The Anatomy of the Myelin Sheath

The immune system attacks this nerve covering in multiple sclerosis

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Targeting Myelin Repair as Treatment for MS
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The myelin sheath is the protective, fatty coating surrounding your nerve fibers, similar to the protective insulation around electrical wires. This coating enables the electrical impulses between nerve cells to travel back and forth rapidly. When myelin becomes damaged, these electrical signals are interrupted and may even stop altogether.

Anatomy

Myelin is made of fat and protein and it's wrapped in numerous layers around many of the nerves in the central nervous system (CNS), which includes your brain, spinal cord, and the optic (eye) nerves, as well as in the peripheral nervous system (PNS), which contains all the nerves outside of the CNS. Myelin is created by specific types of glial cells: In the CNS, these are oligodendrocytes; in the PNS, they are Schwann cells.

If you've ever noticed the jerky, sudden movements babies make, this is because their myelin sheaths aren't fully developed at birth. As they get older and the myelin matures and builds up, their movements become smoother and more controlled. This process continues through adulthood.

However, there are certain rare genetic disorders in which the myelin sheath never develops the way it's supposed to, often resulting in permanent neurologic issues. These conditions include:

Function

In a healthy person, nerve cells send impulses to each other along a thin fiber that's attached to the nerve cell body. These thin projections are called axons and most of them are protected by the myelin sheath, which allows nerve impulses to travel rapidly and effectively. Myelin is vital to a healthy nervous system, affecting everything from movement to cognition.

In multiple sclerosis (MS), the most common disease associated with myelin damage, immune cells attack myelin—and eventually, the axons—in the brain and/or spinal cord. Repeated attacks on myelin eventually lead to scarring. When myelin is scarred, nerve impulses cannot be properly transmitted; they either travel too slowly or not at all. Eventually, axons degenerate (lose their ability to function) as a result of chronic myelin loss, leading to nerve cell death.

Depending on where in the central nervous system myelin is attacked, symptoms like sensory disturbances, vision problems, muscle spasms, and bladder problems begin to manifest. This is why the symptoms of MS vary widely from one person to another, as the location of myelin attacks varies within the central nervous system.

In addition to the variable sites of immune system attacks in your brain and spinal cord, the timing of these attacks is also unpredictable, though there are potential triggers like stress or the postpartum period.

Demyelination

Demyelination is damage to or destruction of the myelin sheath, the protective covering surrounding nerve fibers. This damage causes nerve signals to slow down or stop, resulting in neurological impairment.

Associated Conditions

Damage to myelin can be caused by the following conditions:

  • Stroke
  • Infections
  • Inflammation
  • Metabolic disorders
  • An antibiotic called Myambutol (ethambutol)
  • Immune disorders
  • Excessive alcohol use
  • Carbon monoxide poisoning
  • Vitamin B12 deficiency

CNS Demyelinating Diseases

The most common demyelinating disease of the central nervous system is multiple sclerosis, but others include:

The causes of these conditions are unknown, but they're believed to be related to an autoimmune reaction that causes inflammation, damaging the myelin sheath.

PNS Demyelinating Diseases

There are also demyelinating conditions that mainly affect myelin in the peripheral nervous system, including:

Treatment

Current therapies for multiple sclerosis target your immune system. While they have been found to decrease the number and severity of MS relapses, there's still no cure for MS, so you may continue to become more disabled as your disease progresses. But now, experts are examining therapies that target myelin.

Repairing Myelin: The Next Step in MS Therapy

While current disease-modifying MS therapies focus on how to prevent your immune system from attacking myelin, scientists are looking into how myelin can be repaired once it has been damaged by the immune system. The hope is that if myelin is repaired, your neurological function may be restored and your MS will stop getting worse—or at least slow down.

The good news is that some studies have already shown that preserving and restoring the myelin that surrounds axons can increase nerve cell survival. Since your MS-related disability is linked to the degree of nerve cell death, by repairing myelin and protecting nerve cells, experts hope to eventually be able to stop the progression of disability in people with MS.

The research on restoring function and repairing myelin in multiple sclerosis is still very early. Nonetheless, it's exciting and potentially a step closer to ending MS once and for all.

For example, in a 2017 study, an over-the-counter allergy medication called Dayhist/Tavist (clemastine) was examined to see if it could promote myelin repair in the brains of people with MS. In this study, 50 people with relapsing MS and optic nerve damage were randomly and blindly given either clemastine by mouth twice a day or a placebo pill for 150 days. After 90 of those 150 days, the participants switched therapies, meaning those who received clemastine initially received placebo and vice-versa for the last 60 days.

The participants underwent visual evoked potentials, which measure signal transmission from the retina of the eye through the optic nerve to the visual cortex, the region of the brain that processes images (converting what one sees into an actual picture).

Results revealed that the delay in visual evoked potentials was reduced by 1.7 milliseconds per eye for the time that people were being treated with clemastine. This reduction in the nerve transmission delay suggests that myelin repair occurred along the optic nerve signaling pathway.

One caveat of the study is that the dose of clemastine was higher than the maximum dose that is normally recommended, so not surprisingly, it did cause some fatigue in the participants.

Potential Myelin-Repairing Medications in Early Trials 

Other early studies are recruiting patients or are currently underway regarding medications that may help promote myelin repair and protect nerve cells in the central nervous system. There are a number of treatments being studied, but some examples include:

  • Guanabenz: A medication previously approved by the U.S. Food and Drug Administration (FDA) for treating high blood pressure, guanabenz has been found in animal studies to increase the survival of oligodendrocytes, cells that produce myelin. It has also been found to reduce the number of immune cells that collect in the brain and spinal cord.
  • Ibudilast: A 2018 phase 2 trial at Cleveland Clinic for 255 patients with either primary or secondary progressive MS found that ibudilast slowed the rate of brain atrophy (shrinking) compared to placebo.
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