S.A. node acts as a pacemaker of the hea because of the fact that it

Some cardiac fibers have the capability of self-excitation, a process that can cause automatic rhythmical discharge and contraction. This capability is especially true of the fibers of the hea 's specialized conducting system, including the fibers of the sinus node. For this reason, the sinus node ordinarily controls the rate of beat of the entire heaRef: Guyton and Hall textbook of medical physiology 13th edition. Page: 123