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Sliding filament theory

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During contraction, the myosin doesn't actually get shorter. It is the actin that slides over to decrease the overall length of the sarcomere.


This is a really good diagram for understanding the basic principle of actin (thin filaments) sliding over the myosin (thick filaments) to shorten the sarcomere and achieve contraction. If you understand this, you won't be shocked by the inevitable trippy questions that examiners will hurl at you in the exam about fancy H/I/A bands/lines/zones. These randomly named lines/zones/bits/pieces are totally arbitrary. If you must know, the A band and I band referring to the length covered by myosin and actin respectively were established as a result of how they appear under a microscope.





Another diagram for the visually-inclined.

Let's go back to this for a second:




What happens to the I band during contraction? It gets shorter. What about the A band? It stays the same.

Notice the wiggly heads on the myosin above. They look like branches, but they're wiggly heads trust me. The wiggly heads are actually what enables the myosin to pull along the actin during contraction. Bridges are formed following the ATP breakdown reaction:






The crossbridges formed are called actinomyiosin bridges. It's important that when muscles are relaxed no power strokes occur by the myosin. Otherwise they wouldn't be that relaxed, would they?




Drum roll please for our latest protein addition: tropomyosin. "Tropo" means turn, so I guess you can think of tropomyosin as a turn of myosin in the sense that it either enables it to bind actin or it turns it away... Good one? Shall grab my coat...

Calcium ions are very important in this process as it is their initial binding (to troponin) that results in tropomyosin moving so that the binding sites on actin are exposed.

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