From which structure is the heart's electrical impulse normally initiated?

From which structure is the heart's electrical impulse normally initiated?

• A. AV node
• B. SA node
• C. The brain
• D. The sympathetic nervous system

Answer: (B)

The impulse is normally initiated by the SA node, the heart’s natural pacemaker. Its pacemaker cells automatically depolarize on their own, generating a spontaneous action potential. Because the SA node has the fastest intrinsic firing rate in the conduction system, its impulses set the heart’s rhythm, typically around 60–100 beats per minute at rest. From there, the signal spreads through the atria to the AV node, which provides a brief delay to allow atrial contraction, and then on to the His-Purkinje system to activate the ventricles. The brain can modulate heart rate via autonomic input, and the sympathetic system can increase rate and force, but they do not initiate the impulse. The AV node can take over as a secondary pacemaker if the SA node fails, but its intrinsic rate is slower, so it does not normally start the heartbeat.

The impulse is normally initiated by the SA node, the heart’s natural pacemaker. Its pacemaker cells automatically depolarize on their own, generating a spontaneous action potential. Because the SA node has the fastest intrinsic firing rate in the conduction system, its impulses set the heart’s rhythm, typically around 60–100 beats per minute at rest. From there, the signal spreads through the atria to the AV node, which provides a brief delay to allow atrial contraction, and then on to the His-Purkinje system to activate the ventricles.

The brain can modulate heart rate via autonomic input, and the sympathetic system can increase rate and force, but they do not initiate the impulse. The AV node can take over as a secondary pacemaker if the SA node fails, but its intrinsic rate is slower, so it does not normally start the heartbeat.