And then that impulse moves within special conduction channels through the right atrium and the left atrium. And at the same time it causes these two atrium to actually contract. Now, what exactly happens next? Well, if we notice, if we take a look at the following diagram, we have the SA node and we have another type of node known as the AV node, which stands for the atrioventricular node. Now the atrio ventricular node is also a collection of specialized cardiac muscle cells that is located inside the interatrial septum of our heart. That's basically the wall that separates the two atrium of the heart.