How is heart rate determined by action potentials?

[sameeralord]

I'm bit confused about this. This is my question

This is the action potential duration of a normal cardiac myocyte
[PLAIN]http://www.med.nus.edu.sg/paed/resources/cardiac_thumbnail/background/images/action_pot.gif

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

 

[mommo]

The pacemaker determines the heart rate of the body. In a normal heart, the pacemaker is the piece of cardiac muscle that beats the fastest (the myocytes at the SA node) as it causes the action potential to travel through the heart, depolarizing the muscle. Thus, to answer your question, its because it sets the frequency of the signal.

For 2, nerves release chemicals at the neuromuscular junctions that bind to the channels at the muscle. The myocytes beat on their own naturally due to leakiness in their channel membranes (in particular, the calcium channels, not shown in your graph) and allow for a rhythmic beating to occur. In order to increase the rate of the heart, the chemicals increase this permeability of ions to allow for faster exchange of ions between the extracellular fluid and cytoplasm. Depending on what chemical is deployed, it affects either the sodium, calcium, potassium, or chloride channels.