Myelin: definition, functions and characteristics

When we think of the cells of the human brain and the nervous system In general, we usually come to mind the image of the neurons. However, these nerve cells by themselves cannot 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 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 a prolongation called the axon that serves to reach areas far away. 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 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 thebrainand the spinal cord.

The role of myelin

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

If we were to remove its myelin sheaths from an axon, the electrical signals that travel through it would go much slower or could even be 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 this is not completely continuous along the axon, but between the myelinated segments are regions discovered. These areas of the axon that are in contact with the extracellular fluid are called Ranvier's nodules.

The existence of Ranvier's nodules is important, since without them the presence of myelin would be of no use. In these spaces, the electric current that propagates through the neuron gains strength, since in Ranvier's nodules it is find ion channels that, by acting as regulators of what enters and leaves the neuron, allow the signal to not lose strength.

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

Where is myelin found?

Myelin is found in the axons of many types of neurons, both in the Central Nervous System (that is, the brain and spinal cord) and outside of it. 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 somewhat less known, there is the white matter. The areas in which white matter is found are those in which myelinated neuronal bodies are so abundant that they change the color of those areas seen with the naked eye. That is why the areas in which the nuclei of neurons are concentrated tend to have a grayish color, while the areas through which the axons essentially pass are colored 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. Oligodendrocytes are sausage-shaped traversed end to end by a string (the axon), while Scwann cells wrap around the axons in a spiral, acquiring a cylindrical shape.

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

Diseases due to altered myelin

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

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

Demyelinating diseases

• Isolated clinical syndrome
• Acute disseminated encephalomyelitis
• Acute hemorrhagic leukoencephalitis
• Balo concentric sclerosis
• Marburg disease
• Isolated acute myelitis
• Polyphasic diseases
• Multiple sclerosis
• Optic neuromyelitis
• Spinal optic multiple sclerosis
• Recurrent isolated optic neuritis
• Chronic recurrent inflammatory optic neuropathy
• Recurrent acute myelitis
• Late postanoxic encephalopathy
• Osmotic myelinolysis

Dysmyelinating diseases

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

To learn more about myelin and its associated pathologies

Here is an interesting video about Multiple Sclerosis, in which it is explained how myelin is destroyed in the course of this pathology:

Bibliographic references:

• Boggs, J.M. (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 in myelinated, unmyelinated, and demyelinated fibers". Archives of Neurology.