Sympathetic stimulation of B-2 receptors will result in what effect on bronchial smooth muscle?

• A. Constrict
• B. Relax/dilate
• C. Have no effect
• D. Increase mucus production

Answer: (B)

Sympathetic stimulation of beta-2 receptors in the bronchial smooth muscle leads to relaxation and dilation of the airways. This mechanism is crucial for increasing airflow to the lungs, especially during instances of stress or physical activity when the body requires additional oxygen intake. When catecholamines like epinephrine bind to these beta-2 adrenergic receptors, a series of intracellular events unfolds, resulting in smooth muscle relaxation.

This response is particularly important in conditions such as asthma, where bronchodilators targeting beta-2 receptors are used to relieve bronchoconstriction, allowing patients to breathe more easily. The relaxation of bronchial smooth muscle can help reduce resistance in the airways and improve ventilation.

The other options do not accurately describe the effects of beta-2 receptor activation. Constriction of the airways would be contrary to the action seen with sympathetic stimulation; having no effect contradicts the well-documented physiological role of these receptors; and increased mucus production is more associated with other pathways, often cytokine-driven processes rather than directly linked to beta-2 receptor action. Thus, the choice indicating relaxation and dilation aligns with the established role of beta-2 adrenergic stimulation in the respiratory system.