The sino-atrial node (SAN) is called the pacemaker of our heart because it is responsible for initiating and maintaining the rhythmic contractile activity of the heart.

The nodal musculature has the ability to generate action potentials without any external stimuli, i.e., it is autoexcitable. The SAN can generate the maximum number of action potentials, i.e., 70-75 min⁻¹, compared to other parts of the nodal system.

Since the SAN sets the pace of the activities of the heart by generating the maximum number of action potentials per minute, it is called the pacemaker. Our heart normally beats 70-75 times in a minute (average 72 beats min⁻¹).