Which of the following actions will slow the heart rate via the parasympathetic nervous system?

• A. Release of norepinephrine onto the sinoatrial (SA) node from sympathetic cardiac nerves
• B. Activation of beta-1 adrenergic receptors in cardiac pacemaker cells
• C. Stimulation of the adrenal medulla to release epinephrine into the bloodstream
• D. Release of acetylcholine onto the sinoatrial (SA) node via the vagus nerve

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the role of the autonomic nervous system in heart rate regulation. The sympathetic nervous system increases heart rate, while the parasympathetic nervous system decreases it.
2. Step 2: Identify the neurotransmitters involved. The sympathetic nervous system primarily uses norepinephrine, which increases heart rate by acting on beta-1 adrenergic receptors in the sinoatrial (SA) node.
3. Step 3: Recognize that the parasympathetic nervous system slows heart rate by releasing acetylcholine onto the SA node, primarily via the vagus nerve.
4. Step 4: Analyze each option: release of norepinephrine or activation of beta-1 receptors increases heart rate (sympathetic effect), stimulation of the adrenal medulla releases epinephrine which also increases heart rate, so these do not slow the heart rate.
5. Step 5: Conclude that the correct action to slow heart rate via the parasympathetic nervous system is the release of acetylcholine onto the SA node through the vagus nerve.