And we'll see what that means and how that works in just a moment. So, once again, the essay node is the sino atrial node. It's a collection of specialized cardiac muscle cells found in the upper portion of the right atrium that generates an action potential and impulse. 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?