The Refractory Period: Study with Video Lessons, Practice Problems & Examples

The refractory period during an action potential consists of two phases: the absolute refractory period and the relative refractory period. The absolute refractory period prevents any additional action potentials, ensuring distinct signaling and unidirectional propagation along the axon. It lasts approximately 0.4 to 2 milliseconds. Following this, the relative refractory period allows for a second action potential only if a stronger stimulus is applied, lasting about 5 to 15 milliseconds. This mechanism prevents overexcitation and maintains neuronal health, crucial for effective communication within the nervous system.

Refractory Period

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.

The Refractory Period Example 1

Okay. So this example asks us what factor determines the maximum frequency of action potentials that could be propagated by an axon? Explain why. So the answer here is the absolute refractory period. And that's because the absolute refractory period is basically the amount of time the neuron can't have an action potential for. Right? During that time, however long it is, the neuron cannot fire more action potentials. And so by setting that limit of when it can't fire, it sets the limit of how many it can fire. So, to give you guys kind of an illustration of that, let's say a neuron's absolute refractory period is 0.01 seconds. Okay? Which would mean that we can have one action potential every 0.01 seconds which sets our limit at 100 action potentials per second. And that limit was set by that absolute refractory period. So there you go and I will see you guys in our next video.

During the relative refractory period, a larger-than-normal depolarizing stimulus can __________.

• A. cause a membrane to reject a response to further stimulation.
• B. cause the membrane to hyperpolarize.
• C. bring the membrane to threshold and initiate a second action potential.
• D. inhibit the production of an action potential.

Here’s what students ask on this topic:

What is the absolute refractory period and why is it important?

The absolute refractory period is a phase during an action potential when no additional action potentials can be initiated, regardless of the strength of the stimulus. This period starts when voltage-gated sodium channels open and ends when they return to their resting state. It typically lasts between 0.4 to 2 milliseconds. The absolute refractory period is crucial because it ensures that each action potential is a distinct event, preventing the overlap of signals. Additionally, it ensures unidirectional propagation of the action potential along the axon, preventing it from traveling backward. This period also establishes the maximum rate of neuronal firing, determining how many action potentials a neuron can generate in a given time frame.

What is the relative refractory period and how does it differ from the absolute refractory period?

The relative refractory period follows the absolute refractory period and is a phase during which a second action potential can be initiated, but only if a stronger-than-normal stimulus is applied. This period begins when the voltage-gated sodium channels return to their resting state and some potassium channels remain open, typically during the hyperpolarization phase. It lasts about 5 to 15 milliseconds. Unlike the absolute refractory period, where no action potentials can occur, the relative refractory period allows for action potentials but requires a larger stimulus due to the hyperpolarized state of the membrane. This period helps prevent overexcitation and ensures the neuron has time to reset before firing again.

How does the refractory period ensure unidirectional propagation of action potentials?

The refractory period ensures unidirectional propagation of action potentials by temporarily rendering the membrane unresponsive to additional stimuli immediately after an action potential has passed. During the absolute refractory period, the voltage-gated sodium channels are either open or inactivated, preventing any new action potentials from being initiated. This ensures that the action potential can only move forward along the axon and not backward. The relative refractory period, which follows, requires a stronger stimulus to initiate another action potential, further ensuring that the signal continues to propagate in one direction. This mechanism is essential for the orderly transmission of signals in the nervous system.

Why is a stronger stimulus required during the relative refractory period?

A stronger stimulus is required during the relative refractory period because the membrane is in a hyperpolarized state, meaning it is more negatively charged than the resting potential. During this phase, some potassium channels remain open, causing the membrane potential to be lower than the usual resting potential of -70 mV. To reach the threshold for initiating another action potential, a larger depolarization is needed. For example, if the membrane potential is at -80 mV, a stimulus must depolarize the membrane by 25 mV to reach the threshold of -55 mV. This requires a stronger stimulus compared to the normal resting state, ensuring that only significant signals can trigger another action potential during this period.

What role does the refractory period play in preventing overexcitation of neurons?

The refractory period plays a crucial role in preventing overexcitation of neurons by limiting the frequency at which action potentials can be generated. During the absolute refractory period, no new action potentials can occur, ensuring that each action potential is a distinct event. The subsequent relative refractory period requires a stronger stimulus for another action potential to occur, providing a built-in mechanism to prevent the neuron from firing too rapidly. This period allows the neuron to reset and recover, maintaining neuronal health and preventing the detrimental effects of continuous, rapid firing. This regulation is essential for the proper functioning of the nervous system and overall neuronal health.