involving inflammation and damage within the central nervous system.

Myelin is an important target, but multiple sclerosis can also damage axons and other nervous-system structures. It should therefore not be described as a disease affecting myelin alone.[5]

Guillain–Barré Syndrome

Guillain–Barré syndrome is an acute immune-mediated disorder of peripheral nerves.

Some forms mainly damage peripheral myelin and Schwann-cell-associated structures, while other forms primarily affect axons. Guillain–Barré syndrome is therefore not a single, exclusively demyelinating condition.[6]

Inherited Myelin Disorders

Genetic changes affecting myelin proteins, lipid metabolism, or myelin-producing cells can cause inherited neurological disorders.

Examples include:

• Certain leukodystrophies
• Charcot–Marie–Tooth disease
• Pelizaeus–Merzbacher disease

These disorders differ greatly in their causes, symptoms, age of onset, and progression.

Common Misunderstandings About Myelin

Myelin Is Produced by Neurons

Myelin is produced by glial cells, not by neurons. Oligodendrocytes form CNS myelin, while Schwann cells form PNS myelin.

Myelin Covers Every Axon

Many axons are myelinated, but others remain unmyelinated. Myelination varies according to axon type, diameter, location, and function.

Nerve Signals Literally Jump Through Empty Space

Current spreads beneath the myelin sheath, while the action potential is regenerated at each node of Ranvier. “Jumping” is a simplified description of saltatory conduction.

Myelin Generates Action Potentials

Myelin does not produce nerve impulses. It changes the electrical properties of the axon so that action potentials can travel faster and more reliably.

Myelin Stops Changing After Childhood

Myelination is especially active during development, but some myelin formation and remodeling can continue in adulthood.

• Myelin is a lipid-rich, multilayered membrane surrounding segments of certain axons.
• The myelin sheath reduces electrical current loss and supports rapid nerve conduction.
• Oligodendrocytes produce myelin in the brain and spinal cord.
• Schwann cells produce myelin in peripheral nerves.
• Nodes of Ranvier are gaps where action potentials are regenerated.
• Not every axon is myelinated.
• Myelin supports signal timing, axonal health, and communication between neural circuits.
• Demyelination can slow or block nerve signals.
• Some remyelination is possible, although repair may be incomplete.

Related Articles

• What Is a Neuron?
• What Are Glial Cells?
• What Is an Action Potential?
• What Is the Central Nervous System?
• What Is the Peripheral Nervous System?

References

1. Simons M, Nave KA. Oligodendrocytes: myelination and axonal support. Cold Spring Harbor Perspectives in Biology. 2016;8(1):a020479. doi:10.1101/cshperspect.a020479. PubMed Central
2. Salzer JL. Schwann cell myelination. Cold Spring Harbor Perspectives in Biology. 2015;7(8):a020529. doi:10.1101/cshperspect.a020529. PubMed Central
3. National Institute of Neurological Disorders and Stroke. Multiple Sclerosis. NINDS
4. Bellanti R, Rinaldi S. Guillain–Barré syndrome: a comprehensive review. European Journal of Neurology. 2024;31(8):e16365. doi:10.1111/ene.16365. PubMed