Myelin Sheath

What is a myelin sheath?

A myelin sheath is a sleeve (sheath) that’s wrapped around each nerve cell (neurons). It’s a protective layer of fat (lipids) and protein that coats the main “body” section of a neuron called the axon.

What are the parts of a nerve cell?

A nerve cell is called a neuron. Nerves cells make up your nervous system. Your nervous system is your body’s communication command center. It sends and receives communication from all parts of your body and reacts to changes inside and outside of your body.

Each nerve cell has three sections. The sections of a nerve cell can be thought of like a tree.

• The soma, or cell body, is like the tree’s root system. The roots of the soma, called dendrites, receive the “nutrients” from the soil. The “nutrients” in this case are the chemical messages they receive from other nearby nerve cells. These messages are converted into electric impulses. The soma also contains the cell’s nucleus.
• The axon is like the tree’s long trunk. The nutrients are transported from the tree’s roots up through the tree’s trunk. Similarly, the axon transports the impulses from the soma onward along its path.
• The axon terminal is like the tree buds at the end of the tree’s branches. Leaves and pollen come out of these buds. The electrical impulses are sent (or “released” like pollen from a bud) to be picked up by the next nerve cell. This process keeps repeating as the impulse or message continues in its travels within your brain and spinal cord or out to your body. The result might be muscle fibers contracting to move your arm, help you breathe or keep your heart beating.

What does the myelin sheath do?

The myelin sheath has three functions:

• Its fatty-protein coating provides protective insulation for your nerve cell, like the plastic insulation covering that encases the wires of an electrical cord.
• It allows the electrical impulses to travel quickly and efficiently between one nerve cell and the next.
• It maintains the strength of the impulse message as it travels down the axon.

Myelin is made by oligodendrocytes in your brain and spinal cord (your central nervous system [CNS]) and by Schwann cells in your peripheral nervous system. Your peripheral nervous system is the network of nerves outside of your CNS. These nerves communicate between your CNS and the rest of your body.

What are the gaps in the myelin sheath called?

Your myelin sheath isn’t one solid covering. It’s a lineup of individual sections of myelin, each separated from the next by a tiny gap — like the small amount of space you see between individual box cars on one long train. Each section of myelin is called an internode. Each gap in the myelin sheath — between internodes — is called the nodes of Ranvier. The nodes of Ranvier are rich in positive sodium ions. As the electrical signal or impulse travels along the axon, it jumps from one node to the next. When passing over the gap, the sodium ions recharge the electrical signal so it can continue in its travel without losing its charge or lessening in signal strength.

What happens when the myelin sheath is damaged?

About 100 billion nerve cells are in constant activity, sending and receiving messages that control every aspect of your body’s functioning. When your myelin sheath on nerve cells is damaged, the electrical signal is slowed or stopped.

Myelin can be damaged when your body’s immune cells think that myelin is a foreign substance. Your body’s immune system produces inflammatory substances that damage myelin and eventually kill the cells (the oligodendrocytes and Schwann cells) that make myelin. The location where the myelin is attacked determines your symptoms. The destruction of the myelin sheath is called demyelination.

What diseases cause damage to myelin?

Probably the most well-known disease that attacks the myelin in your central nervous system (brain and spinal cord) is multiple sclerosis.

Other central nervous system (CNS) diseases in which myelin is attacked include:

• Acute disseminated encephalomyelitis. This is a rare, acute, intense immune reaction in the CNS.
• Schilder’s sclerosis. This disease, also called diffuse myelinoclastic sclerosis, is a rare neurodegenerative disease in children.
• Transverse myelitis.This is an autoimmune disorder that causes inflammation in your spinal cord.
• Neuromyelitis optica. This is an antibody-mediated immune attack on your optic nerves and spinal cord.
• Optic neuritis. This is another condition in which your eye nerves become inflamed.
• Tumefactive demyelination. This is a single demyelinating lesion greater than 2 centimeters (cm).

Peripheral nervous system diseases in which myelin is attacked include:

• Guillain-Barre syndrome.This immune disease could be caused by infections, including cytomegalovirus, Mycoplasma pneumonia, Epstein-Barr virus and influenza virus or vaccinations.
• Chronic inflammatory demyelinating polyradiculoneuropathy. This is a slow-developing autoimmune disease that attacks myelin.
• Paraproteinemic demyelinating neuropathy. This is a type of neuropathy associated with the presence of an antibody that is associated with malignancy.
• Charcot Marie Tooth type 1 and type X. This is an inherited neuropathy.
• Copper deficiency. Lack of copper leads to loss of protection of the myelin, making it susceptible to damage.

What other conditions can damage or destroy myelin?

A myelin sheath can also be damaged or destroyed in adults by:

• Stroke.
• Infections, immune and metabolic disorders.
• Poisons.
• Vitamin B12 deficiency.
• Certain drugs, including ethambutol.
• Excess alcohol intake.

In addition, in a few rare inherited diseases, the myelin sheath doesn’t properly form. These diseases include Tay-Sachs disease, Gaucher disease, Hurler syndrome and Niemann-Pick disease.

Can damage to myelin be repaired?

Damaged myelin can be repaired in both your central and peripheral nerve systems. It’s important to manage the factors that cause demyelination such as inflammation and immune suppression/modulation treatment, including steroids, intravenous immune globulin (IVIG) and other agents.

Scientists are making progress in understanding potential ways to promote myelin repair. Although there have been several promising studies, it still isn’t clear if the benefits seen were meaningful improvements for individuals, and there are currently no approved medications for myelin repair. Here are only some examples of studies:

• A small phase II clinical trial has shown that the oral antihistamine clemastine has myelin-repairing qualities. Clemastine improved the transmission of the electrical signal in people with multiple sclerosis who had optic nerve damage. Another early human trial found that the diabetes drug metformin plus clemastine helped regenerate myelin.
• Blocking a molecule in stem cells (the “building blocks” of all cell types) caused oligodendrocytes (the cells that make myelin in the CNS) to repair myelin and allowed mice with multiple sclerosis-like symptoms to achieve some degree of recovery.
• Research studies continue to explore the potential role of stem cells in repairing myelin damage and slowing disease progression.
• Other drugs are showing promise in protecting the nervous system from further damage. Ibudilast, an anti-inflammatory drug, significantly slowed the rate of brain cell death (atrophy) in a phase II trial. An epilepsy drug, phenytoin, was shown to be protective, resulting in 30% less damage to myelin compared with a placebo. Lipoic acid, an antioxidant, may help block nerve fiber damage.

A note from Cleveland Clinic

Myelin is an essential substance that keeps your nerves functioning, sending and receiving communication from all parts of your body. Without the100 billion functioning nerve cells, no part of your body would work. Many diseases and conditions can attack myelin — the best known is multiple sclerosis. Researchers continue looking for ways to protect, repair and regenerate myelin. The good news is that they’re making progress.