For the heart muscles, it is the extracellular Ca2+ that is necessary for the contraction, but for the skeletal muscles, it is intracellular Ca2+.

One of my answers on a test was the following: "In cardiac muscles, an augmentation of the intracellular Ca2+ concentration is necessary for muscle contraction".

Therefore, I'm confused. For the heart, is it extracellular or intracellular Ca2+ that determines contraction?


Perhaps your confusion will be helped by separating for you 2 processes that occur in both skeletal and cardiac muscle contraction. The first process is the electrical change (action potential) in the muscle cells membrane that triggers the contraction and the second is the contraction of the muscle cell itself.

The latter process, contraction of the muscle cell, is basically the same for both muscle types. Simplistically this requires high intracellular Ca in both skeletal and cardiac muscle. In short, when the intracellular Ca in the cell is high all muscle cells will contract and when it decreases they will relax.

The difference between the cell types is what triggers this increase in intracellular Ca when the cells are stimulated I.e. what causes the increase in intracellular Ca. In skeletal muscles the action potential moving through structures called t tubules directly causes release of Ca from the sarcoplasmic reticulum. In cardiac muscle this is different as the action potential here causes Ca to enter the cell from outside and it is this small amount of Ca coming from the extracellular space that triggers release of the far larger amounts of Ca in the cell that are needed for contraction. This is called Ca Induced Ca Release or CICR.

Schematic pictures are the best way to understand the differences in the processes in the 2 muscle types so try these 2 links (Cardiac (https://courses.lumenlearning.com/boundless-ap/chapter/cardiac-muscle-tissue/) & Skeletal (https://opentextbc.ca/anatomyandphysiology/chapter/10-3-muscle-fiber-contraction-and-relaxation/)

You can also access the Textbook of Medical Physiology by Guyton and Hall online or at a school/university library which will give you more detail.


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