Because of the property of originating a cardiac impulse, the sino-atrial node is called the pacemaker of the heart. The sino-atrial node generates the action potential and produces cardiac impulse. This cardiac impulse then spreads over atria and ventricles causing them to contract or relax.
The SA node can generate a maximum number of action potentials which is about 70−75 per minute. It also controls the rhythmic contractile activity of the heart.