Know your spinal cord – Oligodendrocytes
We made it to day forty-eight! As always, the neuroanatomy category will help you find each and every post in the spinal cord series. It’s all really good stuff! We mentioned yesterday that this was coming, we’re going to do a quick breakdown of the types of glial cells just to make understanding the glial scarring post easier. Plus it helps to understand the functions of glial cells in general when we talk about things that could go wrong. With that, let’s get into oligodendrocytes!
If we are going to be talking about oligodendrocytes we need to first talk about myelination. White matter looks white because there is a fatty tissue surrounding the nerve. This tissue acts as an insulator, sort of like the covering around your cables. There are several differences, but for the most part we can think of it like a way to protect the nerve. The more myelination that occurs (the thicker the fatty covering) the faster the signal, not all nerves have the same amount of myelination (some have none). Below is a nerve showing the basic structures and how myelination forms around the nerve.
You will notice a few things, first that the myelination is not uniform, it has small gaps between bands. These gaps are called the node of Ranvier and let ions flow between the cells. You’ll also notice that myelination occurs in bands that look like the layers of an onion. You may also noticed the graphic labels these as produced by Schwann cells when we are talking oligodendrocytes. This complicates things a bit, but there is a somewhat simple answer.
Both schwann cells and oligodendrocytes produce the myelin sheath that covers and protects the axon. Both of these cells are glial cells too. The difference is that schwann cells myelinate the nerves of the peripheral nervous system while oligodendrocytes myelinate the nerves of the central nervous system (which is our focus!). So while they perform the same purpose, they are in two different locations. We should note, they also look different. Below is a split image showing the two different types of cells. Yes, the one above shows schwann cells, but I used it more to show the basic structures and to point out the difference between the two types of cells. The one below shows both, you can see the oligodendrocytes look more like an octopus and wrap several of the bands instead of just a single one.
While the image above shows the oligodendrocytes attached to a single axon, this isn’t always the case, they often wrap around several of them. Below is an image showing what that looks like.
Lastly, let’s look at how it forms around the axon, like the first image of the schwann cell, it wraps around and forms layers like an onion. As we mentioned, impulse speed of myelinated axons increase linearly with the axon diameter. Compare this to the impulse speed of unmyelinated cells, which increases only with the square root of the diameter. The insulation must be proportional to the diameter of the fiber inside. The optimal ratio of axon diameter divided by the total fiber diameter (which includes the myelin) is 0.6.
While there are a lot of other complexities that make up oligodendrocytes, since this is just an introduction I think this covers the basics enough. I think that tomorrow we are going to talk astrocytes. There is so much we could go into that it will be hard to give a good overview, but we will do our best!
Until next time, don’t stop learning!