Final answer:
The pacemaker of the heart is the sinoatrial (SA) node, which is responsible for initiating the sinus rhythm that causes the heart to beat.
Step-by-step explanation:
The sinoatrial (SA) node is known as the pacemaker of the heart. This is because it establishes the normal cardiac rhythm by generating electrical impulses that prompt the heart to beat. Located in the superior and posterior walls of the right atrium near the orifice of the superior vena cava, the SA node has the highest inherent rate of depolarization within the heart's conduction system. It initiates the sinus rhythm, which is the normal electrical pattern that the heart muscle follows when contracting.
When considering the options provided:
- A) SA node - Correct. It is the pacemaker of the heart.
- B) cardiac plexus (nerve fibers) - Incorrect. These are networks of nerve fibers supplying the heart.
- C) AV node - Incorrect. The atrioventricular node delays the impulse, allowing the atria to contract beforehand, but it is not the pacemaker.
- D) muscle fibers of the atria - Incorrect. While these fibers contract in response to the SA node's signal, they do not set the rhythm.