pacemaker potentials are fired not only by sa node, but also by the other foci. however, the other firing frequencies are slower than the one of the sa node (as seen above). normally, all the foci will end up firing at the sa node rate, not their intrinsic rate. the other foci attempt to fire at their intrinsic rate, but they are activated by the sa node before they can fire. this rapid firing causes all the foci to fire faster than their intrinsic rates, a phenomenon known as overdrive-suppression. thus, in the normal, healthy heart, only the sa node intrinsic rate is observable.