 Hey everybody, Dr. O here, and this video I want to talk about the two types of refractory periods, but mainly so I can explain why they matter. So here you see the entire process of an action potential. You start with a resting potential of negative 70 millivolts. You rapidly depolarize the some amount of a neuron to positive 30 millivolts as you turn it on, quote, unquote. Then you repolarize it from positive 30 back to negative 70, but there is some extra potassium does rush out of the cell leading to hyperpolarization. And that's where we're at. So there's a period when an action potential is already in progress. Another one cannot be initiated. This is called the refractory period, and it's because of this hyperpolarized state in the lower right hand corner. So there are two types, the absolute refractory period and the relative refractory period. So at the absolute refractory period, another action potential could not be started no matter what, no matter how much stimulus you were to add. Think about like you can't, you can't fire a gun while it's already firing. The sodium channels literally would not work right now. So there's no way that you can get an action potential, and that's great. That's called the absolute refractory period. The relative refractory period is the period right after that, you see the yellow line is starting to go back up. An action potential could be triggered sooner than normal, but it would take a stronger stimulus than normal to do so. So in the absolute refractory period, you could not cause another action potential no matter what. In the relative refractory period you can, but it takes a stronger stimulus than normal. So for some reason you needed this, this neuron to fire more quickly than usual. You could during that relative refractory period. So why does this matter? Very, very important in an image. Action potentials are propagated or sent down the axon one section, one segment at a time. And you can see what's happening here. So right at the top section there in response to it, I'm just going to read it in response to a signal, the soma or cell body end of the axon becomes depolarized. So it's going to start there at the cell body, what's called the axon hillock. And that first section is going to be depolarized by sodium spreading in. But very important to notice, look at the arrows are going in both directions. Sodium doesn't care where it goes. It's diffusing from an area of high to low concentration as it travels through these sodium channels. So that first section of this neuron has been depolarized. And then now it's going to be repolarized and hyperpolarized as the depolarization spreads. So let me read this. The depolarization spreads down the axon. Meanwhile, the first part of the membrane repolarizes. Because sodium channels are inactivated, and additional potassium channels have opened, the membrane cannot depolarize again, which is great. So you see that while that second section of the neuron is depolarizing, trying to send sodium or sending sodium in both directions, it's only going to actually depolarize that one section. And it's not going to depolarize a section behind it. So why is this so important? This is the reason that nerves all send signals in only one direction. Imagine right there at step B, if the sodium that rushed into the section of neuron actually depolarized the section behind it, you'd have nerve signals that are traveling and then turn around and go the other direction. Imagine if you had a reflex, right? If you touch a step on a Lego and the sensory signal traveling towards your spinal cord to withdraw the leg, withdraw the foot from the Lego, imagine that signal turned around and became a motor command to stomp on the Lego. That's not good if you have kids, you know, that's a bad thing. So it's very important that we have this refractory period where while a section of an axon is repolarizing, it cannot be depolarized. This is the only reason that our nerves fire or send signals in only one direction. Sensory nerves always send information towards your brain and spinal cord, motor nerves are efferents. They're always sending signals away. So I think that's probably everything you need to know. But that's the reason those two types of refractory periods are important is it allows our nerves to function by only sending signals in one direction. I don't think I need to talk about it anymore, so but it's a very important concept. Make sure you understand this and link it to that last video about the action potential. I hope this helps. Have a wonderful day. Be blessed.