What is the sinus node?

• A. A group of cells in the left atrium that inhibits the heartbeat
• B. The AV node in the right atrium that delays conduction
• C. A nerve bundle in the ventricles
• D. A group of cells in the right atrium where the electrical signal is generated that establishes the heartbeat

Answer: (D)

The signal that begins each heartbeat comes from the heart’s natural pacemaker, the sinoatrial (SA) node. This is a small group of specialized pacemaker cells located in the right atrium near where the superior vena cava enters the heart. These cells have automaticity, meaning they depolarize spontaneously and regularly generate electrical impulses. That impulse sets the pace for the heart, stimulating atrial contraction and then traveling to the AV node to coordinate timing with the ventricles, producing a coordinated heartbeat.

It’s not in the left atrium and it doesn’t inhibit the heartbeat, which rules out that option. It isn’t the AV node, which is the structure that delays conduction to allow the atria to empty before the ventricles beat. It isn’t a nerve bundle in the ventricles, such as Purkinje fibers, which carry impulses inside the ventricles rather than initiating them.