So if we take a look at the heart, the heart appears to be striped, it contains these triations shown in red and that's because if we zoom in onto an individual cardiac muscle cell, we contain these myofibrils that consist of sarcomeres. So basically if we take individual sarcomeres and we connect them end to end, we basically form something called a myofibril and we have many of these cylindrically shaped myofibrils in any given cardiac muscle cell. In this case, we have 123-4567 myofibrils in this particular cardiac muscle cell. 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.