So if we take a cross section of our muscle cell, the green region is basically our sarcoplasmic reticulum. So this is our sarcoplasm reticulum. Once it accepts that action potential, once the action potential travels to our sarcolema, it will basically open up these special channels on the membrane that will basically allow calcium ions to flow into our cytoplasm. Remember, the cytoplasm of our muscle cell is also known as our sarcoplasm. So inside our sarcoplasm reticulum, we have a high concentration of calcium. On the outside, in the cytosol, we have a low concentration.