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What is the Purpose of Myelin Sheath?

Vibhav Gaonkar Apr 14, 2019
Myelin sheath is a coating present on the axons of neurons. It reduces capacitance and increases the electrical resistance for speedy electrical conduction. Look at some more aspects about the purpose and function of myelin sheath in the human nervous system.

Canavan Disease

The disease is caused in infants due to the degeneration of myelin. It leads to severe symptoms like intellectual disability, loss of motor skills, paralysis, blindness, megalocephaly, etc.
Neurons are the most fundamental units of our nervous system. Every tiny action performed by us (like reading this information, or scrolling down to read more) is a result of the electrical impulses produced by several neurons. According to their function, neurons can be classified into three types―sensory neurons, motor neurons, and interneurons.
Sensory neurons receive sensory stimulation from tissues and organs, motor neurons receive the data from sensory neurons and plan or devise a certain response, interneurons connect sensory and motor neurons.
A neuron is an electrically charged cell, it transmits messages to other neurons via electrical and chemical impulses. Its long axon extends in order to communicate with other neurons, it carries the electrical impulses to the terminals and transmits them to the dendrites of the next neuron via a chemical reaction.
Myelin present on the axon functions as an insulating material as well as helps increase the speed of conduction.

Let's discuss the role of myelin sheath in detail with a neatly labeled diagram.

Functions of Myelin Sheath

The myelin sheath is responsible for performing two crucial functions; first, speeding up electric conduction of the neuron, and second, insulating the axon from electrically charged atoms and molecules.

Speeding Up Electrical Conduction

➔ A neuron receives chemical neurotransmitters from a neighboring neuron. It then converts these chemical neurotransmitters to electrical impulses, which are transmitted to the next neuron via the axon. The axon is a long tubular canal surrounded by positively charged extracellular fluid. The axon's insides also contain a fluid which is negatively charged.
➔ An impulse traveling down the axon is so strong that it reverses the polarity of every section it passes through. The reversal of polarity is done by allowing the external ions (positively charged) in the cell, and ejecting the internal ions (negatively charged) outside the cell; this in technical terms is called depolarization.
This short-lasting event wherein several parts of the cell rapidly rise and fall in terms of electric charge, is called an action potential.
➔ So how does myelin help? Here's how. Myelin covers the entire axons in patches (called sheaths), and not completely.
Two myelin sheaths sandwich a portion of the axon called the 'Node of Ranvier', these nodes favor depolarization, thus facilitating the action potentials to travel or propagate to the terminals. To make it simpler, consider these nodes as gates. These gates exchange charges during an action potential.
➔ When an impulse passes through the axon, it hops over the sheaths to the next inversely charged node, and then to the next. Likewise, the impulse also travels internally hopping directly to the next inversely charged node.
Take a look at the three sequence of images given earlier, in the first image, the impulse depolarizes the axon membrane beginning from one node to the next; this includes the insides of the axon as well as the outsides. It then depolarizes the next membrane and travels across it to the next one (second image) and so on (third image).
In this manner, the impulse covers the entire axon in a fraction of a second by hopping internally as well as externally as it depolarizes every subsequent membrane of the axon.
➔ In case the axon was a straight tube without myelin, the impulse would have to depolarize every set of ions without any scope to hop, thus making the process slow. This is what happens in diseases like Multiple Sclerosis, Neuromyelitis Optica, Transverse Myelitis, and other neurodegenerative autoimmune diseases.

Insulating the Axon

➔ Practically, both functions of myelin sheath―insulation and speedy conduction―are interlinked.
As seen in the illustration, where no myelin is present, depolarization takes place at every portion of the axon; this happens because the entire axon is exposed to the positively charged extracellular fluid. This ultimately leads to the reduction in conduction capability, thus leading to inefficient sensation and cognition.
➔ In diseases where myelin gets stripped off neurons, it is very well possible that an impulse can be wrongly transmitted to the axon of another demyelinated neuron. This can cause faulty sensation as well as cognition, thus leading to incorrect responses.
➔ Glial cell is a type of cell responsible for maintenance of neurons. Maintenance includes everything from supplying oxygen, insulation, holding neurons in place, and discarding dead neurons. Demyelination in some cases destroys these cells, especially the ones producing myelin (oligodendrocytes).
➔ The only way to restore efficient functioning of neurons is to reconstruct myelin, which is possible only if oligodendrocyte precursor cells exist. The reconstruction of myelin is termed as remyelination.
A lot of research has been conducted over these cells and why they fail to construct myelin in case of neurodegenerative diseases. However, the method of treatment to promote myelin regeneration is still a black hole in neurology.
To put myelin sheath in a nutshell, we can say that it is vital for efficient and healthy functioning of neurons, and ultimately the nervous system. The absence of myelin can cause severe, and in some cases, even fatal diseases.