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

Sourav PanNovember 8, 2024

Answered

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