1.The influx of calcium, triggering the exposure of binding sites on actin This should be obvious... calcium has to expose actin in order for anything to start between actin and myosin.
2.The binding of myosin to actinAlso obvious, because if actin and myosin do not contact one another, no sliding can occur.
Keep in mind, however, that at the time myosin grabs onto actin, it is already "energized." You could also say that it is in its "high-energy state." This means that in order to understand how these six steps to sliding occurs, you have to always keep in mind that they start with an already-energized myosin head. OK? Really keep this in mind, or the rest won't make sense.
3.The power stroke... Since myosin is already energized, once it grabs onto actin it immediately begins to pull. Got it?
Here you have to keep in mind that there is a strong attraction between actin and myosin. They will remain connected unless pulled apart, even after the power stroke is completed.
Another thing about this step is that while the myosin head is bending in the power stroke, this conformational change in the shape of the myosin head causes the ADP + Pi that remains attached to the myosin head (in the ATP-binding site) to fall out. This falling out of the products of ATP hydrolysis gives room for more ATP to bind to myosin.
4.The binding of ATP... The myosin head has already done its work in the last step. Now it is in its "low-energy" state. It needs to use more ATP in order to get ready for another power stroke. The ATP-binding site is free, so any time there is more ATP, it will bind to the myosin head.
The binding of more ATP has an affect on the myosin head-- it causes it to undergo another conformational change. This time, the conformational change prevents the myosin head from staying attached to actin. So, suddenly, the myosin head falls off of the actin.
Note that it rigor mortis is a condition where this cycle stops before this step occurs. Rigor mortis happens because after a person dies, they stop making ATP. So no ATP is available to free the myosin from the actin. Therefore, myosin stays attached to actin and muscles become rigid.
5.The hydrolysis of ATP... Once the myosin head has ATP, it has to hydrolyze it to get its energy. So this has to be the next step. Once the ATP is hydrolyzed, its products, ADP + Pi, remain in the ATP-binding site, and myosin is back in its "high-energy" state.
6.The transport of calcium ions back into the sarcoplasmic reticulum Assuming that the signal from the nervous system for muscle to contract has ended, all the calcium will go back into the SR. Each action potential on the muscle fiber sarcolemma is extremely brief, so right after the calcium ions spill out, they have to be sucked back up again.
I hope that the word "sucked" makes you think of an active process. You see, to get all the calcium back into the SR means that the calcium has to build up to a high concentration within the SR, while leaving almost no calcium (a low concentration) in the sarcoplasm. Think back to modes of transport across a membrane, and you should notice that the movement of calcium is movement from a low concentration to a high concentration. This is NOT diffusion! It is ACTIVE TRANSPORT! So it requires protein pumps and ATP. There is a nice animation of this in the CD on page 25-- check it out! Meanwhile, if you need to remind yourself what active transport is, go back to our unit 1 web page on selective permeability.
Meanwhile, keep in mind that this is a cycle. As with any cycle, it is hard to pick a starting point. Right? To prove this notion to yourself, draw a circle and see if you can figure out where it starts! You can't! So, the six-step cycle above is a continuous process, and although we started it on step #1, you could imagine any step as the starting point... I have tried to illustrate that here for you.