The sliding filament theory is the explanation for how muscles produce force (or, usually, shorten). It explains that the thick and thin filaments within the sarcomere slide past one another, shortening the entire length of the sarcomere. In order to slide past one another, the myosin heads will interact with the actin filaments and, using ATP, bend to pull past the actin.
I describe this only briefly here. It is very nicely described in the Marieb Interactive Physiology CD you have. It is also nicely shown in this cool web page and in this regular web page sent to me by my students last year.
Your textbook describes a simplified version, like the one in the link "this regular web page" above, of the sliding filament theory in Figure 9.10. Instead of learning those 4 steps to sliding filaments, learn these six as identified in the CD:
Let's consider each of these six steps:
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 actin Also obvious, because if actin and myosin do not contact one another, no sliding can occur.
3. The power stroke... Since myosin is already energized, once it grabs onto actin it immediately begins to pull. Got it?
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.
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.
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.
So the important thing is not to memorize these by number, but to pick one step as as starting point for your studying and thoughts and move through the steps from there. Good luck!
© 2011 STCC Foundation Press