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The Myelin Sheath, Schwann Cells & Nodes of Ranvier

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  • 0:05 Data Transfer
  • 0:59 The Myelin Sheath
  • 1:57 Schwann Cells & Nodes…
  • 2:41 Action Potentials
  • 4:52 Multiple Sclerosis
  • 6:38 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
The myelin sheath is an essential part of our nervous system. Learn more about this neuron component, explore the nodes of Ranvier, and discover why Schwann cells are crucial for neuron survival.

Data Transfer

If you have a smartphone, you can play along with the example I'm about to give. If not, don't worry and just look at the picture below. If you've just taken a picture with your smartphone and want to transfer it to your computer, you can take out a cord and plug one end into your smartphone and the other end into your computer. The picture is nothing more than a piece of information. This information - your picture - can be transferred via the cord into the computer.

Cords transfer information from a smartphone to a computer
phone connected to computer

Likewise, your neurons transmit information from one neuron to another through a series of signals that travel along cord-like structures. You'll soon see how this all works as we explore the cells, structures, and functions that make communication between your body's cells and organs possible in the first place.

Neurons transmit information to other neurons through cord-like structures
Neuron Communication

The Myelin Sheath

The soma is the cell body of a neuron. The nerve cells in your body have a protrusion extending out of the soma that we call an axon. If your smartphone is the soma, then the axon sort of looks like the cord coming out of the smartphone. The axon transmits information via electrical impulses from the soma to another cell - in our case, the computer. In order to allow the axon to transmit information as quickly as possible, it has some special upgrades.

Myelin sheaths insulate axons and allow for faster conduction
Myelin Sheath

Just like some cords can transmit data faster than others, the same goes for axons in your body. Not all axons are created equal. The axons that can transmit information the fastest have an electrically insulating layer wrapped around the axon that increases the speed of electric conduction; we call this layer the myelin sheath.

Schwann Cells

The myelin sheath is made of a material called myelin, which is produced by special cells known as Schwann cells. Schwann cells are cells in the peripheral nervous system that form the myelin sheath around a neuron's axon.

Nodes of Ranvier

The key thing to note here is that the cells that produce myelin in order to form a myelin sheath around an axon do not cover the entire axon. Look at the image below. There are unmyelinated gaps between myelin sheaths surrounding an axon that we call nodes of Ranvier. These nodes exist for a reason that I will get into in just a little bit.

Nodes of Ranvier are unmyelinated gaps on an axon
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Action Potentials

Action potentials are electric impulses that travel down axons
Action Potential

Not all of the axons in your body contain a myelin sheath; we call these axons unmyelinated axons. Whether an axon is myelinated or not, the axon will still transmit an electrical impulse from one cell to another.

When a change in voltage across the plasma membrane of an unmyelinated portion of axon occurs, we term this an action potential. An action potential is nothing more than an electric impulse, or signal, traveling down the axon. This electric signal helps to transmit information from one neuron to another cell in the body, be it another neuron or another cell altogether.

When this same change in voltage occurs across the plasma membrane of unmyelinated portions of myelinated axons, we call it saltatory conduction. The reason we call it saltatory conduction is because a change in voltage across a plasma membrane can only occur in areas of the axon that are non-myelinated. Hence, in myelinated axons, the only areas that are non-myelinated are the nodes of Ranvier I just mentioned. Therefore, in saltatory conduction, the action potential essentially 'jumps' from one node to another as it travels down the axon, whereas in fully non-myelinated axons, it travels smoothly from start to finish all the way down the axon.

With this in mind, you'd think that unmyelinated axons are faster because they don't have to stop at every node like in myelinated axons. However, do recall that myelin is an insulating layer, which helps to actually conduct signals through the axon at a much faster speed than unmyelinated axons. Therefore, myelinated axons transmit action potentials faster than unmyelinated axons.

Multiple Sclerosis

Let's think about that last point. Myelinated axons transmit action potentials faster than unmyelinated axons. This means that unmyelinated axons are slower in the conduction of electric signals, and therefore information, than myelinated axons.

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