Question

In discussing the anatomy of the heart and the sequence and timing of cardiac excitation in class, I brought an apparent contradiction to your attention: That the internodal pathways connecting the SA and AV nodal regions are specifically designed to rush the SA "signal" to the AV node, where it is subsequently delayed before being passed on to the ventricles via the Purkinje system. Adopt a speculative posture and address the following questions:

a) Why might the heart go to all of the trouble of rushing the SA signal to the AV nodal region when it's going to be intentionally delayed once it arrives anyway, and

b) Why provide 3 separate internodal pathways, along with an interatrial band?

Answer 1

a. signals from SA node rushes to AV node because, the rate at which these SA node cells send out electrical signals determine the rate of entire heart beat.The impulses in AV node are slowed because this allows the atria to contract a fraction of second before the ventricles

b.there are 3 internodal pathways

• anterior
• middle
• posterior

1. anterior

• leaves anterior surface of sinostrial node
• passes impulses posteriomediallyto interatrial septum and then inferiorly splits to left atrium and posteriorly to torus aorticus to reach AVnode

2. middle

• leaves posterior and superior surface of sinoatrial node
• passes impulses inferiorly and medially around orifice of inferior venacava reaches interatrial septum where it passes inferiorly to atrioventricular node

3. posterior

• leaves posterior and inferior surface of sinoatrial node
• passes impulses inferiorly through the crista terminalis and medially around valve of inferior venacava reaches posterior surface of atrioventricular node