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All the components of the nodal tissue are autoexcitable. Why does the SA node act as the normal pacemaker?

1. SA node has the highest rate of depolarisation.
2. SA node has the lowest rate of depolarisation.
3. SA node is the only component to generate the threshold potential.
4. Only SA node can convey the action potential to the other components.

Correct answer: SA node has the highest rate of depolarisation.

Solution

The SA node has the fastest spontaneous depolarization, so it reaches threshold before other autoexcitable tissues. Its impulses therefore overdrive the rest of the conduction system and set the heart rate.

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