When we think of the cells of the human brain and the nervous system in general, the image of neurons usually comes to mind. However, these nerve cells by themselves cannot form a functional brain: they need the help of many other "pieces" with which our organism is built.

The myelin , for example, is one of those materials without which our brain could not perform its operations effectively.

What is myelin?

When we graphically represent a neuron, either through a drawing or a 3D model, we usually draw the area of the nucleus, the branches with which it connects to other cells and an extension called the axon, which serves to reach remote areas. However, in many cases this image would be incomplete. Many neurons have, around their axons, a whitish material that isolates it from the extracellular fluid. This substance is myelin.

Myelin is a thick lipoprotein layer (made up of fatty substances and proteins) that surrounds the axons of some neurons, forming sausage-shaped sheaths or rolls. These myelin sheaths have a very important function in our nervous system: to allow the transmission of nerve impulses quickly and efficiently between the nerve cells of the brain and the spinal cord .

The function of myelin

The electrical current that passes through the nerve cells is the type of signal with which these nerve cells function. Myelin allows these electrical signals to propagate very quickly through the axons , so that this stimulus reaches the spaces where the neurons communicate with each other in time. In other words, the main added value that these sheaths bring to the neuron is the speed of propagation of the electrical signals.

If we were to remove the myelin sheath from an axon, the electrical signals traveling through it would be much slower or even lost along the way. The myelin acts as an insulator, so the current doesn’t dissipate outside the pathway and goes only inside the neuron.

Ranvier’s nodules

The myelinic layer that covers the axon is called the myelin sheath, but this is not completely continuous along the axon, but between the myelinated segments there are exposed regions. These areas of the axon that come into contact with the extracellular fluid are called Ranvier’s nodules .

The existence of Ranvier’s nodules is important because without them the presence of myelin would be useless. In these spaces, the electrical current that propagates through the neuron gains strength, since in the Ranvier nodules are the ionic channels that, by acting as regulators of what enters and leaves the neuron, allow the signal not to lose strength.

The action potential (nerve impulse) jumps from one node to another because these, unlike the rest of the neuron, are equipped with clusters of sodium and potassium channels, so that the transmission of nerve impulses is faster. The interaction between the myelin sheath and Ranvier’s nodules p allows the nerve impulse to be transferred more quickly, in a jumping manner (from one Ranvier’s node to the next) and with less possibility of error.

Where is the myelin found?

There is myelin in the axons of many types of neurons, both in the Central Nervous System (i.e., the brain and spinal cord) and outside of it. However, in some areas their concentration is higher than in others. Where myelin is abundant, it can be seen without the aid of a microscope.

When we describe a brain it is usual to talk about grey matter, but also, and although this fact is somewhat less known, there is white matter . The areas in which white matter is found are those in which myelinated neuronal bodies are so abundant that they change the colour of those areas seen with the naked eye. This is why the areas in which the nuclei of the neurons are concentrated are usually grey, while the areas through which the axons essentially pass are white.

Two types of myelin sheaths

Myelin is essentially a material that serves a function, but there are different cells that form myelin sheaths. Neurons belonging to the Central Nervous System have layers of myelin formed by a type of cell called oligodendrocytes, while the rest of the neurons use bodies called Schwann cells . Oligodendrocytes are sausage-shaped, crossed from end to end by a cord (the axon), while Scwann cells wrap around the axons in a spiral, taking on a cylindrical shape.

Although these cells are slightly different, both are glial cells with a virtually identical function: to form myelin sheaths.

Myelin disorders

There are two types of diseases that are related to myelin sheath abnormalities: demyelinating diseases and demyelinating diseases .

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Demyelinating diseases are characterized by a pathological process directed against healthy myelin, unlike demyelinating diseases, in which there is inadequate formation of myelin or an impairment of the molecular mechanisms to maintain it in its normal conditions. The different pathologies of each type of disease related to myelin alteration are

Demyelinating diseases

  • Isolated clinical syndrome
  • Acute disseminated encephalomyelitis
  • Acute hemorrhagic leukoencephalitis
  • Concentric Sclerosis of Balo
  • Marburg disease
  • Isolated acute myelitis
  • Polyphasic diseases
  • Multiple Sclerosis
  • Optic Neuromyelitis
  • Optic spinal multiple sclerosis
  • Isolated recurrent optic neuritis
  • Chronic recurrent inflammatory optic neuropathy
  • Recurrent acute myelitis
  • Late Postanoxic Encephalopathy
  • Osmotic myelinolysis

Dysmyelinating diseases

  • Metachromatic leukodystrophy
  • Adrenoleukodystrophy
  • Refsum disease
  • Canavan disease
  • Alexander’s disease or fibrinoid leukodystrophy
  • Krabbe disease
  • Tay-Sachs disease
  • Cerebral-tendon Xanthomatosis
  • Pelizaeus-Merzbacher disease
  • Orthochromic leukodystrophy
  • Leukoencephalopathy with disappearance of white substance
  • Leukoencephalopathy with neuroaxonal spheroids

To know more about myelin and its associated pathologies

Here is an interesting video on Multiple Sclerosis, which explains how myelin is destroyed in the course of this pathology :