So the red portion of the cell is the plasma membrane of the cardiac muscle cell and that is known as the sarcolema. And just like we have a sarcolema with T tubules in the skeletal muscle, we also have the sarcolema with the T tubules inside our cardiac muscle cell. So the T tubules are basically the deep in vaginations, these deep tunnels that begin on the plasma membrane and extend throughout the cell and they are basically responsible for allowing the action potential to propagate quickly and uniformly throughout the entire cardiac muscle cell. Now we also have our green section and that is the sarcoplasm reticulum, that is a specialized type of endoplasm reticulum inside the cardiac muscle cell that contains a high concentration of calcium that is needed for the proper muscle contraction. Now we also have a nucleus and that is shown in blue. But unlike in skeletal muscle that contains many nuclei per given cell, the cardiac muscle cell only contains a single nucleus.