What is the action of Acetylcholine in the respiratory system?

Acetylcholine plays a significant role in the autonomic regulation of the respiratory system. When released by the parasympathetic nervous system, it binds to muscarinic receptors on the smooth muscle of the airways, leading to bronchoconstriction. This is a crucial physiological response, particularly during the regulation of airway tone.

Bronchoconstriction reduces the diameter of the airways, which can increase resistance to airflow and may be significant in conditions like asthma. This action contrasts with substances that promote bronchodilation, which increase airway diameter to enhance airflow. Therefore, the identification of bronchoconstriction as the correct answer underscores the importance of acetylcholine’s role in the regulation of bronchial smooth muscle tone, differentiating it from other factors that contribute to airway dynamics.