Solution

A specialised cardiac musculature, called as the nodal tissue is distributed in the heart. A patch of this tissue is present in the right upper corner of the right atrium, called as the sino-atrial node (SAN).
The nodal musculature has the ability to generate action potentials without any external stimuli, i.e., it is auto excitable. The SAN can generate the maximum number of action potentials, i.e., 70 - 75 beats/min and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore, it is called as the pacemaker of heart.