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Refractory period in action potential

Refractory period in action potential


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I know that the part E in this graph is definitely the part of refractory period.

My question : Will there be any effect on B,C or D if a stimulus is given at time B,CorD respectively ?


If there is a stimulus at E, there will be depolarization (membrane becomes relatively more positive). It is just that no action potentials are fired. Therefore, if a strong stimulus does arrive, it will depolarize the membrane to an extent depending on its strength. It will increase the height of the succeeding B phase or reduce the dip of the succeeding E phase. If it is strong enough, it might persist above the threshold till the refractory period is over, and then an action potential can be fired.


The All or Nothing Law & Refractory Period


In order for an action potential to occur enough sodium ions must enter the cell so that the membrane potential reaches a specific threshold. This then causes the sodium gates to open. However, if not enough sodium enters, the depolarisation is not large enough and no action potential occurs. In other words, there’s no half way point in which some ion channels open and other don’t. This is the all or nothing law.

An action potential is always the same size: it always reaches +40mV. It never decreases, no matter how long the axon of a neurone is. It’s the frequency of an impulse which dictates how strong a stimulus is: the higher the frequency, the stronger the stimulus.


What is the refractory period of a nerve action potential?

The refractory period is composed of two parts. The first part is the absolute refractory period in which another action potential cannot be stimulated in the nerve. In the relative refractory period that follows an action potential can occur but a greater depolarising stimulus than usual is required. The reason for the refractory period is due to the time and voltage characteristics of the Na+ and K+ channels that govern the membrane potential. The absolute refractory period is due to the inactivation of sodium channels where sodium ions cannot enter the channel to depolarise the membrane and occurs after the initiation of action potential spike. During the relative refractory period, sodium channels return to a state where they can conduct sodium but the potassium channels are delayed in closing leading to hyperpolarisation. Therefore more depolarising stimulus is necessary in the relative refractory period.


Electrical Cell Membranes

Cell membranes are electrical. They use ions on either side of the cell – extracellular ions and intracellular ions – to create a charge that runs all the way along the cell membrane. When nothing much is happening the cell membrane is said to have a resting potential. Many products can enter the cell by diffusing through open pores or through the double phospholipid membrane. All cells have membranes with a resting potential. However, not all cells can produce an action potential.

Many particles need help to enter or leave a cell, including charged particles or ions. They require special channels that close and open. The different ways of entering and exiting a cell can be studied in the articles about passive transport and active transport all you need to know in order to understand the action potential is that this is the swapping over from a resting state to an action state through changes in electrical charge caused by changes to the (resting) internal and external concentrations of ions.

A membrane potential describes how an electrical charge is spread across the membrane. It is measured in millivolts (mV). This is most commonly measured by looking at the charge on the outside of the cell (the side where the extracellular fluid is) and comparing this with the charge on the inside of the cell (the cytosol or intracellular fluid). To keep calculations as simple as possible it is supposed that the outer side has zero mV.

Usually, the numbers of negative and positive ions inside the cell and in the surrounding fluid are similar and, therefore, neutral. However, in a region that lies extremely close to the membrane’s inner and outer surfaces a difference can be detected.

In a resting state – resting potential – the channels that allow charged particles to flow in and out of a cell are predominantly closed. There are very specific concentrations of ions close to the surfaces of the inner and outer membrane. You find more positive potassium ions (K + ) inside the cell than outside there are more positive sodium ions (N + ) outside the cell than inside. Negative charges inside the cell are mainly composed of larger proteins called anions that cannot make their way through the ion channels in the membrane. So electrical signaling is the result of the movement of positive ions.


The importance of the refractory period in impulse transmission

I'm towards the end of my revision on nerve impulse transmission and have come across two points in my class notes which I do not understand. I'll post the section below highlight in bold what I do not understand. It would be much appreciated if somebody could explain them to me thankyou.

