ANATOMY

Detailed explanation-1: -SA node pacemaker cells depolarize at a rate of 60 to 100 per minute, while the AV node at 40 to 60 per minute. The pacemaker with the highest rate of depolarization takes over as the primary pacemaker. In healthy individuals, this is the SA node.

Detailed explanation-2: -This depolarization is caused by very small net inward currents of calcium ions across the cell membrane, which gives rise to the action potential.

Detailed explanation-3: -Automaticity: unlike other cardiomyocytes, pacemaker cells do not require external stimulation to initiate their action potential; they are capable of self-initiated depolarization in a rhythmic fashion.

Detailed explanation-4: -Contractile cells demonstrate a much more stable resting phase than conductive cells at approximately −80 mV for cells in the atria and −90 mV for cells in the ventricles. Since the cells re connected by gap junctions, the action potential spreads form cell to cell by ion flow through the gap junctions.