The nodal musculature has the ability to generate action potentials without any external stimuli, i.e. it is autoexcitable. However, the number of action potentials that could be generated in a minute vary at different parts of the nodal system. The SAN (Sino-Atrial Node) can generate the maximum number of action potentials, i.e $70-75 \mathrm{~min}$, i.e. the highest rate of depolarisation and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore, it is called pacemaker.