Which of the following occurs when the peripheral nervous system (PNS) is stimulated in airway smooth muscle?

When the peripheral nervous system (PNS) is stimulated in airway smooth muscle, it leads to increased intracellular calcium levels and bronchoconstriction. This occurs primarily through the action of acetylcholine, which is released from the parasympathetic fibers. Acetylcholine binds to muscarinic receptors on the airway smooth muscle, triggering a series of intracellular events that result in the release of calcium ions from the sarcoplasmic reticulum. As calcium levels rise, the smooth muscle contracts, leading to bronchoconstriction, which narrows the airways.

This physiological response is crucial for various reflex actions, such as narrowing the airways during an asthma attack to limit exposure to irritants. Understanding this mechanism highlights the role of the PNS in managing respiratory responses and the implications it has for conditions such as asthma and chronic obstructive pulmonary disease (COPD).

In contrast to this response, increasing cyclic AMP levels or decreasing intracellular calcium would lead to smooth muscle relaxation, which is associated with sympathetic nervous system stimulation and certain bronchodilator medications rather than PNS activity.