Myelin: definition, functions and characteristics

What is myelin and what is its function within the nervous system?

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

The myelinfor example, is one of those materials without which our brain could not perform its operations efficiently.

What is myelin?

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

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

The function of myelin

The electrical current that passes through neurons is the type of signal by which these nerve cells function. Myelin allows these electrical signals to propagate very rapidly through axons.The main added value that these sheaths bring to the neuron is the speed of propagation of electrical signals. In other words, the main added value that these sheaths bring to the neuron is the speed at which electrical signals propagate.

If we were to remove the myelin sheaths from an axon, the electrical signals traveling along it would be 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 pathway and goes only inside the neuron.

The nodes of Ranvier

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 there are uncovered regions. These areas of the axon that remain in contact with the extracellular fluid are called nodes of Ranvier.

The existence of the nodes of Ranvier is important, since without them the presence of myelin would be useless. In these spaces, the electric current that propagates through the neuron gains strength, since the nodes of Ranvier contain ion 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 the nodules of Ranvier allows the nerve impulse to travel faster, in a jumping manner (from one node of Ranvier's node to another). (from one node of Ranvier to the next) and with less possibility of error.

Where is myelin located?

Myelin is present in the axons of many types of neurons, both in the central nervous system (i.e., the brain and spinal cord) and outside 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 describing a brain it is usual to speak of gray matter, but there is also, although this fact is somewhat less well known, white matter. 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 are usually grayish in 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. Neurons belonging to the central nervous system have myelin sheaths formed by a type of cells called oligodendrocytes, while the rest of the neurons use bodies called Schwann cells. The oligodendrocytes are sausage-shaped, crossed from end to end by a string (the axon), while the Scwann cells wrap the axons in a spiral, acquiring a cylindrical shape.

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

Diseases due to myelin alteration

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, as opposed to dysmyelinating diseases, in which there is an inadequate formation of myelin or an impairment of the molecular mechanisms to maintain myelin in its normal conditions. The different pathologies of each type of disease related to myelin alteration are:

Demyelinating diseases

• Isolated clinical syndrome
• Disseminated acute encephalomyelitis
• Acute hemorrhagic leukoencephalitis
• Concentric Balo concentric sclerosis
• Marburg disease
• Isolated acute myelitis
• Polyphasic diseases
• Multiple sclerosis
• Neuromyelitis optica
• Optic spinal 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 disease
• Canavan disease
• Alexander's disease or fibrinoid leukodystrophy
• Krabbe disease
• Tay-Sachs disease
• Cerebrotendinous xanthomatosis
• Pelizaeus-Merzbacher disease
• Orthochromic leukodystrophy
• Leukoencephalopathy with white matter disappearance
• 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.:

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.