part of membrane depolarization membrane depolarization during cardiac muscle cell action potential atrial cardiac muscle cell action potential membrane depolarization during atrial cardiac muscle cell action potential atrial depolarization GO:0098912 biological_process The process in which atrial cardiac muscle cell membrane potential changes in the depolarizing direction from the negative resting potential towards the positive membrane potential that will be the peak of the action potential. electrocardiogram PR interval atrial cardiac muscle cell depolarization