Now let's zoom in. Let's take a look at the right atrium of the heart. So this is the right atrium and the upper wall of the right atrium of the heart. This section here contains a collection, a bundle of specialized cells that together are known as the sinoatrial note, or simply the SA node. And it's the ACA node. It's the cells within the essay node that are responsible for initiating an action potential by depolarizing.