Discussions about important brain cells usually center on neurons. Electrical signals sent between neurons form our thoughts, feelings, and perceptions. What we often fail to discuss are the cells that make this process possible: neuroglial cells.
Neuroglial cells perform a variety of functions, from “cleaning up” the brain to providing nutrients to the neurons. One structure made by neuroglial cells is particularly fascinating: the myelin sheath. Without it, we would not be able to send messages throughout our brains!
Myelin sheath is composed of two different types of neuroglial cells: Schwann cells and oligodendrocytes. In the peripheral nervous system, myelin is made from Schwann cells. In the central nervous system, myelin is made from oligodendrocytes. Both types of neuroglia form a lipid-rich, multi-layered membrane structure called the myelin sheath.
Where Is Myelin Sheath Located?
Oligodendrocytes are found in specific locations throughout the central nervous system, just as Schwann cells are found in specific locations around the peripheral nervous system. They wrap around axons. Axons are thin fibers that stretch away from a neuron and end with synapses.
As electrical signals move through the axon and to the synapse, they continue their journey in the form of a neurotransmitter to another synapse and neuron. This journey, protected by myelin, is how we see what is in front of us and know how to respond when we are asked a question. It’s involved in everything we do and everything we think!
Why Is Myelin Sheath Important?
Let me put this in simple terms first. By insulating the axon, the myelin sheath allows the neurons to send messages faster and not lose their charge on their journey. We can recall memories faster, react to new sensory information faster, and brainstorm faster!
Okay, now time to make things a little more complicated.
Electrical signals are constantly moving throughout the brain, but they require certain charges to know where to go and have the ability to do so. On the outside of the axon are positively charged sodium ions, while inside the neuron is a negative charge.
When a message is “sent” and a neuron fires, it receives a burst of positive charge that is attracted to the negative charge of the axon. It moves through the negative charge until it reaches the synapse.
Without myelin, the positive charge would die out before it could reach the synapse. Think of sliding a curling stone down the ice. At some point, it stops moving. It needs sweepers to adjust the ice in such a way that the stone can keep moving and get to its destination. Myelin has a similar effect to the sweepers.
But there is actually one more element to this analogy that further explains how the structure of myelin keeps electrical signals moving toward synapses. The “gaps” in myelin sheath could be compared to another person on the curling team taking the stone and pushing it with even more momentum down the ice. With another push or two, the stone can travel much farther than it would if the sweepers were protecting it.
Why Does Myelin Sheath Have Gaps In It?
When you look at images of the myelin sheath, you might notice that myelin leaves gaps along the axon. The gaps, called Nodes of Ranvier, aid electrical signals as they move from neuron to neuron. As action potentials travel, they open sodium channels and allow the positively charged sodium ions to “recharge” the signals and keep them moving down the axon.
The blocks of myelin in between the Nodes of Ranvier are called “internodes.” Saltatory conduction is the process of an action potential hopping from internode to internode, recharging in between.
How Myelin Sheath is Formed
Glial cells in both the peripheral nervous system and central nervous system wrap themselves around an axon to form a block of myelin called an internode. When an internode is wrapped around an axon, the axon is considered to be “myelinated.”
The first instances of myelin being formed in animals happened a whopping five hundred million years ago! Myelin is not found in insects, but it is found in multiple aquatic creatures. When it comes to evolution, scientists believe that myelin was developed around the same time that animals started developing a hinged jaw. There doesn’t seem to be a correlation between the two, but animals that have myelinated nerve fibers also have a hinged jaw. Of course, the myelin sheath and its appearance in animals weren’t discovered until recently, as scientists were able to watch electrical signals move through neurons at the cellular level.
What Happens When Myelin Is Disrupted?
Myelin lasts as long as it is needed to insulate the axon, but there are cases when myelin is damaged or destroyed completely. Myelin that is disrupted or damaged shows up in the brain as sclera. Think of sclera as scars from damage, but in the brain. When this occurs, signals cannot travel from neuron to neuron. Their charge simply runs out without any insulation to keep it moving.
This can result in a variety of symptoms, from speech impairments to muscle weakness to loss of sensation. When many instances of sclera show up in the brain and lead to these symptoms, a person may be diagnosed with multiple sclerosis (MS).
Why does this happen? Why does the body destroy its own myelin? We’re not entirely sure. MS is considered an autoimmune disease, like lupus or rheumatoid arthritis. When a person has an autoimmune disease, their immune system turns on them and attacks certain tissues. Why the immune system decides to attack, disrupt, or destroy the myelin sheath is unknown to researchers.
Can Myelin Sheath Regrow and Heal?
Myelin does have the ability to regrow and heal, giving hope to people who have certain symptoms of MS. People with “relapsing-remitting MS” experience symptoms in short periods of time when the myelin is destroyed, but are able to regrow myelin throughout the brain and body. This happens when glial cells once again wrap themselves around the axon. “Progressive MS,” however, is a type of MS in which the myelin cannot grow back.
In some cases where the myelin attempts to rebuild itself, it can become “bumpy,” twisted, and end up damaging the nerve cells. Fortunately, studies involving myelin and myelin damage are giving hope to scientists who want to repair myelin more effectively. We may have a long way to go before we can help humans repair their own myelin and more properly treat MS, but we are on our way!
All of this research begins with understanding concepts like myelin sheath and how our brains send messages to our bodies. Now that you know more about myelin, you can be on your way to answering puzzling questions about the brain and how it affects our everyday life.