 Hi, welcome to physiology open. Try to solve this question for this video. Which of the following statements is or are true regarding absolute refractory period of cardiac contractile cells? You have to choose all the correct statements. You can pause the video and think about the answer. Okay, first let us see the characteristics of sodium channels. We know that opening of voltage gated sodium channels is responsible for phase zero of action potential. Actually these sodium channels can be in three states, closed, open or inactivated. At resting membrane potential, the sodium channels are in closed state. Upon arrival of a stimulus, as the potential changes towards less negative, these channels start to open up. When potential reaches to a threshold value, suddenly the probability of opening of these channels increases tremendously and hence many channels open together causing lot of sodium to move from outside to inside and leading to further change in potential towards positive, that is phase zero of action potential. Now these channels are very fast to inactivate. Pay attention that here we are using the term inactivate and not close. It's because the structure of the channel protein at this state is not same as that in closed state. In this state also just like in closed state, sodium ions cannot pass through the channels. So functionally it is just as if it is closed. But structurally it is different. This is important because channels can open only from closed state and they cannot open directly from inactivated state. Okay now when the potential starts moving back to resting membrane potential, that is as repolarization occurs, these channels again change their structure such that they are now in closed state. In summary at RMP, sodium channels are closed at threshold they open and then inactivate very fast and with repolarization they again close. As potential reaches to RMP all the channels close. This property of sodium channels is what makes the cells refractory for a certain period of time when action potential occurs. Refractory means unresponsive that is cells do not respond to a new stimulus during that time. See for next action potential to happen we want channels to open and channels can only open from closed state. So during the time the channels are inactivated even if another stimulus comes these channels cannot open. So the new stimulus is basically ineffective. Now from the beginning of phase zero to almost half of phase three of action potential in cardiopontactile cells the cells are refractory because in phase zero the channels are already open they cannot open any further and after that till half of phase three they are inactivated. So this period from phase zero to half of phase three is known as absolute refractory period or effective refractory period. You may argue that even after this point the channels are inactivated you are true but some channels start closing after this point. So at this point some channels are inactivated some are closed. As the potential changes towards RMP more and more channels close and less and less are inactivated. So after this point if a stimulus comes it may stimulate the cell but a higher strength of stimulus is required because number of channels available to open at less. So from half of phase three to RMP is known as relative refractory period that is cell is unresponsive to normal stimulus but responds to a higher strength of stimulus. Now let us come back to our original question which of the following is true regarding absolute refractory period of cardiac contractile cells. See first statement is true that is the duration of absolute refractory period but second statement is incorrect cells are refractory because the sodium channels are in inactivated state not in closed state. Statement three is also wrong because cells respond to a higher strength of stimulus in relative refractory period not in absolute refractory period. So this question has actually only one answer correct even though it was asking for choose all the correct statements. Well this concept of refractory period is very important for understanding arrhythmias and the action of anti arrhythmic drugs. These are dealt in other videos which you can check out here. Thanks for watching the video and do not forget to subscribe to the channel Physiology Open.