Within the myofibrils, this is what's happening:
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a. Without calcium ions (in relaxed state). Because tropomyosin completely covers the sites (indicated in blue) of actin binding to myosin, myosin cannot bind to actin.
b. When calcium ions released from excited muscle cells bind to troponin, the binding sites of actin are exposed owing to the shift of tropomyosin; however, the extent of the shift is not sufficient for myosin to bind to actin.
c. When small amounts of myosin heads bind to the binding sites of actin under state b, tropomyosin shifts further, resulting in the complete exposure of the binding sites of actin.
d. Because the binding sites of actin are completely exposed, many myosin heads can bind to actin, which enables the generation of a large muscle contraction power.
I love that it is showing the myosin heads (green globs) all the way around the actin. Most of the time we look at it as a very 2-dimensional thing, but when you look at the picture below, you can see that it is a very 3-dimensional process, with myosin heads able to bind all the way around. Each of the small dots represent actin, the bigger ones are myosin (seen on the ends of the myofibrils).
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Keep in mind, myofibrils are organelles within a muscle cell. So the sarcoplasmic reticulum is the muscle version of ER, wrapped around each myofibril. Stuff is diagrammed 2D to make it simpler and more understandable. Once you understand it, it's cool to look back and imagine the bigger picture.
Videos!
Love to weed through the options and find a few good ones for you, this time I've included a song, rap, and skit!
Very short and sweet, this is just a good little animation to show how the filaments slide past each other, with the myosin heads moving along at different times.
One recommended by a student. It says it's for invertebrates, but looks like it's accurate for us as well.
Hahaha, this is a fun one. A song/rap explaining the whole process. I probably like it because the background music is Daft Punk (they did the music for Tron Legacy). Anyway! Maybe it's helpful for someone, and it's kinda fun to listen to.
This skit is a great idea. It's hard to read their signs some times, but this was a fun little video and it might help a couple things click for you like it did for me. I want to get a bunch of people to act something like this out some day! :P (Don't worry, no plans to make my study sessions do this...)
The singing on these videos often leaves much to be desired! But the info in this one is pretty good...
Here's Dr. Ashworth's explanation she gave permission to share. Thanks Jessie for recording and sending it.
Enjoy. :)
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