physiology

Myelin

Myelin is an insulating substance with a lamellar structure, consisting mainly of lipids and proteins. At the white-greyish view, with straw-yellow hues, myelin externally covers the axons of neurons; this coating can be simple (monolayer), or composed of various concentric layers, which give rise to a sort of sheath or sleeve.

Components% of dry weight *

Protein

Lipids

ganglioside

Cholesterol

cerebrosides

Cerebroside sulfate (sulfatide)

Phosphatidylcholine (lecithin)

Phosphatidylethanolamine (cephalin)

Phosphatidylserine

sphingomyelin

Other lipids

21.3

78.7

0.5

40.9

15.6

4.

10.9

13.6

5.

4.7

5.1

* Myelin, in vivo, has a water content of about 40%.

Depending on the layers of myelin that surround the axon, we talk about unmyelinated nerve fibers (only one layer with no real sheath) and myelinated nerve fibers (multilayer sleeve). Where there is myelin, the nervous tissue appears whitish; we therefore speak of white matter. Where there is no myelin, the nervous tissue appears grayish; we therefore speak of gray matter.

In the central nervous system the axons are generally myelinated, while at the peripheral level the myelin sheath is missing around most of the sympathetic fibers.

As we will see better later, the formation of myelin sheaths is entrusted to the Oligodendrocytes (for the myelin of the central nervous system) and to the Schwann Cells (for the myelin of the peripheral nervous system). The myelin that surrounds the axons of neurons, in essence, consists of the plasma membrane of Schwann cells (in the peripheral nervous system) and of oligodendrocytes (in the central nervous system).

The main function of myelin is to allow the correct conduction of nerve impulses, amplifying the transmission speed through the so-called "saltatory conduction".

In myelinated fibers, in fact, myelin does not cover the axons uniformly, but covers them at times, forming characteristic chokes that visually give rise to many small "sausages"; in this way the nerve impulse, instead of traveling along the entire length of the fiber, can proceed along the axon, jumping from one "sausage" to the other (in reality it does not spread from node to node, but someone jumps). The interruptions of the myelin sheath, between one segment and another, are defined as Ranvier nodes. Thanks to the saltatory conduction the transmission speed along the axon goes from 0.5-2 m / s to about 20-100 m / s.

A secondary but equally important function of myelin is that of mechanical protection and nutritional support for the axon it covers.

The insulating function is instead important because in the absence of myelin neurons - especially at the CNS level where the neural networks are particularly dense - being excitable, would respond to the many surrounding signals, just as an electric wire without an insulating cover would scatter the current without bringing it to destination.

Examining the composition of myelin, there is a predominant contribution from lipids, especially cholesterol and to a lesser extent phospholipids such as lecithin and cephalin. 80% of proteins is instead made up of a basic protein and a proteolipid protein; there are also minor proteins, among which the so-called oligodendrocyte protein stands out.

Being the body's own components, normally the immune system recognizes myelinated proteins as "self", therefore friendly and not dangerous; unfortunately in some cases, the lymphocytes become "self-aggressive" and attack myelin, destroying it little by little. We are talking about multiple sclerosis, a disease that leads to the gradual loss of the myelin coating, until the death of the nerve cell. When myelin is inflamed or destroyed, conduction along the nerve fibers is damaged, slowed down or stopped completely. The damage of myelin is, at least in the early stages of the disease, partially reversible, but may in the long run result in irreparable damage to the underlying nerve fibers.
For years it was believed that once damaged, myelin could not be regenerated. Recently it has been seen that the central nervous system can remielinize itself, ie form new myelin, and this opens up new therapeutic perspectives in the treatment of multiple sclerosis.

As anticipated, myelin is made up of the plasma membrane (plasmalemma) of particular cells, which wraps itself around the axon several times. At the level of the central nervous system, myelin is produced by cells called oligodendrocytes, while at the peripheral level the same function is covered by Shwann cells. Both cell types belong to the so-called glial cells; myelin is formed when these glial cells envelop an axon with their plasma membranes, squeezing the cytoplasm outward so that each winding corresponds to the addition of two layers of membrane; for instance, the process of myelination can be compared to the wrapping of a deflated balloon around a pencil, or of a double layer gauze around a finger.

Since there are space problems in the CNS, every single oligodendrocyte provides myelin for only one segment, but more axons; therefore each axon is surrounded by myelinated segments formed by different oligodendrocytes. At the peripheral level, instead, every single Shwan cell supplies myelin to a single axon.

Oligodendrocytes and Schwann cells are induced to produce myelin from the axon diameter: in the CNS this occurs when the diameter is 0.3 μm, while in the SNP it starts from diameters greater than 2 μm.

Usually the thickness of the myelin sheath, therefore the number of windings from which it is formed, is proportional to the diameter of the axon and this in turn is proportional to its length.

The structurally unmyelinated fibers consist of small bundles of naked axons: each bundle is wrapped by a Schwann cell, which sends thin cytoplasmic offshoots to separate the single axons. In the unmyelinated fibers, therefore, numerous small diameter axons can be contained in the introflexions of a single Schwann cell.

At the peripheral level, the presence of myelin produced by Shwann cells gives nerve fibers the chance to regenerate itself, something that until a few years ago was considered impossible at the CNS level. Unlike Schwann cells, in fact, oligodendrocytes do not promote nerve fiber regeneration in the event of injury. Recent research, however, has shown that regeneration is difficult but also possible in the central nervous system and that, potentially, "neurogenesis", or the formation of new neurons, is even possible.