What group of cells sends signals to control the heart rate?

The cells of the SA node at the top of the heart are known as the pacemaker of the heart because the rate at which these cells send out electrical signals determines the rate at which the entire heart beats (heart rate). The cells that create these rhythmic impulses, setting the pace for blood pumping, are called pacemaker cells, and they directly control the heart rate. They make up the cardiac pacemaker, that is, the natural pacemaker of the heart. Hope this helps!