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Myelin: definition, functions and characteristics

Myelin: definition, functions and characteristics

June 17, 2024

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

The myelin , for example, is part of those materials without which we could not 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 that serves to reach distant areas. However, in many cases that 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 or roll-shaped sheaths. These myelin sheaths have a very important function in our nervous system: allow the transmission of nerve impulses quickly and efficiently between the nerve cells of the brain and the spinal cord .

The role of myelin

The electrical current that passes through the neurons is the type of signal with which these nerve cells work. Myelin allows these electrical signals to propagate very rapidly through the 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 in the propagation of electrical signals.

If we removed their myelin sheaths to an axon, the electrical signals that travel through it would go much slower or could even get lost along the way. The myelin acts as an insulator, so that the current does not dissipate outside the path and goes only inside the neuron.

Ranvier's nodules

The myelin layer that covers the axon is called the myelin sheath, but it is not completely continuous along the axon, but there are discovered regions between the myelinated segments. These areas of the axon that remain in contact with the extracellular fluid are called Ranvier nodules .

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

The action potential (nervous impulse) is jumping from one node to another because these, unlike the rest of the neuron, are endowed with groups of sodium and potassium channels, so that the transmission of nerve impulses is more fast The interaction between the myelin sheath and the Ranvier nodules p allows the nervous impulse to move with greater speed, in a saltatory way (from one node of Ranvier to the next) and with less possibility of error.

Where is myelin?

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

When we describe a brain it is usual to speak of gray matter, but also, and although this fact is something less known, there is the white matter . The areas in which the white matter is found are those in which the myelinated neuronal bodies abound so much that they change the color of those areas seen with the naked eye. That is why the areas in which the nuclei of the neurons are concentrated tend to have a grayish color, 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. The neurons that belong to the Central Nervous System have layers of myelin formed by a type of cells called oligodendrocytes, while the rest of neurons use bodies called Schwann cells . The oligodendrocytes are shaped like a sausage traversed from end to end by a string (the axon), while the cells of Scwann wrap the spiral axons, acquiring a cylindrical shape.

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

Diseases due to alteration of myelin

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

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

Demyelinating diseases

  • Isolated clinical syndrome
  • Acute disseminated encephalomyelitis
  • Acute haemorrhagic leukoencephalitis
  • Concentric sclerosis of Balo
  • Marburg disease
  • Acute myelitis isolated
  • Polyphasic diseases
  • Multiple sclerosis
  • Optic neuromyelitis
  • Multiple spinal optic sclerosis
  • Recurrent isolated optic neuritis
  • Chronic recurrent inflammatory optic neuropathy
  • Recurrent acute myelitis
  • Late postanoxic encephalopathy
  • Osmotic myelolysis

Demyelinating diseases

  • Metachromatic leukodystrophy
  • Adrenoleukodystrophy
  • Refsum disease
  • Canavan disease
  • Alexander's 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 know more about myelin and its associated pathologies

Then we leave an interesting video about Multiple Sclerosis, which explains how myelin is destroyed in the course of this pathology :

2-Minute Neuroscience: Myelin (June 2024).

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