ECG Signal Overview
The sinoatrial (SA) node acts as the pacemaker of the heart. These cells rhythmically produce action potentials that spread through the muscle fibres of the atria and ventricles. The resulting contraction pushes blood into the ventricles, and then out into the vasculature.
Innervation by the parasympathetic and sympathetic nerves can modify the basic cardiac rhythm e.g. as occurs in sinus arrhythmia. Similarly, heart rate can be influenced by activity of the sympathetic nervous system due to change in emotional and cognitive state.