Sinoatrial Node

The sinoatrial (SA) node is a collection of specialised cells (pacemaker cells), and is located in the upper wall of the right atrium, at the junction where the superior vena cava enters.

These pacemaker cells can spontaneously generate electrical impulses. The wave of excitation created by the SA node spreads via gap junctions across both atria, resulting in atrial contraction (atrial systole) – with blood moving from the atria into the ventricles.

The rate at which the SA node generates impulses is influenced by the autonomic nervous system:

• Sympathetic nervous system – increases firing rate of the SA node, and thus increases heart rate.
• Parasympathetic nervous system – decreases firing rate of the SA node, and thus decreases heart rate.

> 032. Atrioventricular Node