Which effect does the sympathetic nervous system have on bronchioles?

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

Answer: (A)

Activation of the sympathetic nervous system relaxes the smooth muscle of the bronchioles, causing bronchodilation. This mainly happens through beta-2 adrenergic receptors on airway smooth muscle. When these receptors are stimulated by norepinephrine (from sympathetic nerves) or circulating epinephrine, the Gs protein pathway raises cyclic AMP, which reduces intracellular calcium and inhibits contraction, leading to muscle relaxation. The end result is widened airways and easier airflow, which supports increased ventilation during stress or exercise. By contrast, parasympathetic input promotes bronchoconstriction, so the sympathetic system’s dominant effect on bronchioles is bronchodilation.