SA & AV action potentials: Phase ____ represents a slow depolarization from the slow, spontaneous influx of Na+ . This is what gives the SA and AV nodes the property of automaticity. They do not need any stimulus to generate an action potential. They do it automatically. Acetylcholine and the drug adenosine cause a decrease in the rate of depolarization and decrease heart rate. Catecholamines increase the rate of depolarization and increase the heart rate.