Which effect does the parasympathetic nervous system have on bronchioles?

Which effect does the parasympathetic nervous system have on bronchioles?

• A. Bronchodilation
• B. Bronchoconstriction
• C. No effect
• D. Variable

Answer: (B)

Parasympathetic activation causes bronchoconstriction. When the vagus nerve releases acetylcholine onto muscarinic receptors (primarily M3) on airway smooth muscle, it triggers signaling that increases intracellular calcium and leads to smooth muscle contraction around the bronchioles. This narrows the airways and raises airway resistance. The sympathetic system, in contrast, acts on beta-2 receptors to relax these muscles and cause bronchodilation, which is the opposite effect. Histologically, bronchioles contain smooth muscle rings that respond to autonomic input, so parasympathetic tone tightens those muscles and constricts the lumen.

Parasympathetic activation causes bronchoconstriction. When the vagus nerve releases acetylcholine onto muscarinic receptors (primarily M3) on airway smooth muscle, it triggers signaling that increases intracellular calcium and leads to smooth muscle contraction around the bronchioles. This narrows the airways and raises airway resistance. The sympathetic system, in contrast, acts on beta-2 receptors to relax these muscles and cause bronchodilation, which is the opposite effect. Histologically, bronchioles contain smooth muscle rings that respond to autonomic input, so parasympathetic tone tightens those muscles and constricts the lumen.