Myelin

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

Components% of dry weight *

Proteins

Lipids

Gangliosides

Cholesterol

Cerebrosides

Cerebroside sulfate (sulfatide)

Phosphatidylcholine (lecithin)

Phosphatidylethanolamine (cephalin)

Phosphatidylserine

Sphingomyelin

Other lipids

21.3

78.7

0.5

40.9

15.6

4.1

10.9

13.6

5.1

4.7

5.1

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

Depending on the layers of myelin that surround the axon, we speak of unmyelinated nerve fibers (a single layer with a lack of a 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 nerve tissue appears greyish; 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 shall see later, the formation of myelin sheaths is entrusted to the Oligodendrocytes (for the myelin of the central nervous system) and to Schwann cells (for the myelin of the peripheral nervous system). The myelin that surrounds the axons of neurons essentially consists of the plasma membrane of Schwann cells (in the peripheral nervous system) and oligodendrocytes (in the central nervous system).

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

In myelinated fibers, in fact, the myelin does not cover the axons in a uniform way, but covers them at times, forming characteristic constrictions 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 propagate from knot to knot, but skips some). The interruptions of the myelin sheath, between one segment and the other, are called 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 sustenance towards the axon it covers.

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

Examining the composition of myelin, we note a preponderant contribution from lipids, especially cholesterol and to a lesser extent phospholipids such as lecithin and cephalin. The 80% of the 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.

Since these are components of the organism, normally the immune system recognizes the myelin proteins as "self", therefore friendly and not dangerous; unfortunately in some cases, the lymphocytes become "self-aggressive" and attack the myelin, destroying it little by little. speaking of multiple sclerosis, a disease that leads to the gradual loss of the myelin lining, leading to the death of the nerve cell. When myelin is inflamed or destroyed, the conduction along the nerve fibers is damaged, slowed or completely interrupted. The damage of myelin is , at least in the early stages of the disease, partially reversible, but can lead in the long run to 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 re-myelinate itself, that is, 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 several times around the axon. 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 coiling corresponds to the addition of two layers of membrane; to be clear, the myelination process can be compared to wrapping a deflated balloon around a pencil, or a double layer of gauze around a finger.

Since in the S.N.C. there are space problems, each 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, however, each individual Shwan cell supplies myelin to a single axon.

Oligodendrocytes and Schwann cells are induced to produce myelin from the diameter of the axon: 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 coils from which it is formed, is proportional to the diameter of the axon and this in turn is proportional to its length.

Structurally unmyelinated fibers consist of small bundles of bare axons: each bundle is enveloped by a Schwann cell, which sends thin cytoplasmic offshoots to separate the individual axons. In 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 the nerve fibers the ability to regenerate, which until a few years ago was considered impossible at the level of the CNS. Unlike Schwann cells, in fact, oligodendrocytes do not promote the regeneration of the nerve fiber 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.