What initiates the action potential in the heart?

What initiates the action potential in the heart?

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

Answer: (A)

The starting electrical signal in the heart comes from the sinoatrial node, the natural pacemaker. Its pacemaker cells have automaticity, meaning they spontaneously depolarize and generate an action potential on their own. This diastolic depolarization is driven by ion flows, including the funny current, allowing the cells to reach threshold and fire first in every cycle. That impulse then spreads through the atria to the atrioventricular node, which provides a brief delay to let the atria finish contracting, before moving down the His-Purkinje system to activate the ventricles. While other parts of the conduction system can take over if the SA node slows or stops, they do so at a slower intrinsic rate, so the normal initiator remains the SA node.

The starting electrical signal in the heart comes from the sinoatrial node, the natural pacemaker. Its pacemaker cells have automaticity, meaning they spontaneously depolarize and generate an action potential on their own. This diastolic depolarization is driven by ion flows, including the funny current, allowing the cells to reach threshold and fire first in every cycle. That impulse then spreads through the atria to the atrioventricular node, which provides a brief delay to let the atria finish contracting, before moving down the His-Purkinje system to activate the ventricles. While other parts of the conduction system can take over if the SA node slows or stops, they do so at a slower intrinsic rate, so the normal initiator remains the SA node.