 Hello everyone, in this video we will see the ionic basis of generation of action potential. Just a quick revision before we do that for the phases of action potential. See whenever there is an excitatory stimulus of adequate strength that is a threshold stimulus, a graded change in potential occurs which reaches the threshold. This leads to a sudden depolarization phase of action potential which overshoots. Then there is repolarization phase and then hyperpolarization phase in which the membrane potential becomes more negative than that of the resting membrane potential. And then finally the potential comes back to resting membrane potential. So what is the ionic basis of generation of these phases? So see with the excitatory stimulus there is opening of few voltage gated sodium channels. These channels are present on the membranes. So suppose this is the membrane and here are the channels and when these channel open sodium ions enter from outside the cell to the inside of the cell. This happens along their concentration gradient since we know that sodium ions are more outside the cell compared to the inside. So basically this is a facilitated diffusion of sodium ions occurring. However see the cells also have leaky potassium channels through which potassium is always leaking out of the cell because potassium is more inside the cell isn't it? So what happens is a change in potential towards the positive side due to entry of these sodium ions is a bit counterbalanced by the exit of these potassium ions. So that is why the change in potential is not that rapid there is a slow change in potential towards the threshold. However if the stimulus strength is adequate enough that is it is a threshold or supra threshold stimulus and the potential change reaches the threshold. Then at that stage lot of voltage gated sodium channels start opening. This is because at the threshold the probability of opening of these voltage gated sodium channel suddenly increases and more and more voltage gated sodium channels open causing more and more entry of sodium ions. So there is almost an explosive entry of sodium ions which leads to a sudden depolarization phase see it is almost a spike. So this high probability of opening of voltage gated sodium channels leads to depolarization phase of the action potential. Now these channels operate in a positive feedback manner. We are saying that these channels open when the potential changes towards the positive side isn't it that is the depolarization which leads to more opening of voltage gated sodium channels and see because of the entry of the sodium ions which are positive ions the potential change occurs towards a positive side that is more and more depolarization which again is a stimulus for the opening of the voltage gated sodium channel. So this is a positive feedback this is also a reason that why the potential change is so sudden. Now what happens that these channels close in a time bound manner or I should say that they inactivate in a time bound manner. So from the close state they have opened due to the change in the voltage but with time and that too it's very fast they become inactivated. So as soon as they open they start getting inactivated also. So this limits the entry of the sodium ions into the cell and this is one reason why the potential change is limited to this much only to plus 20 millivolts plus along with the inactivation so I should write here inactivation of the sodium channels there is also opening of voltage gated potassium channels. So these are also voltage gated channels so the depolarization causes opening of these channels also but they are quite slow to open that's why they have not opened at the beginning of the phase because they are slow to open it takes time for them to open and they open quite late. So by this time there is opening of voltage gated potassium channels and due to this sodium entry is restricted so sodium stop entering into the cell but potassium ions start going out of the cell more. So apart from the leaky channels now potassium are also going out because of the opening of these voltage gated potassium channels so there is facilitated efflux of potassium ions occurring out of the cell. So this starts bringing back the potential towards the resting membrane potential so this is the repolarization phase caused by opening of the voltage gated potassium channels and inactivation of the sodium channels. Now we said that sodium channels operate in a positive feedback manner these potassium channels operate in a negative feedback manner see the depolarization has caused the opening of these channels and opening of potassium channels leads to efflux of potassium ions and efflux of potassium ions is causing repolarization which is opposite of the depolarization isn't it? So that means now here less and less potassium channels are opening ok so it's a negative feedback which is operating for potassium channels. Anyways what happens I told you that these potassium channels are slow to open also these potassium channels are slow to close also so there is delayed closure of potassium channels. So more potassium goes out of the cell and if this positive ion more potassium ion is going out of the cell the potential actually becomes more negative than that of the resting membrane potential so this phase is known as the hyperpolarization. So if I ask you how hyperpolarization is occurring it is because of the delayed closure of the potassium channels leading to more efflux of the potassium ions out of the cell. Now one thing we should remember here see even though depolarization and repolarization are opposite to each other when we consider the change in voltage. Repolarization is change in voltage towards the positive side and repolarization is change in voltage towards the resting membrane potential. But ionically if you see the reasons are different depolarization is due to entry of sodium ions. Sodium ions are entering into the cell and repolarization is due to efflux of potassium ions. So ionically they are two different ions which are moving in and out. So now we need that this ionic change should be normalized also. So this is brought about by sodium potassium ATPase which throws sodium out of the cell and brings back potassium into the cell. So this sodium potassium ATPase operates corrects the ionic changes which have taken place during the phases of the action potential and also brings back the hyperpolarized potential back to the resting membrane potential. So guys that is all for ionic basis of phases of action potential for somebody the depolarization phase occurs due to the entry of sodium ions, repolarization phase occurs due to the efflux of potassium ions from the cell, hyperpolarization phase occurs due to the delayed closure of potassium channels causing more efflux of potassium ions from the cell and the potential is brought back to resting membrane potential by sodium potassium ATPase. So thank you for watching the video. If you liked the video do press the like button, share the video with others and don't forget to subscribe to the channel Physiology Open. Thank you.