How is heart rate determined by action potentials?

[scilearner]

This is the action potential duration of a normal cardiac myocyte

 

 

Here are the potential pace makers.

 

* SA node: 80 - 100 bpm

* Atrial foci: 60 - 80 bpm

* Junctional foci: 40 - 60 bpm

* Ventricular foci: 20 - 40 bpm

 

1. First of all let's say ventricular foci is the pacemaker. Why does it give a low heart beat? Is it because it decreases the duration of the action potential or frequency of the signal?

2. When heart rate is increased by nerves, the action potential duration decreases, I want to know how this occurs? How can nerves control the ions in the extracellular medium? or do they control the sodium channels?

 

I'm just interested in how an action potential can be shortened.

 

Thank you