UWSA 1 question - contraction velocity (spoilers)

[sharklasers]

Put spoilers because I don't want someone who hasn't taken it yet come into this topic and get a question ruined for them!

Anyways, here it is:

Decreasing which of the following would increase contraction velocity:

Preload
Afterload
Actin and myosin overlap
Extracellular Ca
Passive tension


I guess I should have known that afterload was right. But I thought increasing preload would also can an increase in contraction velocity?

And doesn't an increase in preload represent a decrease in actin and myosin overlap? (with low preload, they are overlapping a lot, with high preload, they aren't overlapping nearly as much because of the pressure against the wall)

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[bobow98]

Don't over think it. If you decrease the pressure you're pushing against, you have a better contraction.

You are right in that increasing preload will increase the velocity of your contraction, but the question asks DECREASING which one will increase contraction velocity.

Hope that helps at least a little.

 

[sharklasers]

Would decreasing overlap between actin and myosin not represent an increase in preload though?

I definitely get why afterload is right. I just feel like decreasing overlap would also do it, no?

 

[pvtbacon]

Would decreasing overlap between actin and myosin not represent an increase in preload though?

I definitely get why afterload is right. I just feel like decreasing overlap would also do it, no?

True, if you look at the length tension curve, increase preload stretch on the muscles would decrease overlap only up to the point where passive tension takes over and resists that stretch. For a given afterload, it appears as though increasing the preload does increase the shortening velocity. Likewise, decreasing preload would decrease shortening velocity.