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The Myelin Sheath & Axon Repair

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

Have you ever wondered how your body repairs itself when you're hurt? What happens to your nerves in the damaged area? In this lesson, we'll be learning about the importance of myelin in axon repair in nerve cells. We'll also look at the importance of myelin for nerve signaling in general.

What Are Neurons?

Most people know the brain is the command center for the body. It controls all of our thoughts, feelings and actions. But have you ever wondered how the brain gets its message out? Neurons, or cells of the nervous system carry messages not only within the brain, but also between the brain and the body. Although neurons in the brain are small, neurons connecting the brain to the body can be very long, reaching from your brainstem all the way to your toes. The network of neurons outside your brain and spinal cord is called the peripheral nervous system. This will be the focus of our lesson today.

Peripheral nervous system
peripheral nervous system

Myelin and Neuronal Signaling

All neurons use electricity to communicate, similar to the electricity that runs through wires in your house. If you look at the wires connecting your computer or your phone to the wall, you'll notice they're coated in plastic. The plastic helps prevent the electricity from diffusing out into the air, and keeps it on track to where it needs to go.

Some neurons actually have a similar coating, called myelin. Myelin is an extension of cells called Schwann cells. These cells wrap their membrane around the neuron to make a coating impermeable to electricity. It helps keep the signal coming from the brain strong and on target to reach its destination.

Schwann cells wrap the axon in myelin to keep electrical signals in the neuron
neuron

Myelin appears white in color, similar to the plastic wrapped around your computer cables. Thus, neurons with myelin are referred to as white matter, and neurons without myelin as gray matter. You might have heard people talking about gray matter in the brain. Cells in your brain send messages over a shorter distance, so many of these neurons lack myelination.

However, without myelin in your peripheral nervous system, signals get lost and the brain can no longer communicate with the body. There are several myelin diseases that present these exact symptoms. In multiple sclerosis the body's immune system attacks the myelin, degrading it. These people have trouble controlling their motions, and sending sensory information to the brain.

Myelin and Axon Repair

As it turns out, myelin isn't just an insulator for neurons, it also is important for repair of neurons in the peripheral nervous system. Although neurons can't divide and make new neurons, some damage to the axon can be repaired. Axons might be crushed by pressure or severed in half during a deep cut. Luckily, some damage can be repaired with the help of Schwann cells. There are four main steps to the process.

1. Demyelination

When the nerve is first injured, signals from the environment cause the currently existing myelin to be degraded. Local Schwann cells and immune cells clean up the debris, picking up the old myelin and recycling it. This process is called demyelination. Although it might seem counterintuitive to destroy something after injury, the old myelin needs to be cleared so the axon can regrow.

2. Schwann Cell Proliferation

Shortly after removing the old myelin, the Schwann cells start to divide and make more of themselves, called proliferation. Although neurons can't divide, Schwann cells can. Scientists think the cell division helps in the regeneration process and provides a new population of Schwann cells to replace any damaged during the injury.

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