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?
Definition & Facts
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].
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
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
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
Demyelinating Disorders
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].
Genetic Myelin Sheath Disorders
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
Other Poorly-Researched Associations
Aside from demyelinating disorders, limited studies have linked the following disorders to white matter or myelin loss or damage:
- PTSD [8]
- ADHD [15]
- Depression [16]
- Chronic Fatigue Syndrome [17]
- Schizophrenia [8]
- Mild cognitive impairment in Alzheimer’s disease [18]
- Traumatic brain injury [19]
- Tourette’s syndrome [8]
- Tone deafness [8]
- Pathological lying [8]
- Guillain-Barré syndrome [20]
- Diabetes [21]
- Nutritional deficiencies (such as B12 deficiency) [22]
- Poisoning with lead, carbon monoxide, or deadly plants like rosary pea [23, 24, 25]
- Drugs (such as the antibiotic ethambutol used to treat tuberculosis) [26]
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 factor. Complex disorders like depression always involve multiple possible factors – including brain chemistry, environment, health status, and genetics – that may vary from one person to another.
Therefore, more research is needed to determine the association between these disorders and myelin or white matter abnormalities.
What Happens to Myelin in Multiple Sclerosis?
Multiple sclerosis is an autoimmune condition in which the person’s immune system attacks the myelin sheath or cells that maintain it [28].
As myelin continues to degrade, symptoms such as impaired balance, and cognition may arise [10].
Cognitive impairment occurs in 40 to 65% of multiple sclerosis patients. The deficits are typically in complex attention, information processing speed, (episodic) memory, and executive functions [29].
Myelin loss can impair or stop the conduction of signals along nerves, eventually causing groups of nerves to wither. Some scientists believe that inflammatory cytokines reduce myelination [30, 10].
Myelin & Intelligence
Do We Have a Clear Link?
Since science discovered that myelin works as a sort of a fine cable for the brain – helping carry electrical signals between neurons at high speeds – some have suggested that it may play a critical role in determining intelligence [31].
Since myelin allows signals to travel faster, researchers think it might make the brain as a whole work better. We know that white matter – nerve fibers coated with myelin – support normal cognitive function, learning, and IQ. But the specifics of the “myelin intelligence” theory have yet to be determined in humans [8, 32].
It’s important to know that studies attempting to find links between brain factors like myelin and intelligence are bound to have inconsistencies and flaws. Intelligence is a complex trait and most studies can only point to possible links. But many factors interact to shape its various aspects: general, verbal, mathematical, emotional, and other types of intelligence [32].
Myelin controls the speed of impulse conduction through axons, and the synchrony of impulse traffic between distant brain regions seems to be critical for optimal mental performance and learning [8].
Limited studies suggest that myelination of appropriate brain regions coincides with the development of specific cognitive functions, such as reading, development of vocabulary, and proficiency in executive decision making [8].
Incomplete myelination of the forebrain until the early twenties has been suggested as a neurological basis for weaker decision-making skills in adolescence, though this association remains controversial [33].
Small studies have suggested a correlation between individual differences in normal cognitive development, IQ, reading skills, working memory, and musical proficiency with differences in white matter in specific brain regions mediating these tasks. Future large-scale studies are needed to clarify this link [33].
In another small study, white matter levels in certain regions were also associated with arithmetic ability, reaction time, and cognitive control [27].
Learning complex skills, such as playing the piano, appear to be accompanied by increased white matter in brain areas involved in musical performance. White matter increased proportionately to the number of hours each subject had practiced the instrument, indicating white matter increases when acquiring certain skills [33].
However, such studies have limitations that make it difficult to determine whether learning a new skill can directly impact myelination.
A couple of studies have noted a positive correlation between white matter volume and intelligence at the level of whole brain white matter volume as well as in specific white matter regions [34].
Additionally, researchers suggest that prefrontal white matter volume is disproportionately larger in humans compared to non-human primates [35].
Summary
To sum it up, white matter and myelin have been associated with the following aspects of intelligence in limited, small studies [27]:
- Working memory
- IQ
- Verbal ability
- Reaction time
- Cognitive control
- Musical ability
- Attention
- Arithmetic capacities
However, larger and better-designed studies are needed before we can draw any clear conclusions.
Takeaway
The myelin sheath is a protective coating that wraps around brain cells, enabling them to send out signals at high speed.
Demyelination refers to a loss or damage of the myelin sheath. The most common demyelinating disorder is multiple sclerosis, though many other disorders have been associated with demyelination.
Symptoms of demyelinating disorders include complex visual, sensory, and other neurological changes. These symptoms vary from person to person depending on the underlying cause. It’s essential to see a doctor to get an accurate diagnosis and treatment.
