in this video we're going to examine the details of what happens during the power stroke and what causes the cessation and the commencement of the power stroke so if we just focus on one of the myosin heads in the thick filaments at rest that myosin has hydrolyzed atp and has bound both the adp and the phosphate from that original atp it has hydrolyzed we're going to call this the first conformation or often times the textbook will call this the cocked position of myosin so let me actually bring you down here there's a much larger version of this so at rest we are in position number one the cox position that myosin is ready to go through a conformational change but it cannot because it is blocked from interacting with actin because of tropomyosin okay so that's happening at rest and as soon as that as soon as the calcium is released into the cytosol again through the events of excitation contraction coupling that calcium signal frees up the binding site on actin for myosin and as soon as that happens the myosin goes into a second conformation it shifts its head forward so shifts the myosin head forward as the phosphate is released now remember the phosphate the inorganic phosphate has a negative two charge so in essence i'm altering the charge of the protein and that's going to cause a conformational change in the protein and as soon as the myosin head shifts forward we then go into the third conformation and this third confirmation we call the rigor state and this is achieved because adp is also released and during this rigor state this is when you have the strongest crossbridge remember the strongest interaction between actin and myosin now we don't stay in the rigor state very long because myosin is an atpase and atp is all over the place we then shift over to confirmation number four the binding of atp to the atp ace domain or the atp binding region of myosin so this reduces the intensity of the interaction between actin and myosin and then because this binding is not is very short-lived we have the hydrolysis of atp and the potential energy that was found in the chemical bonds of atp now is locked in to this cocked position now this is one power stroke cycle if calcium remains in the cytosol which it usually does you don't just go through one power stroke and that's it you're gonna go through the power stroke numerous times as long as the calcium signal is present as long as that tropomyosin has been shifted over however once the calcium pump kicks in and the calcium levels in the cytosol fall below a certain threshold eventually you grind to a halt and you do not pass the cox position back to the second position uh to to continue the power stroke so this is what happens sort of at relaxation now although we show one myosin head kind of bobbing forward and bobbing backwards all the while the thin filaments are being moved closer to the m line so the overall length of the sarcomere decreases because the thin filaments being anchored to the zetas are in essence making the sarcomere smaller so the more times we go through the power stroke the closer we are to the m line the more likely more actins and myosins are interacting to form these strong cross bridges so we can develop tension ultimately in that muscle cell so one circle of this power stroke is using a single atp and since you have hundreds of thousands of myosins this is a very energy intensive process now the term rigor state you might think is synonymous with rigor mortis now there is a similarity in rigor mortis however you are no longer making atp well because you're no longer making atp the transient nature of the rigor state is no longer so you're in essence staying in a rigor state for a very long time and this could be anywhere from 24 to 48 hours depending on temperature and humidity and decomposition of the body once that threshold has been surpassed that 24 to 48 hours bacteria have started to break your proteins down and thus you basically get out of rigor so the power stroke was initiated by a single electrical event so that single electrical event you know the whole epp to action potential this is fairly rapid this is happening in about two milliseconds as large as the skeletal muscle cell is that's pretty fast the mechanical event however is relying a lot upon diffusion the diffusion of calcium in the cytosol the diffusion uh to bind to troponin the pumping of calcium out of the cytosol back into the sr so that part takes much longer okay so cumulatively the electrical and the mechanical is known as the muscle twitch and the muscle twitch is going to be different for skeletal and cardiac and smooth but for skeletal it's about 100 milliseconds so that single electrical event is causing a mechanical event of roughly 100 milliseconds so in the next video we're going to look at the dynamics of the muscle twitch and further dive into other aspects of skeletal muscle physiology