The electrical activity that stimulates and paces the heartbeat is critical. In order to deliver an appropriate blood supply to body tissues, the heart must beat at an adequate rate, and the timing and sequence of muscular contractions must be precisely coordinated.
Your heart’s natural pacemaker is the sinoatrial (SA) node, a microscopic group of specialized electrical cells located at the top of the right atrium. Each heartbeat originates in the SA node when it ?res off an electrical impulse. This impulse travels via specialized pathways to the cells in the muscle tissues of the heart wall. The impulse ?rst stim- ulates the upper chambers, the atria, to contract and squeeze blood out into the ventricles.
Then the impulse moves to another area of electrical cells called the atrioventricular (AV) node, located over the ventricles. This node acts as a relay station, allowing for a brief interval during which the atria empty completely before releasing the impulse along branching pathways that travel to the two ventricles to stimulate ventricular contraction. The ventricles similarly contract and empty, and blood is pumped into the pulmonary artery and the aorta.
The SA node speeds up when your body needs more blood. It also slows down during rest or in response to some medications. The mes- sage to increase or decrease the rate of impulses is controlled by the autonomic nervous system—the part of the nervous system that con- trols unconscious, automatic body functions including heart rate, blood pressure, and breathing. Autonomic nervous system activity regulates the release of the hormones epinephrine and norepinephrine, which act as accelerators for the heart’s electrical impulses during times of stress or exercise.
Your heart’s electrical activity can be followed and recorded on paper as an electrocardiogram (ECG, see pages 122–125). The initial impulse from the SA node is seen as a wave on the ECG, followed by a more static interval. The ECG recording shows spikes as the impulse travels from the AV node through the ventricular pathways and is again fol- lowed by a static interval that is a segment of recovery.