Which structure originates impulses that set the pace of the heart in a healthy adult?

Which structure originates impulses that set the pace of the heart in a healthy adult?

• A. Bundle of His
• B. Purkinje fibers
• C. Sinoatrial node
• D. AV node

Answer: (C)

The sinoatrial node acts as the heart’s natural pacemaker. Its pacemaker cells have automaticity, meaning they spontaneously depolarize and generate impulses, and among the heart’s conduction tissues they fire the fastest under normal conditions. This rapid, intrinsic firing sets the resting heart rate (roughly 60–100 beats per minute) and drives the pacing of the whole heart. From the SA node, impulses spread through the atria to the atrioventricular node, which adds a brief delay to optimize atrial contraction, then travel via the His-Purkinje system to coordinate ventricular contraction. The AV node, Bundle of His, and Purkinje fibers are crucial for conduction, but they do not normally set the pace; the SA node does.

The sinoatrial node acts as the heart’s natural pacemaker. Its pacemaker cells have automaticity, meaning they spontaneously depolarize and generate impulses, and among the heart’s conduction tissues they fire the fastest under normal conditions. This rapid, intrinsic firing sets the resting heart rate (roughly 60–100 beats per minute) and drives the pacing of the whole heart. From the SA node, impulses spread through the atria to the atrioventricular node, which adds a brief delay to optimize atrial contraction, then travel via the His-Purkinje system to coordinate ventricular contraction. The AV node, Bundle of His, and Purkinje fibers are crucial for conduction, but they do not normally set the pace; the SA node does.