What are the pacemaker cells of the Heart and their normal heart rates?

What is the most accurate way to calculate resting heart rate?

What are the difference between contractile cells of the heart and electrical cells responsible for generation and conduction of impulses. Why is the cardiac action potential propagated more slowly in an AV node cell than in an atrial or ventricular myocyte? How are the channels and ions that are responsible for depolarization and repolarization work? How can be AV conduction affected by hypokalemia?

What are the three populations of cells in the heart that can act as pacemakers? How do they differ?