Answer
The primary pacemaker is the sinoatrial, a complex of modified cardiac muscle cells between the superior vena cava and the right atrium. These cells have a less negative resting potential than other muscle cells and are therefore less stable. When a signal is received in the sinoatrial cells, calcium and sodium channels open to decrease the negativity of the membrane potential, in turn generating an action potential that leads to the simultaneous activation of the cardiac cells in the sinoatrial.
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