Parasympathetic Nervous System: Which of the following actions decreases heart rate?

• A. Release of acetylcholine onto the sinoatrial (SA) node via the vagus nerve (muscarinic receptors)
• B. Inhibition of vagal (parasympathetic) output to the heart
• C. Release of epinephrine from the adrenal medulla acting on the heart (beta-1 receptors)
• D. Release of norepinephrine onto the sinoatrial (SA) node from sympathetic postganglionic fibers (beta-1 receptors)

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the role of the parasympathetic nervous system in heart rate regulation. The parasympathetic system primarily decreases heart rate by releasing acetylcholine (ACh) onto the sinoatrial (SA) node, which slows the pacemaker activity.
2. Step 2: Recognize that acetylcholine acts on muscarinic receptors in the SA node, causing hyperpolarization of pacemaker cells and reducing the rate of depolarization, which leads to a decrease in heart rate.
3. Step 3: Contrast this with sympathetic nervous system actions, where norepinephrine and epinephrine bind to beta-1 adrenergic receptors on the SA node, increasing heart rate by enhancing pacemaker depolarization.
4. Step 4: Note that inhibition of vagal (parasympathetic) output would reduce acetylcholine release, thereby removing the slowing effect and resulting in an increased heart rate, not a decrease.
5. Step 5: Conclude that the only action among the options that decreases heart rate is the release of acetylcholine onto the SA node via the vagus nerve acting on muscarinic receptors.