Which nerve carries parasympathetic fibers to the heart?

• A. Phrenic nerve
• B. Glossopharyngeal nerve
• C. Vagus nerve (cranial nerve X)
• D. Hypoglossal nerve

Answer: (C)

Parasympathetic control of the heart is carried mainly by the vagus nerve. Parasympathetic fibers originate in the brainstem and travel in the vagus to the heart, where they form cardiac connections and release acetylcholine onto muscarinic receptors (M2) in the SA and AV node. This slows pacemaker activity and reduces conduction velocity, lowering heart rate. The other nerves listed don’t serve the heart in a parasympathetic way: the phrenic nerve mainly motor for the diaphragm; the glossopharyngeal nerve supplies parasympathetic to the parotid gland but not the heart; the hypoglossal nerve controls tongue muscles. So the vagus is the nerve carrying parasympathetic fibers to the heart.