The heart’s internal pacemaker beats by

The heart’s internal pacemaker beats by: a. an internal implant that sends an electrical impulse through the heart b. the excitation of cardiac muscle cells at the sinoatrial node followed by the atrioventricular node c. the excitation of cardiac muscle cells at the atrioventricular node followed by the sinoatrial node d. the action of the sinusC

The Correct Answer and Explanation is:

correct Answer:
b. the excitation of cardiac muscle cells at the sinoatrial node followed by the atrioventricular node

Explanation:

The heart’s intrinsic conduction system is responsible for creating and propagating the electrical impulses that trigger each heartbeat. The primary components of this system are:

1. Sinoatrial (SA) Node (“the heart’s natural pacemaker”)
   • Located in the right atrial wall near the superior vena cava, the SA node contains specialized pacemaker cells that possess an unstable resting membrane potential.
   • These cells undergo spontaneous depolarization (phase 4 of the pacemaker action potential), driven largely by the “funny” current (If) carried through HCN channels and a gradual increase in intracellular calcium.
   • Once the threshold is reached, an action potential is fired (phase 0), rapidly depolarizing the SA nodal cells and initiating the electrical impulse.
   • This intrinsic rate of SA nodal firing is approximately 60–100 beats per minute, setting the heart’s baseline rhythm.
2. Atrial Depolarization and Conduction to the AV Node
   • The impulse spreads quickly across the atrial muscle via gap junctions, causing atrial contraction (P wave on ECG).
   • It then arrives at the atrioventricular (AV) node, located at the junction of the atria and ventricles in the interatrial septum.
3. Atrioventricular (AV) Node Delay
   • The AV node introduces a physiological delay (roughly 100 ms), which is crucial to allow complete atrial emptying before ventricular contraction.
   • This delay corresponds to the PR segment on the electrocardiogram.
4. His–Purkinje System and Ventricular Contraction
   • From the AV node, the impulse travels down the bundle of His, which bifurcates into the right and left bundle branches, and finally into the Purkinje fiber network.
   • This rapid conduction system ensures a coordinated and forceful ventricular contraction (QRS complex), propelling blood into the pulmonary and systemic circulations.
5. Autonomic Modulation
   • Sympathetic stimulation increases the slope of phase 4 depolarization in the SA node (↑ heart rate), while parasympathetic (vagal) input decreases it (↓ heart rate).

In summary, the SA node initiates each heartbeat by spontaneously generating an electrical impulse, which then propagates through the atria to the AV node, before descending the His–Purkinje system to trigger ventricular contraction. This sequential excitation—SA node → AV node → ventricles—ensures efficient cardiac function and is the basis for option b.