The cardiac electrical impulse normally begins spontaneously in the sa node because

The cardiac electrical impulse normally begins spontaneously in the SA node because this area of the heart has a special type of cells that are depolarized very fast creating an action potential that expands through the rest of the heart causing the contraction of it. Also, the Sinoatrial node is located in the right atrium, which is the first section of the heart that has to be contracted to start the cardiac cycle.        

Explanation:

The sinoatrial node is known as the heart natural peacemaker. It is a group of cells in the heart's walls that produces a spontaneous potential action to contract the heart. The SA node is located in the right atrium, which is the first area of the heart that is contracted in order to start the cardiac cycle.

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explanation:

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