 Hi, let's see what is an action potential and what are the phases of action potential. Action potential is defined as abrupt changes in the membrane potential that sweep along the cell membrane of excitable cells in response to excitation stimuli. So there are certain key terms in this definition. One is there is abrupt changes that is there is sudden change in the membrane potential. So that we'll see in the phases of action potential what is this sudden change. Then it sweeps along the cell membrane so it can travel along the cell membrane as such. So once an action potential is generated it travels as action potential along the entire cell membrane of excitable cells. So excitable cells are those which actually respond to any stimulus. So like nerve, skeletal muscles, smooth muscle basically all muscles in response to excitation stimuli. So note here that the term is excitation stimuli and not inhibition stimuli. So that again we'll see that what is the difference. So that is the definition of action potential. So this diagram is showing the phases of action potential and here x axis you see it is time in milliseconds whenever we are seeing the action potential it is very important to note the time also. So here x axis is in time in milliseconds and y axis is membrane voltage in millivolts. So now see that here the starting point this is RMP resting membrane potential which is shown as minus 70 millivolts. So there is RMP and at this point a stimulus comes. And it is an excitatory stimulus because it is causing the change in potential towards the positive you see the membrane potential is becoming less negative. So that is towards the positive side. So it is an excitatory stimuli and this is causing a graded change in potential. And once threshold is reached this red line is showing the threshold once threshold is reached action potential is fired. So basically what we are saying here is that a stimulus will lead to a graded change in potential. So what is this actually is shown in this diagram where you see that there is a certain stimulus and it is causing the graded change. Now first we'll see towards the positive side and then we'll come towards the negative side. So positive side changes in the membrane potential means that this is an excitatory stimuli and it is causing depolarization the change in potential towards positive side is known as depolarization. And you see that it is shown as a graded change that means increase in the strength of the stimulus is leading to increase in the change of the potential. But this graded change in potential is coming back towards the resting membrane potential there is no firing of the action potential until and unless it is reaching a threshold. So at this minus 55 millivolt is the threshold which is also known as the firing level and it leads to the generation of a propagated signal which is also known as the action potential as we have seen in the definition action potential sweeps along the membrane. So it is a propagated it will travel along the membrane signal. But you see here that graded change in potential can also occur towards a negative side if it is an inhibitory stimulus it will cause more and more negative change in the potential more the strength of the inhibitory signal more the change in the potential towards the negative side. So this is known as hyperpolarization the change in potential more towards the negative side compared to RMP is known as the hyperpolarization. So one thing is clear here that action potential will only occur when the change in potential is towards the positive side so threshold is towards the positive side while graded change in potential can be towards the positive side as well as towards the negative side. Now one small concept here that this thing is true for nerve but not for skeletal muscle because in a skeletal muscle whenever a skeletal muscle is stimulated by a nerve it always causes depolarization it is always an excitatory signal and never ever we see hyperpolarization. Okay let's move further so basically what we are seeing is action potential generation is basically a two-step activation. Stimulus first leads to a graded change in potential and if this graded change in potential reaches a threshold which is the firing level then there will be the generation of the action potential otherwise a change in potential will die down. Now one concept you can remember is generally this threshold is plus 15 millivolts from RMP. So you can safely assume that if RMP is minus 70 millivolts the threshold will be at minus 55 millivolts if RMP is minus 90 millivolts the threshold will be around minus 75 millivolts. So this is just a simple concept you can remember. Okay now let's move to the phases of the action potential. So this diagram same diagram which you saw earlier is shown here but with phases. So this is the site for a stimulus and there is a graded change in potential and once it reaches a threshold you see what is happening there is a abrupt change in the potential you see the slope has changed this is quite less slope and here it is almost a straight line. So this abrupt change in potential towards positive from the threshold is known as depolarization phase of the action potential. Now when this depolarization phase crosses 0 millivolt that means it has become so positive that the membrane inside now has become positive with respect to outside it's known as this part is known as overshoot. So see here that it is the membrane is becoming less and less negative with respect to outside right RMPs membrane is minus 70 millivolt negative with respect to outside. So here it is becoming less and less negative and once it crosses 0 millivolt the membrane has become positive with respect to outside so this component is known as overshoot. Then up to that the potential again starts to come back to resting membrane potential so this phase is known as repolarization phase. So remember this term that when the potential is coming back towards the resting membrane potential from the depolarized state it is known as repolarization because RMP said that it is membrane is polarized membrane is polarized. So coming back towards the polarized state is known as repolarization. Now you see this repolarization phase slope is little less than that the depolarization phase depolarization is almost like a spike. So that's why action potential is also known as spike potential or it is an impulse. So this is the reason because of the slope of the depolarization phase it is so quick. Anyways now once it reaches towards the resting membrane potential what happens that the potential does not stop there instead it becomes more negative. So this change in potential towards more negative than the resting membrane potential is known as hyperpolarization phase of action potential and after that slowly it comes back again to the resting membrane potential. So till now we focused on the change in the membrane voltage but we have not spoken about the time. Now you see the time taken for the action potential is how much it is so less right. So in neuron the duration of action potential is only around 1 to 3 milliseconds it is so less 1 to 3 milliseconds starts and finishes. However in skeletal muscle it ranges from 3 to 5 milliseconds and so whenever you are drawing this diagram please also focus your attention to the time frame in which you are drawing the phases of the action potential. Okay before finishing I think we should have a look on what is refractory period. See the term refractory means that the cell the excitable cell becomes unresponsive to a stimulus that is the English meaning of the term refractory. So there are two types of refractory period there is absolute refractory period now this absolute refractory period starts from the beginning of the action potential till about two-third of repolarization phases finish. So from here to here is absolute refractory period meaning absolute meaning that if another stimulus comes see we are telling stimulus has come here. So suppose there is another stimulus which comes here while action potential is going on there will not be any new action potential it is absolute refractory doesn't respond to any stimulus during this phase so that is absolute refractory period. Then there is relative refractory period relative refractory period is from the end of absolute refractory period to the end of the action potential. So as the term indicates refractory obviously it is unresponsive but relative refractory that means it will respond to a stimulus but not to a normal stimulus it will respond to a higher strength of stimulus during this time. So we need supra threshold stimulus for stimulating an excitable cell during this relative refractory period. So guys that's all for phases of action potential and the refractory period in this video. So thanks for watching if you like the video don't forget to subscribe to the channel Physiology Open. Thank you.