Myocardial and Pacemaker Action Potential

Question

I recently learned the Myocardial and Pacemaker Action Potentials and had some doubts regarding them.

Do structures other than the SAN and AVN (Bundle of HIS and purkinje fibres)exhibit the pacemaker action potential? I have come to understand that they too possess the property of automaticity.

When an action potential/current reaches the lower conduction tissue, let's say the purkinje fibres, do their cells depolarise and repolarise exhibiting the pacemaker action potential? OR do they then behave like normal cardiomyocytes, exhibiting the myocardial action potential?

Answer 1

There are two concept you bring up in your query:

1. Cardiac action potentials.
2. Automaticity.

Cardiac Action Potentials

Any cells that propagates impulses of any kind will have an action potential. The nature of the action potential will depend upon the types of channels and receptors involved. (This is what contributes to the different "shapes" in the diagrams.)

Figure 1: Phases of Cardiac Action Potential and Relevant Receptors

Figure 2: Differentiation of Cardiac Action Potential Based on Cell Location and Properties

Automaticity

All cardiac myocytes exhibit the property of automaticity. That is, any cell in the heart (be it from the myocardium, SA node, AV node, or His-Purkinjie network) can functionally and independently pace the heart. In addition, another important and relevant concept, is that of overdrive suppression. This means that any cell pacing the heart will suppress all other cells that intrinsincly pace at a slower rate. In other words, the cell that paces the fastest will function as the cardiac pacemaker. In a normal, healthy heart, this is the SA node.

SA-sinoatrial, AV-atrioventricular

Resources:

https://www.itaca.edu.es/cardiac-action-potential.htm (Image Source)

https://www.ahajournals.org/doi/pdf/10.1161/01.RES.41.3.269