sentences:  Parasympathetic stimulation (which acts to increase probability of potassium channels opening but decreases the probability of calcium channel opening) decreases the heart rate by the opposite action, that is by shifting the If activation curve towards more negative voltages.When vagally-released acetylcholine (ACh) binds to muscarinic M2 receptors, which promotes dissociation of βγ subunit complexes, leading to direct opening of the G-protein–gated inwardly rectifying K+ channel (Girk/Kir) "IKACh.