Anatomy & Organs

Myelin – Structure, Function & Diseases

Myelin

Myelin is a special biomembrane that is particularly rich in lipids, which mainly encloses axons of the nerve cells of the peripheral nervous system and the central nervous system as a so-called myelin or marrow sheath and electrically insulates the nerve fibers contained therein.

Due to regular interruptions in the myelin sheaths (Ranvier nodes), the electrical stimulus conduction occurs erratically from node to node, which leads to a higher conduction speed overall than with continuous stimulus conduction.

What is myelin?

Myelin is a special biomembrane that encases the axons of the peripheral nervous system (PNS) and central nervous system (CNS) and electrically insulates them from other nerves. The myelin in the PNS is formed by Schwann cells, with the myelin membrane of a Schwann cell always “wrapping” only a section of one and the same axon in several or many layers.

In the CNS, the myelin membranes are formed by highly branched oligodendrocytes. Due to their special anatomy with many branched arms, oligodendrocytes can make their myelin membranes available to up to 50 axons at the same time. The myelin sheaths of the axons are interrupted every 0.2 to 1.5 mm by nodes of Ranvier, which leads to an erratic (saltatory) form of transmission of electrical stimuli that is faster than the continuous form of transmission.

The myelin protects the inner nerve fibers from electrical signals from other nerves and ensures that transmission is as low-loss as possible, even over relatively long distances. Axons of the PNS can reach a length of more than 1 meter.

Anatomy & Structure

The high lipid content in myelin has a complex structure and consists mainly of cholesterols , cerebrosides, phospholipids such as lecithin and other lipids . The proteins contained such as the basic myelin protein (MBP) and the myelin-associated glycoprotein and some other proteins have a decisive influence on the structure and strength of the myelin.The composition and structure of the myelin is different in the CNS and PNS. The myelin oligodendrocyte glycoprotein (MOG) plays an important role in the myelination of the axons of the CNS. The specific protein is not found in the Schwann cells that make up the myelin membranes of the axons of the PNS. Peripheral myelin protein-22 is probably responsible for the firmer structure of the myelin in Schwann cells compared to the structure of the myelin in oligodendrocytes.

In addition to the regular interruptions in the myelin sheaths caused by the nodes of Ranvier, there are what are known as Schmidt-Lantermann notches in the myelin sheaths, also known as myelin incisions. These are cytoplasm residues of Schwann cells or oligodendrocytes, which run through all myelin layers as narrow strips to ensure the necessary exchange of substances between the cells .

They take over the function of gap junctions, which allow and enable the exchange of substances between the cytoplasm of two neighboring cells.

Function & Tasks

One of the most important functions of the myelin or myelin membrane is the electrical insulation of the axons and the nerve fibers running within the axon and the rapid transmission of electrical signals. On the one hand, the electrical insulation protects against signals from other non-myelinated nerves, and on the other hand it requires the fastest possible transmission of nerve stimuli with as little loss as possible.

Transmission speed and “line losses” are of particular importance for axons in the PNS due to their length of sometimes more than one meter. The electrical insulation of the axons and also of individual nerve fibers made possible a kind of miniaturization of the nervous system in the course of evolution. Only the invention of myelination by evolution made powerful brains with a huge number of neurons and an even larger number of synaptic circuits possible. About 50% of the brain mass consists of white matter, i.e. myelinated axons.

Without myelination, brain performance that is even remotely similar in complexity would be completely impossible in such a small space. The optic nerve emerging from the retina , which contains about 2 million myelinated nerve fibers, serves to clarify the proportions . Without the protection of the myelin, the optic nerve would have to have a diameter of more than one meter for the same performance. Simultaneously with the myelination, the saltatoric stimulus transmission developed in evolution, which has a clear speed advantage compared to the continuous stimulus transmission.

Put simply, one can imagine that ion channels are opened and closed via depolarization in order to forward the action potential to the next section (internode). Here the action potential is built up again with the same strength, passed on and at the end of the depolarization section to activate the ion pump again and transfer the potential to the next section.

Diseases

One of the most well-known diseases that is directly related to a gradual breakdown of the myelin membrane of axons is multiple sclerosis (MS). During the course of the disease, the myelin in the axons is broken down by the body’s own immune system , so that MS can be classified in the category of neurodegenerative autoimmune diseases .Unlike Guillain-Barré syndrome , in the course of which the immune system attacks the nerve cells directly despite protection by the myelin membrane, but the body partially regenerates the neuronal damage, the myelin that has been degenerated by MS cannot be replaced again. The exact causes for the occurrence of MS have not (yet) been adequately researched, but MS tends to run in families, so that at least a certain genetic disposition can be assumed.

Diseases that cause the breakdown of myelin in the CNS and are based on hereditary genetic defects are referred to as leukodystrophies or as adrenoleukodystrophy if the gene defect is located at a locus on the X chromosome.

vitamin B12 deficiency disease , pernicious anemia, also known as Biermer’s disease, also leads to a breakdown of the myelin sheaths and triggers corresponding symptoms. The specialist literature discusses the extent to which the development of mental illnesses such as schizophrenia can be causally linked to functional disorders of the myelin membrane.

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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.