 Hi and welcome to Physiology Open. Have a look at this question. Which of the following statements is false regarding ventricular action potential? You can pause the video, think about the answer. We will come back to the question at the end of the video again. Now let's talk about ventricular action potential. In this figure, left side is showing action potential of pacemaker cells, while right side is showing action potential of contractile cells. You see, the resting membrane potential of pacemaker cells is not stable. See, it is not a straight line. Rather, it slowly drifts towards threshold automatically. On the contrary, in contractile cells, this phase 4 is seen as a straight line at minus 90 millivolts. This is resting membrane potential of contractile cells and it is stable. Only when the electrical activity from pacemaker cells travels via gap junctions to the contractile cells, the action potential is generated in the contractile cells. In this video, we will see in detail the ventricular action potential. First, let us see the phases of ventricular action potential. The stable resting potential is phase 4. This phase of rapid depolarization is phase 0. This is phase of early rapid repolarization, also known as phase 1. This phase is plateau or phase 2, where there is hardly any change in potential. That is why you are getting kind of a straight line here. And then there is phase of delayed rapid repolarization or phase 3. Total duration of ventricular action potential is approximately 250 milliseconds. Now let us see how these changes in potential occur in ventricular cells. When the electrical activity from pacemaker cells reaches to the contractile cells via the gap junctions, some ions basically enter into these cells via gap junctions. This changes the potential of ventricular cells to minus 70 millivolts. Now this is the threshold. At this voltage, a lot of voltage-gated sodium channels open suddenly. Now since electrochemical querying for sodium is from outside to inside, a lot of sodium ions enter into the cells from outside. This causes a very rapid change in potential towards positive from minus 70 millivolts to around plus 20 to plus 30 millivolts. These sodium channels which are responsible for this phase are very fast to inactivate also. So as the potential changes to plus 20 millivolts, this sodium channels become inactivated. Due to this, sodium stops entering into the cells. And also by this time, voltage-gated potassium channels open, through which potassium ions start moving out. So as they move out, membrane potential starts decreasing. This leads to early rapid repolarization. Now action potential of contractile cells has a phase known as platuphase, in which there is hardly any change in potential. What happens is that by this time, voltage-gated potassium channels also open due to which calcium starts entering into the cells. Now see at this point, potassium channels as well as calcium channels are open, potassium is moving out due to which membrane potential tends to move towards negative. While calcium ions are moving in due to which membrane potential tends to move towards positive. So basically both effects cancel each other and the membrane potential remains constant. So that's why we get a plateau. Then these calcium channels close while potassium channels remain open. These potassium channels are kind of slow channels. They remain open for much longer time. Anyway, since calcium channel close while potassium channels remain open, calcium stops entering into the cells while potassium keeps moving out of the cells. This causes membrane potential to come back to its original resting membrane value. This phases delayed rapid repolarization. So in short, potassium efflux continues throughout phase one, two and three, but is accompanied by calcium influx in phase two. In summary, phase zero is due to entry of sodium, phase one due to exit of potassium, phase two due to balance between entry of calcium and exit of potassium, phase three due to exit of potassium and phase four is resting membrane potential. Now let's come back to our original question. So question was which of the following statements is false regarding ventricular action potential? Yeah, point one is correct. Total duration of ventricular action potential is approximately 250 milliseconds. This occurs because of the plateau phase. Now this is in contrast to nerve action potential, which is only about one to two milliseconds in duration and skeletal muscle action potential, which is around three to five milliseconds in duration. Point two is also correct as we have already seen in the video. Now for point three, we have to think a little bit. See potassium channels are blocked, potassium will not be able to go out of the cells and since going out of potassium is responsible for repolarization, blocking the channels will slow the repolarization. So it will prolong the duration of action potential, not decrease the duration of action potential. This concept we have discussed further in the video on anti-arhythmic drugs, which you can check out here. Thank you for watching the video. Don't forget to subscribe to the channel Physiology Open. Bye.