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ADVANCED HUMAN BIOLOGY
MUSCULAR SYSTEM
Activity #3
Contraction of Muscle Cells
Sliding Filament Theory
CONTRACTION
SLIDING FILAMENT THEORY
Let's consider each of these six steps:
(Courtesy of: http://faculty.stcc.edu/AandP/AP/AP1pages/Units5to9/unit8/sliding.htm)
1. ____________________________________________________________
This should be obvious... calcium has to expose actin in order for anything to start
between actin and myosin.
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2. ____________________________________________________________
Also 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. ____________________________________________________________
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 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 effect 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. ____________________________________________________________
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 ATPbinding site, and myosin is back in its "high-energy" state.
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6. ____________________________________________________________
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.
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!
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QUESTIONS
1. What chemical – ATP or Ca2+ – triggers sliding of the muscle filaments?
2. What ions enter the muscle cell during action potential generation?
3. Which is a cross-bridge attachment more similar to: a precise rowing team or a person pulling a
bucket on a rope out of a well?