Final answer:
The atrioventricular (AV) node is part of the heart's electrical conduction system and imposes a critical pause on the electrical impulse from the sinoatrial (SA) node, allowing the atria to pump blood into the ventricles before they contract.
Step-by-step explanation:
The atrioventricular (AV) node is an essential component of the cardiac intrinsic conduction system, located in the inferior part of the right atrium within the atrioventricular septum. Its primary role is to receive the electrical impulse from the sinoatrial (SA) node and to impose a critical delay before passing it to the atrioventricular bundle. This delay or pause is around 100 milliseconds (ms) and is crucial as it ensures that the atria have enough time to contract, effectively pumping blood into the ventricles before the ventricles begin to contract.
The small diameter of the cells within the AV node and the less efficient conduction between these cells contribute to this delay. The AV node can transmit impulses at a maximum rate of 220 per minute under extreme stimulation by the SA node, establishing the maximum heart rate for a young healthy individual. When the heart is damaged or stimulated by drugs, it can contract at even higher rates. However, beyond these rates, the effectiveness of the heart's blood-pumping action diminishes.
The cardiac impulse spreads from the SA node through three specialized internodal pathways before it reaches the AV node. In addition to these pathways, there is also Bachmann's bundle, which conducts the impulse directly between the right and left atria. Nevertheless, it is anatomically designed so that the impulse can reach the ventricles only through the AV node.
Figure 19.19 illustrates the cardiac conduction system, initiating with the SA node and spreading the impulse through the atria, experiencing the critical delay at the AV node, and then moving through the atrioventricular bundle towards the ventricles. The entire sequence of these events ensures a well-coordinated contraction of the heart muscles, thereby optimizing the heart's function as a pump.