Myelinated and Unmyelinated Axons

00:01 Now axons, as I mentioned earlier can be myelinated or unmyelinated.

00:10 Here is a high magnification image on the left hand side of a nerve that has being treated by a fixed nerve where the myelin has been retained. And you can see that the axon is a very clear structure and it is surrounded by myelin sheaths staining black here. And on the right-hand side, you can see the myelin sheath taken by an electron microscope viewing. It is made up of little lamellae or sheaths and sheaths of myelin and on the right-hand side of that you can see an axon. And look very carefully inside the axon, you can see some little dots.

00:54 Those little dots represent myotubules. The microtubules are very important for transport of substances particularly neurotransmitters up and down the nerve axon. And that large stained structure on the right-hand side of the myelin sheath is the Schwann cell nucleus. Again if you look very carefully at the image on the left, you can see some myelinated axons, but also you can see some that are not myelinated at al. And on the right-hand side, you can see the nucleus of a Schwann cell and you can see that the Schwann cell has wrapped up axons and also you can see little tiny dots between these axons. That represents the collagen fibers of the endoneurium. So the Schwann cell does invest the axon, but it does not myelinate these axons. So in the peripheral nerve then, you have some axons that are myelinated and some that are not myelinated. And you would also noticed that axons vary in their size, their diameter and that often reflects their speed of conduction ability.

02:23 Sometimes that myelin also, in fact not sometimes, when the myelin sheath wraps around these axons, the axon is myelinated by a whole large number of Schwann cells. So when you look right down the pathway of the axon, right down the length of the axon, they have myelin sheaths wrapping all the way around them. And there's gaps between these myelin sheaths and those gaps are called the node of Ranvier. And they are very important because they allow the nerve impulse to jump from one node to another and speed up the right of impulse conduction.

03:10 It is called saltatory conduction.