intracellular vs extracellular Ca in muscle contraction

[mytoechondriac]

I'm confused about the extracellular concentration of Ca vs the intracellular concentration of Ca in different types of muscles. I thought all 3 types(smooth, skeletal and cardiac) have an initially high extracellular Ca concentration BEFORE the action potential, and when the AP comes, it depolarizes the muscle cell and allows intracellular Ca concentration to increase. Anyway, here is the official question:

Which of the following ions is invovled in initiating cardiac action potential but not skeletal muscle action potential?

A)high extracellular [Na]
B)high intracellular [K]
C)high extracellular [Cl]
D)high extracellular [Ca]

The answer is supposed to be D, but I don't know why. Is it because the skeletal muscle gets its Ca from sarcoplasmic reticulum, which is INSIDE the skeletal muscle? Can someone clarify this for me? Thanks!!

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[engineeredout]

The answer is supposed to be D, but I don't know why. Is it because the skeletal muscle gets its Ca from sarcoplasmic reticulum, which is INSIDE the skeletal muscle? Can someone clarify this for me? Thanks!!

You got it. Skeletal muscle has its Ca in the sarcoplasmic reticulum, which is not extracellular.

 

[mytoechondriac]

You got it. Skeletal muscle has its Ca in the sarcoplasmic reticulum, which is not extracellular.

Thanks, engineered!!

 

[chreesteene]

Ca2+ in skeletal muscle contraction is from the SR (released when the action potential/increase in voltage travels down the T-tubule, conformationally "activates" the dihydropyridine (DHP) receptor, which in turn opens the ryanodine receptor that is plugging up the SR. this leads to Ca2+ release from the SR).

what happens in cardiac muscle is that the voltage increase causes the DHP receptor to open, allowing an influx of Ca2+ into the cardiac myocyte, which opens the ryanodine receptor. this in turn stimulates the SR to release Ca2+ as well. so just remember that cardiac muscle requires a trigger by calcium-dependent-calcium release.