Which ONE of the following functions is not affected by the parasympathetic nervous system?

Question

BlackTom AI · Accepted Answer

Parasympathetic activity promotes “rest-and-digest” responses: it increases salivation, enhances gastrointestinal motility and secretion, and causes bronchoconstriction (thus altering airway patency), but it does not provide the principal vasomotor tone for most systemic blood vessels.

1. Relevant knowledge points and concepts
- The autonomic nervous system has two main divisions: sympathetic (fight-or-flight) and parasympathetic (rest-and-digest).
- Parasympathetic fibers release acetylcholine acting on muscarinic receptors to increase secretions and smooth muscle activity in the gut and airways.
- Vascular tone of most systemic blood vessels is primarily under sympathetic control (through norepinephrine on α-adrenergic receptors); parasympathetic influence on systemic vasculature is minimal or absent, except in special regions (e.g., erectile tissue).

2. Step-by-step reasoning
- Evaluate each option for parasympathetic influence:
  a) Production of saliva: Parasympathetic stimulation strongly increases salivary gland secretion (via facial and glossopharyngeal nerves).
  b) Patency of the respiratory airways: Parasympathetic stimulation causes bronchoconstriction and increased bronchial secretions, thus affecting airway patency.
  c) Tone of most blood vessels: Most blood vessels’ basal tone is maintained by sympathetic vasomotor fibers; parasympathetic fibers generally do not innervate most systemic arterioles.
  d) Digestion: Parasympathetic activity enhances peristalsis and digestive secretions, strongly affecting digestion.

3. Calculations/intermediate steps
- Not applicable (conceptual physiology question; no numeric calculations required).

4. Why the correct answer is correct
- "Tone of most blood vessels" is not affected by the parasympathetic nervous system because vasomotor control for most systemic vessels is mediated by sympathetic adrenergic pathways. Parasympathetic vasodilation occurs only in limited sites (e.g., genital erectile tissue), so it does not influence the tone of most vessels.

5. Why the other options are incorrect
- Production of saliva, patency of the airways (via bronchoconstriction), and digestion are all directly promoted or modulated by parasympathetic action, so they are affected by the parasympathetic system.

6. Conclusion
- Therefore, the correct choice is "Tone of most blood vessels," which matches the provided answer.

Which ONE of the following functions is not affected by the parasympathetic nervous system?单项选择题

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