Myelin is a special, particularly lipid-rich, called biomembrane, which encloses mainly as a so-called myelin or medullary axons of nerve cells of the peripheral nervous system and the central nervous system and the nerve fibers contained electrically isolated.

Because of regular interruptions of the myelin sheaths (Ranvier-Schnürringe), the electrical conduction is abruptly from lacing to Schnürring, which leads to an overall higher conduction velocity than with continuous conduction.

What is myelin?

Myelin is a special biomembrane that envelops the axons of the peripheral nervous system (PNS) and the central nervous system (CNS) and isolates them electrically from other nerves. The myelin in the PNS is formed by Schwann cells, the myelin membrane of a Schwann cell always "wrapping" only a portion of one and the same axon in several to 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 provide their myelin membrane with up to 50 axons simultaneously. The myelin sheaths of the axons of the are broken every 0.2 to 1.5 mm of Ranvier laces, resulting in a leaky (saltatory) transmission of electrical stimuli that is faster than the continuous transmission.

The myelin protects the internal nerve fibers against electrical signals from other nerves and causes low-loss transmission over relatively long distances. Axons of the PNS can reach a length of over 1 meter.

Anatomy & Construction

The high lipid content in myelin has a complex structure and consists mainly of cholesterols, cerebrosides, phospholipids such as lecithin and other lipids. The contained proteins such as the myelin basic protein (MBP) and the myelin-associated glycoprotein and several other proteins have a decisive influence on the structure and strength of the myelin.

The composition and structure of myelin varies in CNS and PNS. Myelin Oligodendrocyte glycoprotein (MOG) plays an important role in the myelination of axons of the CNS. The special protein does not occur in the Schwann cells, which form the myelin membrane of the axons of the PNS. Peripheral myelin protein-22 probably contributes to the firmer structure of Schwann cell myelin, compared to the structure of myelin of oligodendrocytes.

In addition to the regular interruptions of the myelin sheaths by the Ranvier-Schnürringe, located in the myelin sheath so-called Schmidt-Lantermann notches, also called Myelininzisuren. These are cytoplasmic residues of Schwann cells or oligodendrocytes, which stretch as narrow strips across all myelin sheaths to provide the necessary mass transfer between cells.

They perform the function of gap junctions, which allow and facilitate 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 lies in the electrical isolation of the axons and the nerve fibers running inside the axon and the rapid electrical signal transmission. On the one hand, electrical isolation protects against signals from other non-myelinated nerves, and it causes the least possible loss of transmission of the nerve stimuli.

Transmission speed and "line losses" are of particular importance for axons in the PNS because of their length, sometimes over one meter. The electrical isolation of the axons and also of individual nerve fibers allowed in the course of evolution a kind of miniaturization of the nervous system. Only the invention of myelination through evolution made powerful brains possible with a huge number of neurons and an even greater number of synaptic interconnections. About 50% of the brain mass consists of white matter, ie myelinated axons.

Without the myelination, even an approximately similarly complex brain performance in such a small space would be completely impossible. To clarify the proportions, the optic nerve emerging from the retina, which contains about 2 million myelinated nerve fibers, serves. 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 saltatory stimulus transmission, which has a clear speed advantage over the continuous excitation conduction, developed in the evolution.

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

Diseases

One of the most well-known diseases, which 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 of the axons is broken down by the immune system, so that MS can be classified in the category of neurodegenerative autoimmune diseases.

Unlike the Guillain-Barré syndrome, in which the immune system attacks the nerve cells directly despite protection by the myelin membrane, but whose neuronal damage partially regenerates the body, the degenerated myelin can not be replaced again. The exact causes of the occurrence of MS are not (yet) sufficiently researched, however, MS occurs frequently in families, so that at least a certain genetic disposition can be assumed.

Diseases that cause myelin breakdown in the CNS and are caused by inheritable genetic defects are called leukodystrophies or adrenoleukodystrophy if the gene defect is at a locus of the X chromosome.

A vitamin B12 deficiency disease called pernicious anemia, also called Biermer disease, also leads to a breakdown of myelin sheaths and triggers corresponding symptoms. The literature discusses the extent to which the development of mental illnesses such as schizophrenia can be causally related to dysfunction of the myelin membrane.

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