 When we stimulate an amphibian nerve muscle preparation with an electrical stimulus what we get is a simple muscle twitch. So here we are showing this S1 means the stimulus is given on the nerve and that is stimulus is also recorded and this is showing the response of the muscle that is simple muscle twitch. Now the simple muscle twitch as we know has two phases that is the phase of contractions so if we draw a line like this from the start of the contraction this is the contraction phase to the maximum height of the contraction then from the maximum height of contraction to the end of relaxation so this is the relaxation phase and from the point of the stimulus to the beginning of the contraction there is another period that is the latent period. So there are three periods when we record a simple muscle twitch latent period contraction period and relaxation period. Now in this video we will see what will be the effect on simple muscle twitch if we give two successive stimuli that is two stimuli one after the other. So in this picture what we are seeing two stimuli are given and you see with the first stimulus we have recorded a simple muscle twitch and then after a sufficient time has passed second stimulus is given so this is S2 that is the second stimulus and again we get a simple muscle twitch. So is there any difference between these two simple muscle twitches no they are exactly the same so in this particular picture we don't see any effect of second stimulus when the second stimulus is given after a sufficiently long time after the first stimulus. Now let us keep the second stimulus little earlier so you see in this picture what we are seeing that this is S1 right and this is a simple muscle twitch and as soon as the simple muscle twitch has ended that is a relaxation has occurred the second stimulus is given and what is the effect here what we see is that that there is a increase in the height of contraction okay so with the second stimulus given just after the end of the simple muscle twitch resulting from first stimulus what we get is that there is increase in height of contraction now suppose if we give it little bit more early so suppose the muscle is relaxing and here only we give the second stimulus then what will happen we will obtain a graph something like this right so the muscle was relaxing here but because of the second stimulus another nerve action potential has generated which has gone to the muscle causing muscle action potential which ultimately causes the muscle contraction so the muscle was relaxing but it is not able to relax due to the second stimulus there is start of new contraction right plus this new contraction now has more height so what we are seeing here in which phase the second stimulus is given it is given in the relaxation phase of the simple muscle twitch fine now let's bring the second stimulus much earlier let's give it during the contraction period itself then what is going to happen well you see here this graph is showing that just here in this part what we have shown is just a simple muscle twitch obtained by a single stimulus but here what we are seeing is that two successive stimuli are given such that the second stimulus falls in the contraction phase of the simple muscle twitch so you see this graph is similar to a simple muscle twitch however we get a increased height of contraction so that is the reason that why I've shown two graphs here because if you see it alone then you will not understand that what is the effect of the second stimulus you will think that there is only one response here correct there is one response but you see the height of contraction has more this is because when the muscle was contracting due to the first stimulus due to second stimulus we get another response from the muscle right and both the response get added up causing increase in the height of the contraction so there is summation of contraction in the muscle right so this is summation of contraction fine now if we give the second stimulus till earlier right so as soon as the first stimulus is over maybe in the latent period itself if we give the second stimulus what will be the response what we see is that there is no effect of the second stimulus right so we are getting only a single simple muscle twitch so what we saw that when it was given in the contraction period we got an increased height but in this one you see it is the same height as that of the simple muscle twitch obtained by a single stimulus so just let us quickly see what is the reason for all these effects in the first one what we saw that it was given at the end of the relaxation period second stimulus was given at the end of the relaxation period and we get increased height of contraction so basically this effect on the simple muscle twitch due to the second stimulus is due to beneficial effect caused by the first contraction so this effect increase in height of contraction which we are seeing is due to the beneficial effect and what is this beneficial effect actually what is happening that due to the stimulus and then the muscle action potential there is a release of calcium in the muscle that we are aware that calcium is important for the excitation contraction coupling and this calcium causes the contraction of the muscle and for relaxation this calcium needs to be moved out of the muscle and then the muscle will relax right so here what happens that the calcium is being released and here the calcium has started moving out of the muscle and it is going down but when the second stimulus is given all the calcium which has entered into the cytoplasm of the muscle has not gone back into the psychoplasmic reticulum okay so some extra calcium is remaining in the cytoplasm and due to which the second stimulus leads to adding up of the calcium because second stimulus is also going to release calcium and this calcium will add up to the remaining calcium and there will be more calcium in the cytoplasm okay and as we know calcium is directly linked to the strength of the contraction so increased calcium means increase in the height of the contraction so that is why what we are seeing is in this one if we see that when the second stimulus is given after sufficient duration such that all the calcium which was released with the first stimulus it has entered back into the cytoplasmic reticulum in that case we get the same height of contraction okay so beneficial effect we get due to increased calcium second reason for beneficial effect is increase in temperature you see whenever there is use of ATP there is some heat released also and this causes increase in temperature increase in temperature decreases the resistance of movement and that is why we get increase in height of contraction now same is the reason for all the increase in height which we are seeing when the stimulus is given in our relaxation period or in contraction period only thing you remember if we compare it with the stimulus given in say here then the height of contraction which we will see increase height of contraction it will be little less only maybe this much height has increased the earlier we give more is the increase in the height of the contraction can you guess why well yes because more is the increase in the calcium isn't it if we give second stimulus here then you see very less calcium has it turned back to the cytoplasmic reticulum that is why the increase in height of contraction will be much more if the second stimulus is given earlier fine so same is the case in this case also where the stimulus is given in the contraction period we are not seeing any relaxation because the muscle has not started relaxing all the calcium is in the cytoplasm only and with the second stimulus more calcium is added right so we get an increased height of contraction now let's come to the final part that why here we are not getting any effect drop the second stimulus well you see this is because the second stimulus has fallen into the refractory period now when we talk about simple muscle twitch actually this is the mechanical activity this is a very important concept you should remember I see many students getting confused in this the simple muscle twitch is mechanical activity while when we talk about refractory period we are talking about electrical activity so what we are saying is that when the first stimulus arose there will be a action potential in the nerve right and then there will be a muscle action potential so muscle action potential will be something let me redraw this muscle action potential will start here and it will be something like this and what we see is that this second muscle action potential first initially in our action potential then it travels it will generate muscle action potential it will fall in the refractory period of the first action potential right so that means actually in the muscle there will be no new action potential two stimulus are causing only one action potential and all the previous crap what happens at the second stimulus leads to generation of a new action potential and that new action potential is responsible for contraction here because S2 is falling in the refractory period we will not get a new action potential and that is why we get a single simple muscle twitch and before I end I just want to ask one question which I generally ask in these graphs that is the effect of simple muscle twitch which we are seeing here in other graphs is it violating all or none law because we are seeing with second stimulus there is increase in height of contraction so is this effect violating all or none law you can type your answers in the comment section below well thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel physiology open thank you