Now, the rate at which the SA node actually contracts varies from about 60 to 100 beats every single minute. And this is actually pretty high. So what the parasympathetic nervous system does is it uses the vagus nerve to actually decrease the rate at which the heart actually contracts. 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.