Not all muscles have a spark plug at the tip delivering its own signal to pulse. The sinoatrial node does this in the heart, and it is a reciprocal system like a v8 engine where one side firing begets the other side firing next. When these signals to fire go out of whack, you get fibril movement, which is when a defibrillator is deployed, it shocks the SA node forcing a 'reboot' of sorts and if all goes well, normal sinus rhythm resumes.