 And when we talk about contractility, we have to talk about action potential. You might remember that from basic physiology class that there are three different stages, which are polarization, depolarization, and repolarization. And starting with polarization is also known as the resting membrane potential. So there is no electrical activity at all. This is a cardiac muscle cell here. And inside we have positive charge and outside there is a negative charge. But there is no electrical activity. The plus remains within the cell. The negative remains outside of the cell and nothing is happening. No electrical activity is taking place.