Which of the following statements about the parasympathetic nervous system is correct?

Question

BlackTom AI · Accepted Answer

Parasympathetic preganglionic neurons use acetylcholine (ACh) as their primary neurotransmitter, releasing it onto nicotinic receptors on the postganglionic neuron; the postganglionic parasympathetic fibers then also release ACh onto muscarinic receptors on target organs.

1) Relevant knowledge points:
- Autonomic nervous system has two-neuron chains: preganglionic (CNS → autonomic ganglion) and postganglionic (ganglion → effector).
- Both sympathetic and parasympathetic preganglionic fibers release ACh onto nicotinic (N-type) receptors on postganglionic neurons.
- Parasympathetic postganglionic fibers release ACh that acts primarily at muscarinic receptors on effector tissues.
- The adrenal medulla is innervated by sympathetic preganglionic fibers and functions like a modified sympathetic ganglion, releasing epinephrine/norepinephrine into the bloodstream.

2) Step-by-step reasoning to the correct choice:
- The statement to verify: “The primary transmitter released by parasympathetic preganglionic neurons is acetylcholine.” Based on the anatomy and pharmacology above, parasympathetic preganglionic neurons indeed release ACh onto nicotinic receptors in the ganglion. Therefore this statement is true.

3) Calculations: none required; reasoning is conceptual.

4) Why the correct answer is correct:
- It matches established neurophysiology: parasympathetic preganglionic axons are cholinergic (ACh-releasing). This is a fundamental distinguishing feature of autonomic neurotransmission.

5) Why the other options are incorrect:
- Option A: Parasympathetic preganglionic axons do not innervate the adrenal medulla; sympathetic preganglionic fibers do, and they stimulate catecholamine release.
- Option B: While ACh from preganglionic fibers acts via nicotinic receptors in ganglia, ACh released by parasympathetic axons at organs acts mainly through muscarinic receptors — so the statement is false as worded.
- Option D: Innervation density varies by organ; it is not true that every organ has greater parasympathetic than sympathetic innervation.

6) Conclusion: The correct answer is “The primary transmitter released by parasympathetic preganglionic neurons is acetylcholine,” which matches the provided answer.

Which of the following statements about the parasympathetic nervous system is correct?Single choice

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