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The sino-atrial or the sinus node (SAN) is a specialized bundle of neurons generating action potential which produces a cardiac impulse without any exterior stimuli i.e., it is auto excitable. SAN can generate a maximum action potential of approximately 70 to 75 in a minute. It is responsible to initiate and maintain the rhythmic contractile activity of the heart. Due to these capabilities, SAN is referred to as the pacemaker.