Neurons generate their electric currents by moving ions across the membrane and down along the axon to propagate their action potentials. Now, the diameter of an axon will actually influence the speed of propagation of an action potential. Basically, the larger a diameter is the lower the resistance, meaning faster action potentials. So, larger diameter, faster action potential. But diameter is not the only way to speed up an action potential. In fact, our axons are almost all covered in myelin. Now, this is a fatty substance that insulates the axon and actually speeds up the action potential. And this is very similar to how wires, for example, are wrapped in plastic or rubber to insulate their electric currents and cause them to move with lower resistance, move faster. Now, the myelin sheath, as it's called, this fatty covering over axons, is generated by glial cells, but not by a single glial cell, and it's also not continuous. It has these gaps that we call nodes of Ranvier, named after the guy who discovered them, and it actually takes multiple glial cells to cover the axon of a neuron. So here you can see the glial cell of the central nervous system responsible for myelination. That's just another way of saying an axon being covered in myelin. Write it down for you. Myelination. These are oligodendrocytes and you can see one right here. This is an oligodendrocyte. It will actually myelinate multiple axons, and you can see that it is currently myelinating in this image, 1, 2, 3 different axons. And this stands in contrast to Schwann cells, which are glial cells of the peripheral nervous system responsible for myelination. However, they myelinate a single axon. And I'm just gonna jump out of the way here. You can see that we have Schwann cells along our axon. Right? And it actually takes multiple Schwann cells to myelinate the axon of this neuron. And you can also see the gap there, that is a node of Ranvier. We also have nodes of Ranvier here. This is another node of Ranvier. So this myelin helps speed up action potentials. But hopefully you're thinking and you're saying, wait, if we're covering the axon how do we have ion channels there that can exchange ions with the extracellular fluid? The answer is those ion channels are packed in at the nodes of Ranvier. And this basically leads us to the crux of how an action potential moves along the axon. Now, it's termed saltatory conduction, and basically this is just a fancy way of saying that the action potential more or less jumps between these nodes of Ranvier along the axon, and it essentially moves from one node of Ranvier to the next, and what this sort of looks like is here. So here we have our open ion channels, right. Our action potential is currently here and it's moving this way. Now, because of these open ion channels, which are, if you can see here, at a node. Right? This is a node right here. This opening in the myelin. So here we have the action potential. Right? The interior of the cell has become positive, the exterior is negative. That is our action potential and it's moving along. So literally, these sodium ions are going to diffuse down the membrane to carry this action potential. Those ions are moving. Right? Electric current is a flow of electric charges. Those ions are moving. And when they get to the next node of Ranvier, they're gonna cause depolarization that will open the ion channels there and allow those sodium ions to rush in so that the action potential can keep moving along the axon. Now what's really cool that gets back to that whole concept of inactivation of the sodium channels, is, if you think about this, there's nothing preventing the action potential from moving backward. Right? Nothing except inactivated sodium channels. You see, where the action potential has just been, there are going to be inactivated sodium channels. Where the action potential is headed, those sodium channels are only closed, they're not inactivated. Meaning that even though as these sodium ions rush in at the location where the action potential currently is in the axon, and because there's nothing preventing them from diffusing in either direction, even though we want the action potential moving this way, there's nothing preventing them from diffusing in either direction in the axon. However, even if they get over here, these channels are inactivated, meaning these sodium ions causing depolarization aren't going to do anything. The action potential can only move this way because only closed sodium channels will open when stimulated by a depolarization by those sodium ions moving along the axon. So again, this results in saltatory conduction, which is basically just the propagation of the action potential along myelinated axons where it hops from one node of Ranvier to the next.
So with that, let's go ahead and flip the page.