Question

Which is the correct sequence of a cardiac impulse? A. SA node ? AV node ? Purkinje fibers ? bundle of His ? ventricles B. AV node ? bundle of His ? Purkinje fibers ? SA node ? ventricles ? atria C. SA node ? atria ? AV node ? bundle of His ? Purkinje fibers ? ventricles D. SA node ? AV node ? atria ? bundle of His ? Purkinje fibers ? ventricles

Short answer

C. SA node ? atria ? AV node ? bundle of His ? Purkinje fibers ? ventricles

Step-by-step solution

Identify the origin of the impulse

The cardiac impulse typically originates in the Sinoatrial (SA) node, which is the natural pacemaker of the heart.

Determine the next destination

From the SA node, the impulse travels to the atria, causing them to contract.

Locate the secondary node

The impulse then reaches the Atrioventricular (AV) node, which acts as a relay station.

Move to the next stage

From the AV node, the impulse moves down the bundle of His.

Travel through the branches

The bundle of His branches into the left and right bundle branches, carrying the impulse to the Purkinje fibers.

Final destination of the impulse

The Purkinje fibers carry the impulse to the ventricles, causing them to contract.

Confirm the correct sequence

Review the options to see which one matches the sequence: SA node ? atria ? AV node ? bundle of His ? Purkinje fibers ? ventricles.

Compare sequences

Option C follows this sequence: SA node ? atria ? AV node ? bundle of His ? Purkinje fibers ? ventricles.

Key concepts

Sinoatrial Node

The cardiac impulse begins in the Sinoatrial (SA) node, which is often referred to as the heart's natural pacemaker. This small group of specialized cells is located in the right atrium. The SA node generates electrical impulses that spread across the walls of the atria, causing them to contract and push blood into the ventricles.

• Location: Right atrium
• Function: Initiates the heartbeat
• Importance: Ensures the heart maintains a regular rhythm

The SA node sets the pace for the entire heart, making it critical for maintaining a consistent and effective heartbeat.

Atrioventricular Node

After the impulse from the SA node causes the atria to contract, it reaches the Atrioventricular (AV) node. The AV node is situated at the junction between the atria and ventricles. This node acts as a relay station that momentarily delays the impulse, ensuring that the atria have emptied their blood into the ventricles before they contract.

• Location: Between atria and ventricles
• Function: Delays impulse to coordinate heartbeats
• Role: Synchronization ensures efficient blood flow

The delay provided by the AV node is crucial since it ensures the ventricles have enough time to fill with blood before they pump it out.

Bundle of His

Upon leaving the AV node, the cardiac impulse travels down the Bundle of His. This bundle is a pathway of specialized fibers located in the interventricular septum, which is the wall between the left and right ventricles. The Bundle of His transmits the impulse rapidly to ensure the coordinated contraction of the ventricles.

• Location: Interventricular septum
• Function: Conducts electrical impulses

The Bundle of His ensures the impulse moves quickly towards the lower chambers, setting the stage for the final conductive pathways.

Purkinje Fibers

The impulses from the Bundle of His are then conducted through the left and right bundle branches to the Purkinje fibers. These fibers are a network of specialized cells that spread throughout the ventricles. They carry the electrical impulses at high speed, ensuring the coordinated contraction of ventricular muscle fibers.

• Location: Ventricular walls
• Function: Efficiently distributes the impulse

The main role of Purkinje fibers is to ensure that the ventricles contract effectively, pushing blood out of the heart to the lungs and the rest of the body.

Ventricular Contraction

Once the impulse reaches the Purkinje fibers, the ventricles contract. This powerful contraction is what pumps the blood out of the heart.

• Left ventricle: Pumps oxygenated blood to the entire body through the aorta
• Right ventricle: Sends deoxygenated blood to the lungs through the pulmonary artery

Ventricular contraction is the final and essential step in the cardiac impulse sequence, ensuring that blood circulates efficiently throughout the body.