Question about Muscle Contraction

[ajumobim]

In the first chapter of TBR, they explained muscle contraction as Ach binds to receptor, Na+ comes in action, action potential is eventually generated, spreads across T-tubules, causes the release of Ca2+ from sarcoplasmic reticulum. But in the passage explanations, like #58, they allude to the fact that Ca2+ enters into the muscle cell. I've looked at an anatomy textbook and the steps for muscle contraction do not include entrance of Ca2+ into the muscle cell.

---

Members don't see this ad.

 

[s5260205]

Sarcoplasmic reticulum = smooth endoplasmic reticulum within myocytes, otherwise known as muscle cells. The passage is just referring broadly to the movement of calcium ions into the muscle cells without specific reference to the cellular organelle involved.

 

[sazerac]

In the first chapter of TBR, they explained muscle contraction as Ach binds to receptor, Na+ comes in action, action potential is eventually generated, spreads across T-tubules, causes the release of Ca2+ from sarcoplasmic reticulum. But in the passage explanations, like #58, they allude to the fact that Ca2+ enters into the muscle cell. I've looked at an anatomy textbook and the steps for muscle contraction do not include entrance of Ca2+ into the muscle cell.

I don't have the text of #58 in front of me, but "calcium entering the cell" could mean calcium entering the cytoplasm of the cell, from the sarcoplasmic reticulum which was an organelle in the cell.

Kind of like you can spray a can of "air freshener into a room", even though the air freshener was already in a can in the room.

 

[ajumobim]

I actually found this online:
"Unlike skeletal muscle, smooth muscle is dependent on two sources of calcium in order to initiate contraction. These two sources are:

calcium sequestered in the S.R. of the smooth muscle cell
extracellular calcium that can enter the smooth muscle cell via calcium channels on the membrane of the smooth muscle cell."

I don't know at which point calcium enters the cell, but at least I found out for sure that extracellular calcium concentration plays a role in contraction.

Thanks to the both of you.