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. So this is the first important difference between cardiac muscle cells and skeletal muscle cells. Now the cytoplasm of our cardiac muscle cell is known as the sarcoplasm. So what about some other differences between cardiac muscle cells and skeletal muscle cells?