Use precise geolocation data. Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. Measure content performance. Develop and improve products. List of Partners vendors. 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. 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. 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. 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. Repeated attacks 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 as a result of the chronic myelin loss, leading to nerve cell death.
Demyelination is the term used to describe the 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.
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. Research through the s showed how myelin ensures that the signal is maintained and transmitted.
In the s, French physician Louis-Antoine Ranvier noted that the myelin sheath is discontinuous, covering most of the nerve fiber but with gaps at regular intervals along the axon. In the s and s, scientists found that this passage of ions helps maintain the electrical signal, allowing it to travel quickly down an axon.
In the s, researchers used animal models to assess how electrical nerve signals are altered after axons were stripped of the myelin demyelinated. When researchers chemically induced myelin loss in the spinal cords of cats, they found that signals moved more slowly along the nerve fiber and often failed to make it to the end of the axon.
Around the same time, scientists also made breakthroughs in identifying many of the components of myelin, like the major protein elements of the myelin sheath and the genes that encode them. Researchers developed mouse models that had defective myelin proteins, resulting in a myelin deficiency. Loss of myelin is a problem for many CNS disorders, including stroke, spinal cord injury, and, most notably, multiple sclerosis MS.
MS is a chronic, disabling disease of the CNS that affects more than 2. MS results from the accumulation of damage to myelin and the underlying nerve fibers it insulates and protects. Current research indicates that MS involves an autoimmune response.
Scientists think that immune cells, which normally defend the body against bacteria and viruses, mistakenly attack the myelin sheath, stripping it away and exposing the nerve fibers underneath. In addition, recent research suggests that axon damage occurs early on in the course of the disease.
Once damaged, the ability of nerve cells in the brain and spinal cord to communicate with each other and with muscles is compromised, leading to a variety of unpredictable symptoms that vary from person to person.
These symptoms, which can be temporary or permanent, range from fatigue, weakness, and numbness to blindness and even paralysis. Research understanding the components of myelin, how it is produced, and how it functions has paved the way for new therapeutic possibilities in myelin-degenerative diseases like MS.
Repairing and protecting myelin is one of the approaches to treating demyelinating disease like MS. This approach focuses on 1 repairing the damage that has already occurred and 2 preventing further injury to nerves and axons. Several drugs that are currently approved for treating MS follow the second strategy. They work by suppressing or changing the activity of the immune system, protecting myelin from unwarranted attacks. However, to date none of the available medications address regeneration of lost myelin.
Stem cell therapy is one avenue being explored in the search for treatments for MS. These new stem cells were then infused into the spinal cords of mice models of MS where they secreted factors that helped the myelin-producing cells survive. Consequently, these mice had more myelination and less axonal damage compared to mice that did not receive stem cell infusions. While the results are promising, much more work will need to be done in human clinical trials to determine the therapeutic efficacy.
Continued research efforts funded by public and private institutions worldwide seek to understand how myelin is compromised in diseases like MS, revealing new possibilities for treatment and offering hope to the millions of people affected by these diseases.
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Personalize your monthly updates from BrainFacts. In contrast, transgenic mice that overexpress neuregulin 1 become hypermyelinated. Although several reports show that oligodendrocytes respond to neuregulin 1 in vitro, analyses of a series of conditional null mutant animals lacking neuregulin 1 showed normal myelination Brinkmann et al.
It is still unclear how myelination is regulated in the CNS. How does myelin enhance the speed of action potential propagation? It insulates the axon and assembles specialized molecular structure at the nodes of Ranvier. In unmyelinated axons, the action potential travels continuously along the axons. For example, in unmyelinated C fibers that conduct pain or temperature 0.
In contrast, among the myelinated nerve fibers, axons are mostly covered by myelin sheaths, and transmembrane currents can only occur at the nodes of Ranvier where the axonal membrane is exposed. At nodes, voltage-gated sodium channels are highly accumulated and are responsible for the generation of action potentials. The myelin helps assemble this nodal molecular organization. For example, during the development of PNS myelinated nerve fibers, a molecule called gliomedin is secreted from myelinating Schwann cells then incorporated into the extracellular matrix surrounding nodes, where it promotes assembly of nodal axonal molecules.
