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I'm trying to understand how muscles contraction/tension works but getting loss in on the cellular level. From my understanding, when muscle tissue need to contract, the cells are flooded with calcium? Does that mean they get filled up with calcium? If so, are the cells hollow or does the calcium replace the fluid that was originally there? Or does the calcium cause the reaction in the cell for it to firm up?

I'm asking because there's an alien character who's quite thin but is also very strong and I was wondering about the science of that. I know them being alien means that their biology wouldn't necessarily follow earth rules but they could also share similar traits.

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Calcium ($\ce{Ca^{2+}}$) is a signalling molecule and it acts like a chemical switch that tells other chemicals in the cell to enact the contraction. It's not accurate to think of the cell "filling up" with $\ce{Ca^{2+}}$ since $\ce{Ca^{2+}}$ wouldn't significantly change the volume of the cell alone *. Also, during contraction, the muscle cells contract in on themselves - they don't swell out.

It's also not accurate to think of the cells as hollow. Other chemicals inside the cell (such as actin and myosin) act like machinery that enact the contraction. Instead of thinking of the cell filling up like a balloon, I think it would be better to think of it containing a chemical winch (as in the figure below) whose pulling force causes the contraction. Similarly, instead of thinking of $\ce{Ca^{2+}}$ as a fluid, think of it as a bunch of particles that cause a chemical reaction to turn on this winch when they're allowed into the cell.

enter image description here


Reference: https://www.ncbi.nlm.nih.gov/books/NBK21670/

* During a $\ce{Ca^{2+}}$ spike, intracellular $\ce{Ca^{2+}}$ rises around 10-100x from 100 nM. Assuming $\ce{Ca^{2+}}$ has a 100 pm ionic radius, the volume of the cell ($x$) is only increased by around $10^{-9}x$ to $10^{-8}x$, which is negligible.

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