How the SA and AV Nodes Generate RHR
The sinoatrial (SA) node, or the pacemaker of the heart, generates action potentials in a rhythmic fashion. When we put non-stimulated SA node cells in a petri dish, we observe an inherent contraction rate of approximately 100 bpm. However, the typical resting heart rate (RHR) is much lower, around 70 bpm. This reduction comes as a result of signal delay at the atrioventricular (AV) node, which has an inherent contraction rate of around 40 bpm.
The SA node increases the rate of contraction of the AV node via cation leakage. By this mechanism, cations from SA node depolarization assist in AV node depolarization. Thus, the SA node is able to increase the rate of AV node contraction past the inherent AV node value.
The AV node also works to decrease the rate of SA node contraction. The AV node is the electrical gateway between the atria and ventricles. Each AV node contraction allows passage of electrical impulses from the atria to the ventricles. As a result, slow AV node contraction at around 40 bpm is key in delaying the passage of the electrical impulses necessary for ventricular contraction. So, because the AV node does not allow rapid passage of impulses from the SA node to the ventricles, the AV node is a key regulator of ventricular contraction rate.
Ventricular contraction must be delayed because of the need to make sure the atria are fully emptied before the next contraction; otherwise, the atrial capacity available for blood transport will not be optimally used. From the interplay of the SA and AV nodes, the heart is able to generate muscular contractions to establish the resting heart rate.
Learn more:
This super helpful video that I am really thankful for (watch it and everything will click): https://youtu.be/RYZ4daFwMa8
A longer video which also covers the basics: https://youtu.be/NijiLTONHxg
On node innervation: after all, the SA and AV nodes aren’t always going to contract at 100 or 40 bpm! Learn about nervous regulation of heart rate:
https://my.clevelandclinic.org/health/body/22279-vagus-nerve
https://emedicine.medscape.com/article/1922987-overview - this article also has other stuff about the SA/AV nodes (anatomy, pathophysiology, etc). Highly recommend!
