Myelin: definition, functions and characteristics

When we think of the cells of the human brain and the the nervous system in general, the image of neurons usually comes to mind. However, these nerve cells alone cannot form a functioning brain: they need the help of many other “parts” that our bodies are built with.

the myelinFor example, it is one of those materials without which we could not ensure that our brain cannot perform its operations efficiently.

What is myelin?

When we represent a neuron graphically, whether by means of 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 an axon and which serves to reach zones. However, in many cases this image would be incomplete. Many neurons have a whitish material around their axons that isolates them from extracellular fluid. This substance is myelin.

Myelin is a thick layer of lipoprotein (made up of fats and proteins) that surrounds the axons of certain neurons forming sausage or roll-shaped pods. These myelin sheaths play a very important role in our nervous system: allow the rapid and efficient transmission of nerve impulses between the nerve cells of the brain and the spinal cord.

The function of myelin

The electrical current flowing through neurons is the type of signal that these nerve cells work with. Myelin allows these electrical signals to travel very quickly through axons, So that this stimulus arrives in time to the spaces in which the neurons communicate with each other. In other words, the main added value that these pods bring to the neuron is the speed of propagation of electrical signals.

If we removed its myelin sheaths from an axon, the electrical signals passing through it would go much slower or might even get lost along the way. Myelin acts as an insulator, so the current doesn’t dissipate outside the pathway and only goes inside the neuron.

Ranvier nodules

The myelin sheath that covers the axon is called the myelin sheath, but it is not completely continuous along the axon, but between the myelinated segments are open regions. Those areas of the axon that come into contact with extracellular fluid are called Ranvier nodules.

The existence of Ranvier nodules is important, because without them the presence of myelin would be useless. In these spaces, the electric current which propagates through the neuron gains in force, as in the nodules of Ranvier are the ion channels which, by acting as regulators of what enters and leaves the neuron, allow the signal not to lose by force.

The action potential (nerve impulse) jumps from one nodule to another because these, unlike the rest of the neuron, are endowed with clusters of sodium and potassium channels, so that the transmission of nerve impulses is done more quickly. The interaction between the myelin sheath and Ranvier’s nodules it allows the nerve impulse to move faster, in a jumping manner (From one Ranvier nodule to another) and with less risk of error.

Where is myelin found?

There is myelin in the axons of many types of neurons, both inside and outside the central nervous system (i.e. the brain and spinal cord). However, in some areas its 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 common to speak of gray matter, but also, and although this fact is a little less known, there is the white matter. The areas in which white matter is found are those in which myelinated neural bodies are so abundant that they change the color of those areas seen with the naked eye. This is why the areas where the nuclei of neurons are concentrated are usually grayish in color, while the areas through which axons predominantly pass are white.

Two types of myelin sheaths

Myelin is basically a material that performs a function, but there are different cells that form myelin sheaths. Neurons that belong to the central nervous system have layers of myelin formed by a type of cell called oligodendrocytes, while other neurons use bodies called Schwann cells. The oligodendrocytes are shaped like a sausage crossed from end to end by a rope (the axon), while the Scwann cells surround the axons in a spiral, acquiring a cylindrical shape.

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

Diseases due to impaired myelin

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

Demyelinating diseases are characterized by a pathological process directed against healthy myelin, unlike demyelination, in which insufficient formation of myelin or an alteration of the molecular mechanisms to maintain it in its normal conditions. The different pathologies of each type of disease linked to the alteration of myelin are:

demyelinating diseases

  • Isolated clinical syndrome
  • Acute disseminated encephalomyelitis
  • Acute hemorrhagic leukoencephalitis
  • Belo’s concentric sclerosis
  • Marburg disease
  • Isolated acute myelitis
  • polyphasic diseases
  • multiple sclerosis

  • neuromyelitis optic
  • Spinal optic multiple sclerosis
  • Recurrent isolated optic neuritis
  • Chronic recurrent inflammatory optic neuropathy
  • Acute recurrent myelitis
  • Late postanoxic encephalopathy
  • Myelinotic osmotic

demyelinating diseases

  • Metachromatic leukodystrophy
  • Adrenoleukodystrophy
  • Refsum disease
  • Canavan disease
  • Alexander disease or fibrinoid leukodystrophy
  • Krabbe disease
  • Tay-Sachs disease
  • Cerebrotendinous xanthomatosis
  • Pelizaeus-Merzbacher disease
  • Orthochromic leukodystrophy
  • Leukoencephalopathy with disappearance of white matter
  • Leukoencephalopathy with neuroaxonal spheroids

To find out more about myelin and its associated pathologies

Here is an interesting video on multiple sclerosis, in which it is explained how the myelin is destroyed during this pathology:

Bibliographical references:

  • Boggs, JM (2006). “Myelin Basic Protein: A Multifunctional Protein”. Cell Mol Life Sci.
  • Swire M, Ffrench-Constant C (May 2018). “Seeing is Believing: Myelin Dynamics in the Adult CNS.” Neuron.
  • Waxman SG (October 1977). “Conduction of myelinated, unmyelinated and demyelinated fibers”. Neurology Archives.

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