Parasympathetic nerves related to heart function mainly arise from which nerve?

• A. Increase heart rate.
• B. Nerves that regulate digestion.
• C. Nerves that decrease heart rate; arise from the vagus nerve and influence pacemaker cells.
• D. Nerves that originate from the spinal cord.

Answer: (C)

Parasympathetic control of the heart is mainly through the vagus nerve. This cranial nerve carries preganglionic parasympathetic fibers to the heart and synapses on the pacemaker cells in the SA and AV nodes. When acetylcholine is released onto these nodes, it makes pacemaker cells less likely to fire quickly, slowing the rate of depolarization and reducing conduction through the AV node. The result is a lower heart rate. This parasympathetic path originates in the brainstem, not the spinal cord, and while the vagus also serves digestive organs, its heart-specific effect is to decrease heart rate by acting on the nodal pacemaker cells.