When an action potential has just taken place at a particular point in a neuron, that point is unable to fire another action potential immediately. the time while the membrane is unable to respond to another depolarisation is called the refractory period. During this time, the sodium ion voltage gated channels are not capable of opening. The refractory period is important for two reasons:
1. the action potential can only be propagated in one direction, thus preventing it from spreading in both directions along the neuron
2. a second action potential will be separated from the first by the refractory period that sets an upper limit to the frequency of impulses along the neuron

Thanks for any help

Not what you're looking for? Try&hellip

The refractory period is the time period where the voltage gated ion channels in the neuron membrane become unresponsive.

A nerve impulse is propagated by the continuous activation of these ion channels in adjacent sections of the axon in the direction of the nerve impulse.

So imagine that at a particular point of the axon the ion channels open and sodium ion diffuses in. The sodium ions will diffuse both ways along the axon, in the same direction as the impulse and the opposite direction as well. They will depolarise the membrane sections that are nearby. However, because of the refractory period, the voltage gated ion channels that are in the opposite direction of the impulse will be unresponsive, so they wont open and allow sodium to enter. The ion channels in the direction of the impulse have not yet been activated, so they will be activated, allowing sodium ions to rush in and repeat the cycle. As such, the refractory period prevents the backward propagation of an impulse.

Similarly, if a section of the axon has already been activated, another impulse cannot be sent as it would be unresponsive.

(Original post by barrinalo)
The refractory period is the time period where the voltage gated ion channels in the neuron membrane become unresponsive.

A nerve impulse is propagated by the continuous activation of these ion channels in adjacent sections of the axon in the direction of the nerve impulse.

So imagine that at a particular point of the axon the ion channels open and sodium ion diffuses in. The sodium ions will diffuse both ways along the axon, in the same direction as the impulse and the opposite direction as well. They will depolarise the membrane sections that are nearby. However, because of the refractory period, the voltage gated ion channels that are in the opposite direction of the impulse will be unresponsive, so they wont open and allow sodium to enter. The ion channels in the direction of the impulse have not yet been activated, so they will be activated, allowing sodium ions to rush in and repeat the cycle. As such, the refractory period prevents the backward propagation of an impulse.

Similarly, if a section of the axon has already been activated, another impulse cannot be sent as it would be unresponsive.

I had to read that a few times to understand it but I understand it now! However, which part relates to 'a second action potential will be separated from the rest by the refractory period that sets an upper limit to the frequency of impulses along the neuron'?

Contents

After initiation of an action potential, the refractory period is defined two ways: The absolute refractory period coincides with nearly the entire duration of the action potential. In neurons, it is caused by the inactivation of the Na + channels that originally opened to depolarize the membrane. These channels remain inactivated until the membrane hyperpolarizes. The channels then close, de-inactivate, and regain their ability to open in response to stimulus.

The relative refractory period immediately follows the absolute. As voltage-gated potassium channels open to terminate the action potential by repolarizing the membrane, the potassium conductance of the membrane increases dramatically. K + ions moving out of the cell bring the membrane potential closer to the equilibrium potential for potassium. This causes brief hyperpolarization of the membrane, that is, the membrane potential becomes transiently more negative than the normal resting potential. Until the potassium conductance returns to the resting value, a greater stimulus will be required to reach the initiation threshold for a second depolarization. The return to the equilibrium resting potential marks the end of the relative refractory period.


What is the significance of the refractory period?

These transitory changes make it harder for the axon to produce subsequent action potentials during this interval, which is called the refractory period. Thus, the refractory period limits the number of action potentials that a given nerve cell can produce per unit time.

Also, what does the refractory period prevent? It is initiated by paced or sensed events after a sensed event, the refractory period prevents double counting the same event, whereas after a paced event, it prevents sensing the pacing stimulus, its after-potential, or the evoked response. Events within the refractory period do not reset the LRI.

Furthermore, what is the refractory period of a wave and why is it important?

