Myelin Sheath Definition, Function & Demyelinating Diseases

Myelin Sheath Definition, Function & Demyelinating Diseases

The myelin sheath helps insulate the nervous system and is vital for optimal cognitive function and brain health. Read on to learn more about its purpose and diseases associated with its loss or dysmyelination . What Is the Myelin Sheath?

The myelin sheath is a cover made out of fats and proteins that wraps around the axons (projection) of nerve cells. It insulates neurons so they can send electrical signals faster and more efficiently. This supports brain health and nervous system function [ 1 , 2 ].

Here are some quick facts about myelin: About 80% fats/cholesterol and 20% proteins.

Considered an outgrowth or extension of a type of glial cell (oligodendrocyte – CNS, Schwann cell – PNS).

Continues to grow throughout adolescence and even into our early 20s.

Myelinated axons are white in appearance, hence the term “white matter” of the brain.

Function

Myelin improves the conduction of action potentials, which are needed to send information down the axon to other neurons [ 3 ].

The myelin sheath increases the speed of impulses in neurons. It facilitates conduction in nerves while saving space and energy [ 1 ].

Myelin helps prevent the electrical current from leaving the axon. It allows for larger body sizes by maintaining efficient communication at long distances.

When babies are born, many of their nerves lack mature myelin sheaths. As a result, their movements are jerky, uncoordinated, and awkward. Scientists think that, as myelin sheaths develop, movements become smoother, more purposeful, and more coordinated [ 4 , 5 ].

Research suggests that myelination might improve children’s cognitive performance improves as they grow and develop [ 6 ].

Additionally, when a peripheral fiber is severed, the myelin sheath provides a track along which regrowth can occur [ 7 ].

The myelin sheath enables neurons to conduct action potentials, increasing the speed of their transmission. When Does Myelination Stop?

Researchers think that myelination occurs most significantly during childhood, but some brain imaging studies suggest it may continue until 55 years of age and possibly even throughout life [ 8 ]. Oligodendrocytes vs. Schwann Cells

Oligodendrocytes and Schwann cells are types of cells that produce, maintain, and repair myelin [ 9 ].

Schwann cells normally produce myelin in peripheral nerves (outside the brain), but can enter the brain when needed [ 9 ].

On the other hand, oligodendrocytes are found solely in the brain. They are responsible for the formation of new myelin in both the injured and healthy adult brains [ 9 ]. Symptoms and Conditions Linked With Myelin Loss or Damage

Demyelination refers to myelin damage or loss. It disrupts signals between neurons and may result in a diverse range of neurological symptoms. These depend on whether peripheral (outside the brain) or central (in the brain and spinal cord) neurons are affected, and to what extent [ 10 ].

Symptoms differ from patient to patient and have different presentations, depending on the specific demyelinating disorder. The most common demyelinating disorder affecting the central nervous system is Multiple Sclerosis [ 10 ].

Thus, symptoms shown here are commonly associated with demyelinating disorders. This list is not exhaustive. The most important step is to see your doctor or other health care professional to get an accurate diagnosis and treatment [ 11 ]. Blurred vision that may affect only one eye

Double vision

Loss of vision/hearing

Odd sensation in legs, arms, chest, or face, such as tingling or numbness (neuropathy)

Muscle weakness

Cognitive dysfunction, including speech impairment and memory loss Heat sensitivity Loss of dexterity Difficulty coordinating movement and/or balance Difficulty controlling bowel movements and/or urination Fatigue Tinnitus Symptoms of demyelinating disorders include complex visual and sensory changes that vary from person to person depending on the underlying cause.Multiple sclerosis is the most common demyelinating disorder. The cause of multiple sclerosis is unknown, though many contributing factors have been proposed [ 10 ].The following are more rare types of demyelinating disorders [ 10 , 12 ]: Acute disseminated encephalomyelitis Acute hemorrhagic leukoencephalitis Neuromyelitis optica Chronic inflammatory demyelinating polyneuropathy Central pontine myelinosis Inherited demyelinating diseases such as leukodystrophy Charcot-Marie-Tooth disease Adrenoleukodystrophy and adrenomyeloneuropathy Leber hereditary optic neuropathy Progressive multifocal leukoencephalopathy The exact cause of many demyelinating disorders is often an enigma. Science suggests that certain primary demyelinating disorders develop after a viral infection or vaccination against viral infection [ 10 ].Some researchers hypothesize that this might be because a virus or another substance somehow triggers the immune system to attack the body’s own tissues (autoimmune reaction). The autoimmune reaction results in inflammation, which damages the myelin sheath and the nerve fiber under it [ 10 , 12 ].However, this hypothesis holds only for specific, rare demyelinating disorders [ 10 , 12 ].HIV infection can also cause white matter abnormalities, including myelin damage [ 10 ].Multiple sclerosis is the most common demyelinating disorder. It causes progressive loss of the myelin sheath.The following are some genetic disorders of myelin [ 13 , 14 ] Adrenoleukodystrophy Tay-Sachs disease Niemann-Pick disease Gaucher disease Hurler syndrome Canavan disease Charcot-Marie-Tooth disease Krabbe’s leukodystrophy Phenylketonuria Aside from demyelinating disorders, limited studies have linked the following disorders to white matter or myelin loss or damage: Nutritional deficiencies (such as B12 deficiency) [ 22 ] According to some theories, reduced white matter in the brain is a contributing factor to some brain-related conditions. Also, scientists think that certain conditions are caused by white matter reductions. At other times, science suggests that specific conditions themselves may cause white matter reduction [ 27 ].However, many of these links are purely investigational and lack large-scale human data as support.Additionally, the majority of studies that focused on these conditions dealt with associations only, which means that a cause-and-effect relationship hasn’t been established.For example, just because depression has been linked with altered white matter (made up of myelin) in certain brain areas doesn’t mean that depression is caused by myelin damage. Data are lacking to make such claims.Also, even if a study did find that poor myelination contributes to depression, myelin is highly unlikely to be the only causative […]

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