Catecholamines *; Atropine - blocks Acetylcholine; Quinidine - inhibits vagal effects
Slows: Acetylcholine * *; Digitalis - central vagal (parasympathetic) stimulation; Inhibitors of acetylcholine esterase, Ca⁺⁺ antagonists (e.g. verapamil); An increased number of impulses arriving at AV junction increases refractoriness