Based on what you know about the effects of activating these autonomic pathways, which of the following statements most accurately describes the situation?

Question

BlackTom AI · Accepted Answer

Muscarinic M2 receptors on the sinoatrial (SA) node slow pacemaker activity by coupling to Gαi proteins, so stimulating a parasympathetic neuron that releases acetylcholine will decrease heart rate.

1) Relevant concepts
- Autonomic control of heart rate: sympathetic stimulation increases heart rate via norepinephrine acting on β1-adrenergic receptors (Gαs → increased adenylyl cyclase activity and cAMP). Parasympathetic stimulation decreases heart rate via acetylcholine acting on muscarinic M2 receptors (Gαi → decreased adenylyl cyclase activity and cAMP).
- Electrophysiological mechanisms: reduced cAMP lowers the funny current (If) and reduces L-type Ca2+ channel activity; Gβγ subunits from Gαi open GIRK (IKACh) K+ channels, hyperpolarizing SA nodal cells and slowing firing.

2) Step-by-step reasoning
- Identify neuron B as the parasympathetic (vagal) efferent to the heart (it must be the cholinergic neuron that slows the heart).
- Stimulating neuron B releases acetylcholine at the SA/AV nodes.
- Acetylcholine binds M2 muscarinic receptors coupled to Gαi proteins.
- Gαi inhibits adenylyl cyclase → intracellular cAMP falls → If and Ca2+ currents fall → pacemaker depolarization slows.
- Gβγ opens GIRK channels → membrane hyperpolarizes → firing rate decreases.
- Net effect: heart rate decreases.

3) Intermediate “steps” (no numeric calculation required)
- Ligand (ACh) → M2 receptor → Gαi activation → ↓adenylyl cyclase → ↓cAMP → ↓If & ↓ICa,L
- Gβγ → open K+ channels → hyperpolarization → slower pacemaker rate

4) Why the correct answer is correct
The chosen option explicitly states decrease in heart rate via muscarinic receptors and Gαi signalling, which matches the well-established parasympathetic pathway and molecular mechanism described above.

5) Why other options are incorrect
- Option 2 (increase via β-adrenergic/Gαs): describes sympathetic signalling, not what parasympathetic neuron B does.
- Option 3 (decrease via β-adrenergic/Gαi): mixes receptor type and G-protein incorrectly—β receptors couple to Gαs, not Gαi.
- Option 4 (increase via muscarinic/Gαs): muscarinic M2 causes decrease and couples to Gαi, not Gαs, so this is wrong.

6) Conclusion
Therefore, stimulating neuron B will decrease heart rate via activation of muscarinic receptors and the Gαi signalling pathways, which matches the provided correct answer.

Based on what you know about the effects of activating these autonomic pathways, which of the following statements most accurately describes the situation?Single choice

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