Cardiac fxn: Parasympathetic effects

Parasympathetic fibers originating in the dorsal vagal nucleus and nucleus ambiguus of the medulla synapse on cardiac tissue via the vagus nerve (Cranial nerve X). Branches of the right vagus largely innervate the SA node and left vagus largely innervates the AV node. Parasympathetic activation via the vagus nerve primarily affects the atria given there is sparse innervation of the ventricle. With parasympathetic activation acetylcholine release activates muscarinic receptors in the SA and AV nodes resulting in hyperpolarization. This causes a decrease in heart rate (chronotropy), conduction velocity through the AV node (dromotropy), and atrial contractility. At rest, “vagal tone” predominates over sympathetic activity of the SA node, resulting in a heart rate between 60-80 beats per minute.

Regulatory feedback loops are modulated by the parasympathetic nervous system. When baroreceptors in the carotid sinus and aortic arch sense elevated pressure, signals are sent via the vagus and recurrent laryngeal neve leading to vagal nerve activation resulting in decreased heart rate. The Bezold-Jarisch reflex utilizes the vagal nerve to detect myocardial injury/inflammation/ischemia resulting in decreased heart rate, vasodilation, and decreased blood pressure.