Myelin is the name given to a special, particularly lipid-rich, biomembrane that mainly acts as a so-called myelin sheath or medullary sheath, enclosing axons of nerve cells of the peripheral nervous system and the central nervous system and electrically insulating the contained nerve fibers. Because of regular interruptions of the myelin sheaths (Ranvier’s cord rings), electrical conduction of stimuli occurs abruptly from cord ring to cord ring, resulting in a higher overall conduction velocity than in continuous conduction.
What is myelin?
Myelin is a special biomembrane that coats the axons of the peripheral nervous system (PNS) and central nervous system (CNS) and electrically insulates them from other nerves. Myelin in the PNS is formed by Schwann cells, and the myelin membrane of a Schwann cell “wraps” only one section of the same axon at a time in several to many layers. In the CNS, myelin membranes are formed by highly branched oligodendrocytes. Due to their special anatomy with many branched arms, oligodendrocytes can provide their myelin membranes to up to 50 axons simultaneously. The myelin sheaths of the axons of are interrupted every 0.2 to 1.5 mm by Ranvier laced rings, resulting in an erratic (saltatory) mode of transmission of electrical stimuli that is faster than the continuous mode of transmission. Myelin protects the internally running nerve fibers from electrical signals from other nerves and conditions transmission with as little loss as possible, even over relatively long distances. Axons of the PNS can reach a length of more than 1 meter.
Anatomy and structure
The high lipid content of myelin exhibits a complex structure and consists mainly of cholesterols, cerebrosides, phospholipids such as lecithin, and other lipids. The proteins it contains, such as basic myelin protein (MBP) and myelin-associated glycoprotein and some other proteins, have a decisive influence on the structure and strength of myelin. The composition and structure of myelin is different in the CNS and PNS. In myelination of CNS axons, myelin oligodendrocyte glycoprotein (MOG) plays an important role. This particular protein is not found in the Schwann cells that form the myelin membranes of PNS axons. It is likely that peripheral myelin protein-22 provides the firmer structure of Schwann cell myelin compared to the structure of oligodendrocyte myelin. In addition to the regular interruptions of the myelin sheaths by the Ranvier cord rings, there are so-called Schmidt-Lantermann notches, also called myelin incisures, in the myelin sheaths. These are cytoplasmic remnants of Schwann cells or oligodendrocytes that extend as narrow stripes through all myelin sheaths to provide for the necessary exchange of material between the cells. They perform the function of gap junctions that allow and enable the exchange of substances between the cytoplasm of two adjacent cells.
Function and tasks
One of the most important functions of myelin, or the myelin membrane, is to electrically insulate the axons and the nerve fibers running within the axon and to provide rapid electrical signal transmission. On the one hand, the electrical insulation protects against signals from other non-myelinated nerves, and on the other hand, it requires the transmission of nerve impulses to be as low-loss and fast as possible. Transmission speed and “conduction losses” are particularly important for axons in the PNS because of their length, which sometimes exceeds one meter. In the course of evolution, the electrical insulation of axons and also of individual nerve fibers enabled a kind of miniaturization of the nervous system. Only the invention of myelination by evolution made powerful brains with a huge number of neurons and an even larger number of synaptic connections possible. About 50% of the brain mass consists of white matter, i.e. myelinated axons. Without myelination, even remotely similar brain 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 illustrate the proportions. Without the protection of myelin, the optic nerve would have to be more than one meter in diameter for the same output. Simultaneously with the myelination, saltatory stimulus conduction arose in evolution, which has a clear speed advantage over continuous excitation conduction.Simplified, one can imagine that ion channels are opened and closed via depolarization in order to transmit the action potential to the next section (internode). Here, the action potential is again built up in the same strength, forwarded and at the end of the section via depolarization again to activate the ion pump and pass the potential to the next section.
Diseases
One of the best-known diseases directly related to a gradual degradation of the myelin membrane of axons is multiple sclerosis (MS). As the disease progresses, the myelin of axons is degraded by the patient’s own immune system, placing MS in the category of neurodegenerative autoimmune diseases. Unlike Guillain-Barré syndrome, in the course of which the immune system directly attacks the nerve cells despite protection by the myelin membrane, but whose neuronal damage is partially regenerated by the body, the myelin degenerated by MS cannot be replaced. The exact causes for the occurrence of MS are not (yet) sufficiently researched, but MS occurs in families, so that at least a certain genetic disposition can be assumed. Diseases that cause degradation of myelin in the CNS and are based on heritable genetic defects are called leukodystrophies or adrenoleukodystrophy if the genetic defect is located on a locus of the X chromosome. A vitamin B12 deficiency disease, pernicious anemia, also called Biermer’s disease, also leads to degradation of the myelin sheaths and causes corresponding symptoms. The literature discusses the extent to which the development of mental illnesses such as schizophrenia can be causally related to myelin sheath dysfunction.
