Sinoatrial Node
The electrical impulse is usually at a frequency of 60–100 bpm.
The SA node functions as the normal pacemaker of the heart.
The sinoatrial (SA) node possesses spontaneous automaticity.
This impulse spreads rapidly through the atria.
Spread of the depolarization wave through the right and left atria induces contraction of these chambers.
The depolarization waves converge at the atrioventricular node which is normally the only conduction pathway between the atria and ventricles.
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Schematic of the cardiac conduction system
The sinus node is located in the subepicardial tissue on the lateral wall of the right atrium near the junction of the SVC and the right atrium in the sulcus terminalis.
It is on average 10 to 20 mm in length. Its vascular supply is from the SA nodal artery, a branch of the right coronary artery (~60% of people) or the left circumflex artery (~40% of people).
It contains the principal pacemaker cells that spontaneously depolarize (automaticity), causing atrial depolarization. It has extensive input from both the parasympathetic and sympathetic nervous systems via the vagus nerve and the sympathetic chain, respectively. The parasympathetic effects on the sinus node, via acetylcholine, are negative chronotropy, prolongation of intranodal conduction time, and increased refractoriness. The concentration of acetylcholine in the atria, and the SA node in particular, is significantly greater than that in the ventricles, and there is tonic parasympathetic stimulation of the SA node. Sympathetic effects include positive chronotropy via an increase in the rate of sinus node discharge and decreased refractoriness.
The SA node pacing rate would be about 100 beats per minute.
The impulse from the SA node is spread to the right and left atria, followed by atrial contraction.
The electrical impulse travels across the atria to the atrioventricular (AV) node, the electrical communication between the atria and ventricles.
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