After an action potential initiates, the cardiac cell is unable to initiate another action potential for some duration of time (which is slightly shorter than the "true" action potential duration). This period of time is referred to as the refractory period, which is 250ms in duration and helps to protect the heart.

What is the importance of a long refractory period in cardiac muscle?

The relaxation is essential so the heart can fill with blood for the next cycle. The refractory period is very long to prevent the possibility of tetany, a condition in which muscle remains involuntarily contracted. In the heart, tetany is not compatible with life, since it would prevent the heart from pumping blood.


Refractory period

+rep

Edit: My bad..Refractory period (physiology).. it might have been a confusion for some people :P

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It also means that the impulse can only travel in one direction, because the area behind the impulse (within the refractory area) is depolarised and so the impulse is only able to travel in the intended direction, so it will get to the intended destination. Obviously synapses also help with this but makes the impulse go in the right direction always.

I think that is all you really need to know

The refractory period is a period of time after the ion channels have been opened and then closed during which they cannot then reactivate.

If the ion channels are unable to reactivate immediately after an impulse then there must be a gap between impulses.

The length of time of this refractory period will also influence the maximum frequency that the ion channels can be activated. If the refractory period is 1 second then the maximum frequency of transmission will be 1hz, if its 0.1 seconds then 10hz etc.

Of course this also assumes that the activation of ion channels is instantaneous, which it isn't, so it isn't only the refractory period that determines the maximum hz.

I'd suggest that the best way to think about it is as a dynamic picture, with the area of neurone which has just depolarised being unable to do so again for a short while as the action potential travels along. If you have this picture in your mind, you'll see why conduction must be unidirectional. Additionally it will make sense that since this process takes a finite amount of time, it will impose a maximum limit on the number of action potentials which can occur in any given time period, i.e. frequency.

Only a very small number of ions move in one action potential. The ion pump is needed to set up the gradients in the first place, but if you poison the pump (e.g. with oubain) then the neurone can still fire many thousands of action potentials before equilibrium is reached.

(Original post by DeeWave)
^ post above seems useful

I'd suggest that the best way to think about it is as a dynamic picture, with the area of neurone which has just depolarised being unable to do so again for a short while as the action potential travels along. If you have this picture in your mind, you'll see why conduction must be unidirectional. Additionally it will make sense that since this process takes a finite amount of time, it will impose a maximum limit on the number of action potentials which can occur in any given time period, i.e. frequency.

Only a very small number of ions move in one action potential. The ion pump is needed to set up the gradients in the first place, but if you poison the pump (e.g. with oubain) then the neurone can still fire many thousands of action potentials before equilibrium is reached.


The slow recovery of sodium channels through their inactive phase is what causes the refractory period when the neurone won't re-excite - as opposed to it being to allow their recovery, caused by something else.

(Original post by DeeWave)
^ post above seems useful

I'd suggest that the best way to think about it is as a dynamic picture, with the area of neurone which has just depolarised being unable to do so again for a short while as the action potential travels along. If you have this picture in your mind, you'll see why conduction must be unidirectional. Additionally it will make sense that since this process takes a finite amount of time, it will impose a maximum limit on the number of action potentials which can occur in any given time period, i.e. frequency.

Only a very small number of ions move in one action potential. The ion pump is needed to set up the gradients in the first place, but if you poison the pump (e.g. with oubain) then the neurone can still fire many thousands of action potentials before equilibrium is reached.


The slow recovery of sodium channels through their inactive phase is what causes the refractory period when the neurone won't re-excite - as opposed to it being to allow their recovery, caused by something else.

Well explained :
But not sure about the 2nd point !" ensure impulses separated"

The sodium channel can exist in three states - either closed and ready, open, or closed and inactive. The channel has to pass through these three states in that order - it takes a certain amount of time at each stage and can't skip any stage out - this is just because of the shape of the protein.

