Schwann cells

In this video, I want to talk about Schwann cells. Schwann cells are glia of the peripheral nervous system derived from neural crest cells and named after a person who described them. Schwann cells come in a couple of shapes. Some are fairly shapeless cells that have little troughs on their surface. And the axons of neurons that have small diameter axons often just sit inside these troughs. So these are neurons with a soma. And I'm leaving off the dendrites. And these have small axons, small diameter axons. And they'll just kind of sit in these little troughs on the surface of these Schwann cells. And these are called nonmyelinating Schwann cells. They do provide some support to these peripheral neuron axons that are small diameter. But they don't myelinate them. So they're nonmyelinating Schwann cells. Now, peripheral neurons that have a larger axon-- and let me just draw this one with a little larger axon-- usually have a myelin sheath. And just like in the central nervous system, there will be a sheath going all the way down the axon, that's regularly interrupted by these little gaps called the nodes of Ranvier. And these little segments of myelin in the myelin sheath of peripheral neurons have the same structure and function as the myelin sheath in central neurons. However, the Schwann cells create the myelin sheath in the peripheral nervous system, as opposed to the oligodendrocytes of the central nervous system. Now while the structure and the function of the little myelin segments is the same, a big difference between the Schwann cells in the peripheral nervous system and the oligodendrocytes in the central nervous system is that a Schwann cell only produces the myelin sheath for one segment of only one axon. It's not myelinating multiple neurons, like oligodendrocytes do. Now, let's take a little closer look at one of these areas of myelin sheath. And let's cut through this segment of myelin, just like this. And we'll look at it this way, like we're looking down from the end of the axon. And this is going to look just like the myelin sheath for a central axon. Here's the axon. And we're looking at it end-on. And the cell membrane of the Schwann cell, which is this material we call myelin, that's very rich in lipid, is just going to be wrapped again and again and again, very thinly and very tightly, like a role of tape, around the axon. Now for a Schwann cell, this is almost the entirety of its cell membrane. And then it's just going to have this little lump on the outside that's going to be sort of like a soma because it's going to have a nucleus and it's going to have most of the cytoplasm of a Schwann cell. But most of its membrane is actually wrapped around the axon as the myelin sheath. In addition to these functions, Schwann cells also appear to influence neurons, and vice versa, through exchange of a variety of substances.