So what these tubules basically do is they allow the action potential to very quickly and evenly spread throughout the entire muscle cell. So once our action potential travels into the cell, it will basically cause the activation of these channels found on the membrane of the sarcoplasmic reticulum. So remember, the sarcoplasmic reticulum is basically the specialized type of endoplasmic reticulum found inside our muscle cell. 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.