The sinoatrial node is a group of specialized cardiac cells that normally initiates the electrical rhythm of the heart. Its pacemaker cells gradually undergo depolarization between action potentials instead of remaining at a stable resting voltage. Their activity spreads into surrounding atrial cardiac muscle through electrical coupling. [ref]
Anatomy and Physiology 2e by J. Gordon Betts and others. Section 19.2, Autorhythmicity of Cardiac Cells and the Sinoatrial Node
The rhythm can arise without a separate motor nerve triggering every beat. Autonomic inputs modulate its rate, while conduction through other cardiac structures organizes the order of activation. Pacemaker activity, propagation and mechanical contraction are therefore different stages: recording the first does not by itself prove that every later stage proceeds normally.
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