Breathing is mediated by the autonomic nervous system. What occurs during parasympathetic control? What occurs during sympathetic control?

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Airway diameter is controlled by the autonomic nervous system, with opposite effects on the bronchial smooth muscle. Parasympathetic activation causes bronchial smooth muscle contraction, leading to bronchoconstriction, and can increase mucus secretion. Sympathetic activation, on the other hand, relaxes the bronchial smooth muscle via beta-2 adrenergic receptors (responsive to epinephrine/norepinephrine), causing bronchodilation and opening the airways. So, during parasympathetic control you get bronchoconstriction, and during sympathetic control you get bronchodilation. This is why beta-2 agonists are used to treat bronchoconstriction in conditions like asthma.