Talking, eating, walking, and all the other activities we do without thinking are something we can take for granted.
The brain receives large amounts of information from our five senses (touch, taste, sound, smell, and vision). It processes these details, controls them, and orders our muscles to react to them.
How does it do this so efficiently?
Myelin – a complex membrane structure.
Below, we’ll discuss what role the myelin sheath plays in our day-to-day lives, how it works, and how it can become damaged by certain conditions, e.g., multiple sclerosis (MS).
Myelin Sheath Definition
The myelin sheath is a sleeve that’s composed of lipids and protein (a plasma membrane) that’s wrapped around fibers called axons. Located within the central nervous system, which is responsible for carrying electrical messages to and from your brain and other parts of your body, the function of the myelin sheath is to protect these fibers and enhance their performance.
What is an Axon?
Axons carry nerve signals to glands, muscles, and other nerve cells from your main neuronal body. They can vary dramatically in length (from 1 millimeter to 1 meter), and when they’re bundled together, they establish a network of nerves that create a pathway for various electrical nerve impulses to travel around your body.
That’s why myelin sheath function to insulate and protect these, so the transmission of electrical impulses is enhanced throughout the body. Essentially, it acts rather like the insulating cover you’ll find around the electrical wires in your home.
Just like these electrical wires, if myelin becomes damaged, the transmissions are slowed down – a process that’s witnessed in a number of neurological conditions, like MS.
How Does the Myelin Sheath Work?
As we’ve already seen, myelin surrounds axons and insulates them. However, each myelin-generating cell will only build myelin for a single segment of a particular axon. This creates periodic interruptions in myelin, where parts of the axon remain exposed – these are known as the nodes of Ranvier.
In a myelinated axon (one that’s covered in myelin), the sheath prevents the electrical currents from moving across the membrane of the nerve. This then directs the current downward to flow through the nerve and on toward the nodes of Ranvier, which are unmyelinated and feature high concentrations of ion channels.
When these ion channels are stimulated, they propel the action potential (the nerve impulse) to the next node. This creates a “jumping” action as the action potential is regenerated at each node – a process that’s known as saltatory conduction.
In the case of an unmyelinated axon, the action potential is propelled along the entire axon, moving continuously and fading as it travels through the membrane in a depolarized section.
So the myelin sheath
and nodes of Ranvier function to create a fast-flowing, efficient current through the nerves.
For example, in the unmyelinated fibers that conduct temperature or pain, the conduction velocity is 0.5-2.0 m/s, which is about the speed you walk or jog. In the most efficiently myelinated axons, the conduction velocity is 70-120 m/s, which is equivalent to the speed of a racing car.
What Happens When Myelin is Damaged?
The significance of how the myelin sheath functions becomes even more apparent in a variety of diseases where defective myelination is the main problem. When myelin sheaths become damaged and lost, this is known as demyelination – one of the primary causes of multiple sclerosis and other neurological diseases.
Triggered by a number of things, including metabolic and inflammatory causes, the loss of myelin causes severe nerve dysfunction because the electrical impulses are blocked or slowed down. This damages the information that’s being sent between the body and brain, or even in the brain itself.
After demyelination occurs, new myelin can re-cover the exposed axon (remyelination). However, the new myelin sheaths that are generated during this process tend to be shorter and thinner, which means they aren’t as effective.
In some cases, demyelination may be unsuccessful, which can leave axons susceptible to degeneration – sometimes, even the whole neuron is affected. Therefore, patients who suffer from demyelinating diseases have a number of neurological symptoms.
One of the most well-known demyelinating diseases is MS – an autoimmune condition. In this case, myelin sheaths are attacked by the body’s immune system, stripping the myelin from the axons they are protecting. Not only are the unprotected nerves less efficient at transmitting electrical signals, but hard scar tissue (sclerosis) forms in the areas where the nerves have been damaged.
Sclerosis causes further disruption to the function of the nerve and is often what doctors use to detect cases of MS and diagnose their severity. How severe a person’s MS is depends on whether or not the myelin has been completely or only partially removed from the nerve fibers.
The key role myelin plays in our bodies and in cases of MS is why doctors are looking for ways the myelin sheath can be regenerated. This could be through drugs or it may be through the use of stem cells that reverse the damage caused by conditions like MS.