 while during depolarization the cardiac muscle cells get stimulated by sodium flowing into the cell and then this action potential kind of goes down like a wave So sodium kind of flows in and causes that cardiac muscle contraction and but This is only the electrical event So this is the electrical event from the positive and negative charges that are Expected to cause a mechanical event and the mechanical event being the contraction here Now there are certain dysrhythmias where there is an electrical event But it's not causing the contraction and that would be PE a Also known as postless electrical activity, which is why we learn in basic CPR class that we always have to check the patient's pulse when they become unresponsive Because on the EKG monitor, we might see a waveform We might see this electrical activity But because no contraction is happening, we are not able to fill a pulse So this postless electrical activity Electricity is flowing through the heart, but it's not stimulating these muscle cells and therefore the patient Doesn't have a pulse. There's no circulation and then certain measures will have to be taken and this depolarization here on an EKG the P wave represents atrial depolarization and the QRS complex represents Ventricular depolarization So the contraction of the atria comes first Followed by the contraction of the ventricles and of course the ventricles are going to be stronger Have a stronger and longer contraction because they are much bigger and they are the ones that produce the cardiac output