An increase in sympathetic activity to the bronchi will cause what?

• A. Constriction of the bronchi
• B. No change in the bronchi
• C. Increased mucus production in the bronchi
• D. Dilation of the bronchi

Answer: (D)

Sympathetic stimulation of the bronchi causes bronchodilation. This happens when nerves release norepinephrine that binds to β2-adrenergic receptors on airway smooth muscle. The receptor activates a Gs protein, increasing cAMP, which lowers intracellular calcium and relaxes the smooth muscle, widening the airways and improving airflow. This is the mechanism behind fast-acting beta-agonist inhalers used to relieve bronchospasm. The other options don’t fit because constriction is driven by parasympathetic/cholinergic activity, mucus production is not driven by sympathetic β2 activation, and there is indeed a change in airway caliber with sympathetic stimulation.