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Sourav PanNovember 8, 2024

How does the electrical signal from the SA node stimulate contraction in the heart chambers?

How does the electrical signal from the SA node stimulate contraction in the heart chambers?

Sourav Pan
Sourav PanNovember 8, 2024

Answered step-by-step

The sinoatrial (SA) node plays a critical role in initiating the heartbeat and stimulating contraction in the heart chambers through a well-coordinated electrical conduction system. Here’s how the process works:

1. Generation of Electrical Impulses

  • The SA node, located in the upper wall of the right atrium, generates electrical impulses (action potentials) spontaneously at a rate of 60 to 100 beats per minute under normal conditions. This makes it the heart’s natural pacemaker.

2. Propagation of Impulses

  • Once an impulse is generated, it spreads through the atrial myocardium:
    • Atrial Contraction: The electrical signal travels across the atria, causing them to contract (atrial systole) and push blood into the ventricles. This contraction is visible on an electrocardiogram (ECG) as the P wave.
    • The impulse spreads via specialized pathways, including Bachmann’s bundle, which conducts signals to the left atrium, ensuring simultaneous contraction of both atria.

3. Transmission to the Atrioventricular (AV) Node

  • After spreading through the atria, the electrical impulse reaches the AV node, located at the junction between the atria and ventricles. Here, there is a brief delay in conduction:
    • Delay for Ventricular Filling: This delay allows time for the ventricles to fill with blood from the atria before they contrac.

4. Conduction Through Bundle of His and Purkinje Fibers

  • Following the AV node, the impulse travels down the bundle of His, which splits into right and left bundle branches that extend into each ventricle:
    • Rapid Conduction: The impulse then spreads through a network of Purkinje fibers, which rapidly conduct the electrical signal throughout the ventricular myocardium, causing both ventricles to contract almost simultaneously.
    • This coordinated contraction pushes blood from the right ventricle into the lungs and from the left ventricle into systemic circulation.

5. Functional Syncytium

  • The heart muscle cells are connected by gap junctions, allowing electrical impulses to propagate quickly between adjacent cells. This property enables the myocardium to function as a functional syncytium, meaning that all cardiac muscle cells contract in unison, ensuring effective pumping action

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