So this video is going to be about the refractory period. Now the refractory period is a time that takes place during our action potential. And during this time our cell either can't respond to a stimulus or it's going to require a stronger stimulus in order to evoke a response. So the refractory period has 2 parts or 2 phases and they come one after the other. So the first part is the absolute refractory period and this it's called that because during this time absolutely no additional action potentials can be evoked. So I always think of it like our neuron is focusing on this action potential. It wants to give that all of its attention and it's not going to accept any other action potentials coming in at this time. So the absolute refractory period basically starts when our voltage-gated sodium channels open and it's going to end around the time those channels return to their resting state. So not just closed but an actual resting state. So if we scooch down here to our figure, you can see this peachy orange color is our absolute refractory period and it's going to start, like I said, around the time those voltage-gated channels are opening and it's going to end around the time we get back to resting potential, around negative 70 millivolts. So this period is usually pretty quick as action potentials are. Right? So they're usually from between 0.4 milliseconds to about 2 milliseconds long. Just depends on the neuron. And despite how short they are, they serve some very important functions. So the first is that they ensure that every action potential is a distinct event. So if you think about how much electricity and how many signals and messages are getting sent in our brain at any given time, it's a lot. Right? And it would be very dangerous for those signals to get jumbled together. So this refractory period makes sure, like I said, that our neuron is focusing entirely on this action potential, on sending this one signal in a nice clean way and nothing else can interfere with it, jumble up with it, anything like that. So it's going to ensure that each action potential is a distinct clean event. The refractory period also serves the purpose of ensuring unidirectional propagation down the axon. And you guys may remember me saying that action potentials can't go backwards and it's because of the refractory period. So this basically puts the membrane in a state where the depolarization can't spread backwards. It can only spread forwards. So this will make sure that our action potential can only travel from the initial segment toward the axon terminals down the axon. It can never go backward toward the soma. Now the absolute refractory period also serves the very important task of establishing the maximum rate of neuronal firing. So by basically establishing the period of time in which our neuron can't fire, it inherently establishes how many action potentials it can fire in a given time. So for example, the number of action potentials our neuron can send in one second is determined by the absolute refractory period. Now immediately following the absolute refractory period is our relative refractory period, and this is a time where only a larger than normal stimulus is going to evoke an action potential. So the big difference between these two periods is that during our absolute refractory period, no more action potentials can take place no matter what. And during our relative refractory period, we can have a second action potential but only if the stimulus coming in is very very strong and I'll explain why in just a second. Now the relative refractory period is going to begin right after the absolute refractory period. So during this time all of our voltage-gated sodium channels are in their resting state and usually some potassium channels are still open. So this is basically our hyperpolarization phase. That's how I always think of it. So you can see we're in the relative refractory period throughout this entire hyperpolarization and it's going to end around the time we get back to resting potential. So the relative refractory period is a little bit longer than the absolute refractory period, usually between 5 and 15 milliseconds, And it serves some very important functions as well. So just like the absolute refractory period, this is going to ensure the unidirectional propagation of our action potential, making sure it travels down the axon, and this is also going to prevent overexcitation. And overexcitation is basically our neuron firing too rapidly. It's just constantly firing. It never has a chance to rest and that's not healthy for a neuron or a person. Right? That's just not healthy. So I always think of this as the neuron kind of saying like, hey, like, I just worked hard. I just sent that action potential. I am taking my 5 millisecond break, and if you send me a super important signal, I'll send it, but if not, I'm taking my break. Right? The neurons have unionized. They want their breaks. So this basically gives our neuron time to kind of, you know, get back to resting potential reset before sending more signals. Keeps them nice and healthy and everything working well. Now just to kind of explain why we do need that larger than normal stimulus coming in, you can think about, you know, when we are having a regular action potential, what we're doing is depolarizing our membrane from negative 70 millivolts up to negative 55 millivolts. So we need a depolarization of about 25 millivolts. But if we're all the way down here in our hyperpolarization, we would need a depolarization of, you know, 30 millivolts, 40 millivolts, 45 even, and that would take a really strong stimulus. So that's why we would have to have a really large stimulus coming in in order to kind of break us out of that hyperpolarization and get us to threshold. Alright. So that is the absolute and relative refractory periods kind of in a nutshell. So as a reminder, this is a period that takes place during our action potential and during this time we either can't have a second action potential or we're only going to have one if that incoming signal is very very strong. Alright. I will see you guys in our next video. Bye bye.
Table of contents
- 1. Introduction to Anatomy & Physiology5h 40m
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11. Nervous Tissue and Nervous System
The Refractory Period
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