What normally suppresses the automaticity foci in the heart?

What normally suppresses the automaticity foci in the heart?

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

Answer: (B)

The normal suppression of automaticity foci in the heart is primarily due to the dominant pacemaker, which is the SA node. The SA node has the highest intrinsic firing rate compared to other pacemaking cells in the heart, such as those located in the AV node, Bundle of His, and Purkinje fibers. When the SA node generates electrical impulses, these signals travel through the atria and reach the AV node, effectively "overriding" the automaticity of the other foci.

This suppression is significant because it ensures a coordinated heartbeat with a regular rhythm. The other areas, while capable of initiating an impulse, are normally quiescent because they are being regularly stimulated by the impulses from the SA node. Only if the SA node fails or its signals are interrupted will these other pacemaker cells become active, resulting in a backup conduction rhythm.