The Mighty Myelin Sheath

In this mini episode, Hillary nerds out about the anatomy of the myelin sheath. The myelin sheath is the protective, fatty coating surrounding your nerve fibers along the axon, similar to the protective covering around electrical wires. It allows electrical impulses between nerve cells to travel back and forth very, very quickly. Tune in to learn more about how the myelin sheath works, and what happens to our body when it doesn't work.

Sources
https://www.ncbi.nlm.nih.gov/books/NBK27954/

https://www.verywellhealth.com/myelin-sheath-4129059

https://www.thieme-connect.de/products/ejournals/pdf/10.4103/0976-3147.127887.pdf

https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Life-and-Death-Neuron

Transcript

Hello and welcome to The Anatomy Nerd Podcast. I am your host, Hillary, and in today’s mini episode, I will be nerding out a little bit about the myelin sheath. Some of you already know that I am a huge neuro nerd and I am fascinated by the brain and how it controls the human body. Our nervous system is amazing in its ability to process information and send signals in a fraction of a second. I could nerd out about the brain for quite some time, but for today, I am going to focus on an important part of the nervous system, the myelin sheath. 

The information I gathered for this episode are from credible sources including articles by the American Association of Neurological Surgeons, The Journal of Neurological Sciences, and more. As always, you can find the articles in the show notes if you would like more information. 

Alright! Let’s dive in! Before we talk about myelin, let’s go over what a neuron is and its structure. Neurons are cells that send information messages to each other. They have three basic parts, the cell body, an axon, and axon terminals. Within the cell body is a nucleus, which controls the cell’s activities and contains the cell’s genetic material. Cell bodies have dendrites, they look like the branches of a tree sticking out and receive messages for the cell. The axon looks like a long tail and transmits messages from the cell. And finally, the axon terminals, also known as the synaptic bouton or terminal bouton, is the most distal portion of a neuron’s axon and is critical for neural communication. Neurons communicate with each other by sending chemicals, called neurotransmitters, across a tiny space, called a synapse, between the axon terminals and dendrites of adjacent neurons. That is your basic structure and function of neurons. 

So, what is the myelin sheath? It is the protective, fatty coating surrounding all of your nerve fibers along the axon, similar to the protective covering around electrical wires. The myelin sheath allows electrical impulses between nerve cells to travel back and forth very, very quickly. Which is important for our everyday activities. For example, when we accidently touch something hot, we need our brain to send a message to our hand to pull away quickly. If it took a long time for the message to get from our brain to our hand, we would unfortunately keep our hand on the hot object for too long, and sustain a very bad burn. That is just one example of why myelin is important. When myelin gets damaged, the electrical signals are slowed, interrupted and may even stop altogether. But before we talk about that, let’s nerd out a little bit about the anatomy of myelin. 

Myelin is made of fat and a little bit of protein and is wrapped in numerous layers around many (not all, but many) of the nerves in the central nervous system, which includes your brain, spinal cord, as well as in the peripheral nervous system, which contains all the nerves outside of the central nervous system. Myelin is created by specific types of glial cells. Glial cells—also called glia or neuroglia—provide structural support for neurons. Fun fact, the word glia literally means "neural glue." The three types of glial cells are microglia, astrocytes, and oligodendrocyte lineage cells. All of them have their own functions and constitute a large portion of our brain. Then we have the nodes of ranvier, which aren’t really nodes at all. They are intermittent interruptions where small gaps of the axon are left uncovered by myelin, and are critical to the functioning of myelin, as they are often where sodium ion channels are located.

Myelin is vital to a healthy nervous system, affecting everything from movement to cognition. So, what happens when the myelin sheath and neurons are damaged? In multiple sclerosis (MS), which is the most common disease associated with myelin damage, our immune cells attack myelin—and eventually, the axons that myelin protects—in the brain and spinal cord. After so many attacks, eventually they lead to scarring. When myelin is scarred, nerve impulses cannot be properly transmitted; they either travel too slowly or not at all. And then eventually, axons degenerate as a result of the chronic myelin loss, leading to nerve cell death. 

We often use the term demyelination to describe the destruction of the myelin sheath. In MS, depending on where in the central nervous system myelin is attacked, symptoms like sensory disturbances, vision problems, muscle spasms, and bladder problems begin to manifest. This is why the symptoms of MS can vary from one person to another, as the location of myelin attacks varies within the central nervous system.

In addition to the variable sites of immune system attacks in your brain and spinal cord, the timing of these attacks is also unpredictable, though there are potential triggers like stress.

Other than multiple sclerosis, damage to myelin can be caused by any number of common and uncommon conditions. These include, Stroke, Infections, Inflammation, Metabolic disorders, Immune disorders, Excessive alcohol use, Carbon monoxide poisoning, and Vitamin B12 deficiency. 

The most common demyelinating disease of the central nervous system is multiple sclerosis, but others include: optic neuritis, which is inflammation in the optic nerves of the eyes. Transverse myelitis, which is an autoimmune disorder that causes inflammation in the spinal cord. And adrenoleukodystrophy and adrenomyeloneuropathy, which are rare genetic disorders. The causes of these conditions are unknown. Some are believed to be autoimmune, but we still don’t have a definitive answer for all of the diseases. 

There are also demyelinating conditions that mainly affect myelin in the peripheral nervous system, including: Guillian-Barre syndrome, chronic inflammatory demyelinating polyneuropathy, and other peripheral nerve polyneuropathies. And last but not least, there are also rare genetic disorders in which a breakdown of myelin or a defective myelin sheath can cause permanent neurological damage.

Current therapies for multiple sclerosis target your immune system. While they have been found to decrease the number and severity of MS relapses, there's still no cure for MS. 
However, experts are examining therapies that target myelin, and hopefully their findings can help cure most if not all of the demyelinating diseases. 

That is all I have for you on myelin sheath. I hope you enjoyed the episode. Thank you for tuning in. Until next time, hasta luego.