Explain why the SA node generates action potentials at a frequency of approximat

The average heart rate is lower than the sinoatrial node's ability to generate action potentials due to the vagus nerve and the parasympathetic nervous system.

Your brain limits your heart rate inside your body to approximately 70 beats per minute. This allows your cardiovascular centres in your brain a little bit more latitude as far as controlling your heart rate.

If you were to remove a beating heart from someone's body you would find that the heart is no longer being suppressed by the parasympathetic nervous system and it will accelerate to the same pace as the sinoatrial node.

The SA node generates action potentials approximately 100 times per minute due to the physiology of the heart, its own permeability to sodium and calcium via its channels, and a host of other things. Anywhere between 80 to 100 beats per minute is normal for the SA node.

Because that's the refractory period of the cells in the SA node in isolation. In vivo, the SA node is subject to modification by the action of the nervous system; vagus nerve tone lowers the average resting heart rate to 70.

I think it's because of the longer refractory period of cardiac muscle. The heart beat has to have completed before the end of the refractory period, so the heart cannot tetanize.