So the channel is sitting in the membrane closed and ready. An action potential comes along and the change in membrane voltage causes the channel to open, as it is voltage-dependant. The channel then moves into the open state and briefly pauses. It is as though you've triggered the process into action. Once the channel has been opened by the change in voltage, it will always behave in a certain way over then next couple of milliseconds.

So the channel is open letting sodium ions pass through into the cell, but time ticks away. After a short pause in the open state the channel moves into a closed and inactive state. It stops letting ions pass through, so it's closed, but unlike in the beginning, it won't open to a change in voltage. It's as though there's a plug or stopper in the channel to prevent stuff passing through whatever the channel is trying to do. During this period then, if another action potential was to come along, the channel would not open, regardless of the voltage change. Nothing would pass through. So the action potential would not be transmitted.

But time is still ticking away, and after a short pause in that inactive state the channel will, if the membrane has repolarised to resting potential, change back into the original state, closed but ready, so that when another action potential arrives the whole process can start again.

So that explains why after one action potential another cannot pass for a short period of time. This is the refractory period.

Do let me know if I can clear anything up

HTH

(Original post by DeeWave)
Yeah sure .

The sodium channel can exist in three states - either closed and ready, open, or closed and inactive. The channel has to pass through these three states in that order - it takes a certain amount of time at each stage and can't skip any stage out - this is just because of the shape of the protein.

So the channel is sitting in the membrane closed and ready. An action potential comes along and the change in membrane voltage causes the channel to open, as it is voltage-dependant. The channel then moves into the open state and briefly pauses. It is as though you've triggered the process into action. Once the channel has been opened by the change in voltage, it will always behave in a certain way over then next couple of milliseconds.

So the channel is open letting sodium ions pass through into the cell, but time ticks away. After a short pause in the open state the channel moves into a closed and inactive state. It stops letting ions pass through, so it's closed, but unlike in the beginning, it won't open to a change in voltage. It's as though there's a plug or stopper in the channel to prevent stuff passing through whatever the channel is trying to do. During this period then, if another action potential was to come along, the channel would not open, regardless of the voltage change. Nothing would pass through. So the action potential would not be transmitted.

But time is still ticking away, and after a short pause in that inactive state the channel will, if the membrane has repolarised to resting potential, change back into the original state, closed but ready, so that when another action potential arrives the whole process can start again.

So that explains why after one action potential another cannot pass for a short period of time. This is the refractory period.

Do let me know if I can clear anything up

HTH


What happens during the absolute refractory period?

In physiology, a refractory period is a period of time during which an organ or cell is incapable of repeating a particular action, or (more precisely) the amount of time it takes for an excitable membrane to be ready for a second stimulus once it returns to its resting state following an excitation.

Secondly, what occurs during the refractory period quizlet? a brief time period after an action potential has been initiated during which an axon is either incapable of generating another action potential. The excitable plasma membrane recovers at this time and becomes ready to respond to another stimulus.

Subsequently, one may also ask, how long is the absolute refractory period?

This is the time during which another stimulus given to the neuron (no matter how strong) will not lead to a second action potential. Thus, because Na + channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials. The absolute refractory period takes about 1-2 ms.


Refractory Period

Refractory periods are a short phase in time following an action potential where another action potential cannot be generated. There are two types of refractory periods:

  1. The absolute refractory period is a period where it is completely impossible for another action potential to be activated, regardless of the size of the trigger (stimulus). This is because the sodium channels are inactivated and remain that way until hyperpolarisation occurs. In the cardiovascular mechanism, this refractory period is sometimes called effective refractory period (ERP) [1] .
  2. The relative refractory period is the period that occurs during the undershoot phase where an action potential can be activated but only if the trigger (stimulus) is large enough. This is because some of the sodium channels have been reactivated and have recovered but it is a difficult process due to the counter-acting potassium flow as some potassium ion channels are still open [2] .

A fibre first enters the absolute refractory period directly after an action potential has been fired, then the relative refractory period. The absolute refractory period of a human muscle fibre is typically between 2.2 and 4.6 ms [3] .


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