the crossbridge cycle so what I've drawn here for you is one filament of myosin and then in yellow is actin and at rest Acton doesn't bind to myosin and that's because troponin is in the way now you may have heard of troponin because this protein is measured in after someone suspected of having a heart attack and you might wonder why why is that well we can think about it this way if cardiac muscle is damaged then the actin and myosin start to break down a little bit and the troponin that is smaller than actin and sits on it is breaks free and fragments of it can actually be released into the blood from the damaged cells and then this protein will actually start to appear in the blood which is abnormal and there's a specific kind of troponin that is characteristic of cardiac muscle as opposed to skeletal muscle so after a patient comes in and they want to see if heart muscle damage has occurred one of the things they look for in the blood is troponin okay so that that's at rest actin is not bound to myosin because of troponin but then depolarization releases calcium from the SR remember this is the circle plasmic reticulum and then that released calcium binds to the troponin and the actin and allows so here's the big key point here calcium allows myosin to find actin so what allows myosin to bind actin calcium does calcium allows myosin to bind actin key take home idea okay now meanwhile myosin has been attached to a molecule called adenosine diphosphate or ADP and that ATP doesn't fit anymore when myosin goes to bind actin so let's look at the second step so here's the myosin again here's the actin and once calcium is available and binds to troponin then it kind of shifts the formation in such a way that myosin is able to bind to actin and as myosin binds to actin three is a crowd and it forces the ADP off ATP is ejected and this happens as myosin binds to the actin three is a crowd so ATP is ejected and I consider this sort of like a squid with jet propulsion the ATP is ejected this way which causes the myosin to do a power stroke this way so the myosin does its power stroke powerstroke moves the actin so I'm gonna put a little extra drawing down here we'll come back to that in just a second but I want to remind you of something so if you think of if you go back to the page where we looked at the sarcomeres and you think about the actin like this say so here's a z line actin and a z line with actin and then myosin is right here oops sorry myosin is right here here are the myosin heads I'm actin then after the power stroke the actin has been shifted just a little bit closer than it was before the power stroke so see how there's a space here and then we go back up here you can see then that the actin got moved a little bit and that is how the muscle could be shortened by isotonic contraction and if the muscle is stretching at the same time this could be an eccentric contraction where the power stroke helps keep the actin from ripping away of the muscle so now we have a tired out bias and head so this next picture shows myosin all spent in all worn out see oh it's all stretched out after the power stroke so it can't move the actin anymore anybody could look at this and say whoa you've got to pull the myosin back this way again before it can move the actin again right it's pushed it as far as it can but it's stuck so it's not able to pull back and recaulk the head at this point it's stuck so in this picture myosin oops myosin is stuck on actin until and then what we're looking for after that is ATP until ATP or adenosine triphosphate binds myosin so ATP is going to come in and find and then we see the next picture ATP causes myosin to fall off of actin so now three's a crowd again but now the crowd is eight Acton which gets left behind so you can sort of see what myosin prefers to bind to its it loves to find ATP and we'll leave Acton behind to bind ATP but if asked to choose between actin an ADP it's gonna choose actin every time and that generates the powerstroke so ATP causes myosin to fall off actin okay so up here we had a key idea calcium allows myosin to bind actin and then here another key idea ATP is the molecule that causes myosin to fall off actin and that's gonna be important if this is a continual cycle this is gonna happen this whole process is gonna happen thousands of times and a typical muscle contraction we've got to have a way to fall off and then not surprisingly you have to recog the myosin head move it back this way so that it can bind to actin and move the actin a little bit more so what causes the reaking is the hydrolysis of ATP to ADP that's called the hydrolysis so you can do the whole thing over again so if you were gonna have myosin and actin and then I said okay well what other ingredients do you really need you would should be able to know right away well you've got to have calcium and you've got to have ATP and sure enough calcium availability is a key ingredient for how long and how powerfully you can contract because the more calcium the more myosin heads can be involved and the more ATP the longer you can keep doing this cycle and that might make you wonder a little bit about rigor mortis so in rigor mortis shortly after someone dies all of the myosin heads are stuck on actin and they can't move because the body has finally run out of ATP and then rigor mortis doesn't last forever though right it might last a few hours spending on how warm it is and that kind of thing and then after a while though the myosin head is the protein is literally breaking apart and no longer able to stay on actin and so the body loses that rigor that it has