Due to the presence of the insulating myelin sheath at internodes and voltage-gated sodium channels at nodes, the action potential in myelinated nerve fibers jumps from one node to the next. This mode of travel by the action potential is called "saltatory conduction" and allows for rapid impulse propagation Figure 1A. Following demyelination, a demyelinated axon has two possible fates. The normal response to demyelination, at least in most experimental models, is spontaneous remyelination involving the generation of new oligodendrocytes.
In some circumstances, remyelination fails, leaving the axons and even the entire neuron vulnerable to degeneration. Remyelination in the CNS: from biology to therapy.
Nature Reviews Neuroscience 9, — All rights reserved. Figure Detail What happens if myelin is damaged? The importance of myelin is underscored by the presence of various diseases in which the primary problem is defective myelination.
Demyelination is the condition in which preexisting myelin sheaths are damaged and subsequently lost, and it is one of the leading causes of neurological disease Figure 2. Primary demyelination can be induced by several mechanisms, including inflammatory or metabolic causes. Myelin defects also occur by genetic abnormalities that affect glial cells.
Regardless of its cause, myelin loss causes remarkable nerve dysfunction because nerve conduction can be slowed or blocked, resulting in the damaged information networks between the brain and the body or within the brain itself Figure 3.
Following demyelination, the naked axon can be re-covered by new myelin. This process is called remyelination and is associated with functional recovery Franklin and ffrench-Constant The myelin sheaths generated during remyelination are typically thinner and shorter than those generated during developmental myelination.
In some circumstances, however, remyelination fails, leaving axons and even the entire neuron vulnerable to degeneration. Thus, patients with demyelinating diseases suffer from various neurological symptoms. The representative demyelinating disease , and perhaps the most well known, is multiple sclerosis MS. This autoimmune neurological disorder is caused by the spreading of demyelinating CNS lesions in the entire brain and over time Siffrin et al.
Patients with MS develop various symptoms, including visual loss, cognitive dysfunction, motor weakness, and pain. Approximately 80 percent of patients experience relapse and remitting episodes of neurologic deficits in the early phase of the disease relapse-remitting MS. There are no clinical deteriorations between two episodes. Approximately ten years after disease onset, about one-half of MS patients suffer from progressive neurological deterioration secondary progressive MS.
About 10—15 percent of patients never experience relapsing-remitting episodes; their neurological status deteriorates continuously without any improvement primary progressive MS. Importantly, the loss of axons and their neurons is a major factor determining long-term disability in patients, although the primary cause of the disease is demyelination.
Several immunodulative therapies are in use to prevent new attacks; however, there is no known cure for MS. Figure 3 Despite the severe outcome and considerable effect of demyelinating diseases on patients' lives and society, little is known about the mechanism by which myelin is disrupted, how axons degenerate after demyelination, or how remyelination can be facilitated.
To establish new treatments for demyelinating diseases, a better understanding of myelin biology and pathology is absolutely required. How do we structure a research effort to elucidate the mechanisms involved in developmental myelination and demyelinating diseases? We need to develop useful models to test drugs or to modify molecular expression in glial cells. One strong strategy is to use a culture system. Coculture of dorsal root ganglion neurons and Schwann cells can promote efficient myelin formation in vitro Figure 1E.
Researchers can modify the molecular expression in Schwann cells, neurons, or both by various methods, including drugs, enzymes, and introducing genes , and can observe the consequences in the culture dish. Modeling demyelinating disease in laboratory animals is commonly accomplished by treatment with toxins injurious to glial cells such as lysolecithin or cuprizone. Autoimmune diseases such as MS or autoimmune neuropathies can be reproduced by sensitizing animals with myelin proteins or lipids Figure 3.
Some mutant animals with defects in myelin proteins and lipids have been discovered or generated, providing useful disease models for hereditary demyelinating disorders. Further research is required to understand myelin biology and pathology in detail and to establish new treatment strategies for demyelinating neurological disorders.
Myelin can greatly increase the speed of electrical impulses in neurons because it insulates the axon and assembles voltage-gated sodium channel clusters at discrete nodes along its length.
Myelin damage causes several neurological diseases, such as multiple sclerosis. Future studies for myelin biology and pathology will provide important clues for establishing new treatments for demyelinating diseases. Brinkmann, B. Neuron 59 , — Franklin, R. Remyelination in the CNS: From biology to therapy.
Nature Reviews Neuroscience 9 , — Nave, K. Axonal regulation of myelination by neuregulin 1. Current Opinion in Neurobiology 16 , —
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