sinoatrial node
The sinoatrial node is the heart's built-in metronome. It is a small patch of specialized tissue, only a few millimeters across, tucked into the wall of the right atrium near where the superior vena cava enters. Left entirely to itself, it ticks out a steady beat, and every normal heartbeat begins as a tiny electrical spark right here.
What makes the sinoatrial node special is that its cells leak ions in a way that lets their voltage drift upward on its own until it crosses a threshold and fires. Because this node drifts up fastest, it reaches threshold before any other part of the heart and so sets the pace for everyone else — a property called overdrive suppression. The intrinsic rate is roughly 60 to 100 beats per minute, but it is constantly tuned by the autonomic nervous system: vagal (parasympathetic) signals slow it down, while sympathetic signals speed it up.
From the sinoatrial node the impulse spreads across both atria and converges on the atrioventricular node, the only normal electrical doorway into the ventricles. When the heart's rhythm is being driven by this node at a normal rate, clinicians call it sinus rhythm. If the node fails or slows abnormally — as in sick sinus syndrome — a backup pacemaker lower in the heart usually takes over, but at a slower, less reliable rate.
On an ECG, each impulse from the sinoatrial node produces a P wave, and the steady, regular march of P waves before every QRS complex is the hallmark of normal sinus rhythm.
Reading the node's signature on the ECG.
The sinoatrial node is supplied by a small dedicated artery (the SA nodal artery), which arises from the right coronary artery in most people. This is one reason a right coronary blockage can cause bradycardia.