Cardiac Pacemaker
The contraction of heart (cardiac) muscle in all animals with hearts is initiated by electrical impulses. The rate at which these impulses fire, controls the heart rate. The cells that create these rhythmical impulses are called pacemaker cells, and they directly control the heart rate.
In humans, and occasionally in other animals, a mechanical device called an artificial pacemaker (or simply "pacemaker") may be used after damage to the body's intrinsic conduction system to produce these impulses synthetically.
The pacemaker is located in the wall of the right atrium.
Read more about Cardiac Pacemaker: Primary (SA Node), Secondary (AV Junction & Bundle of His), Generation of Action Potentials