The activation of muscarinic receptors in bronchiolar smooth muscle is associated with
**Question:** The activation of muscarinic receptors in bronchiolar smooth muscle is associated with
A. bronchoconstriction
B. bronchodilation
C. bronchial secretions
D. decreased airway resistance
**Correct Answer:** A. bronchoconstriction
**Core Concept:**
Muscarinic receptors are a type of receptor found on bronchiolar smooth muscle cells. They are part of the parasympathetic nervous system and are activated by acetylcholine, which plays a crucial role in the regulation of airway smooth muscle tone.
**Why the Correct Answer is Right:**
Activation of muscarinic receptors in bronchiolar smooth muscle leads to the following effects:
1. Increased intracellular calcium concentrations, which results in the contraction of smooth muscle cells and bronchoconstriction.
2. Activation of phospholipase C, which leads to the production of inositol trisphosphate (IP3) and diacylglycerol (DAG). IP3 causes the release of calcium from sarcoplasmic reticulum, contributing to bronchoconstriction. DAG activates protein kinase C, which also contributes to bronchoconstriction.
**Why Each Wrong Option is Incorrect:**
A. **Bronchodilation (Option B)** is the opposite of the correct answer (bronchoconstriction). This is incorrect because muscarinic receptor activation leads to bronchoconstriction, not bronchodilation.
B. **Bronchial secretions (Option C)** is not directly related to the activation of muscarinic receptors in bronchiolar smooth muscle. While secretions may occur as a secondary effect, the primary action is bronchoconstriction.
D. **Decreased airway resistance (Option D)** is not the direct result of muscarinic receptor activation. Instead, it is bronchoconstriction that leads to increased airway resistance, not decreased resistance.
**Clinical Pearl:**
Muscarinic receptors play a crucial role in the regulation of bronchopulmonary smooth muscle tone. Understanding this concept is essential for understanding respiratory system physiology and pathophysiology, particularly in conditions like asthma, where bronchoconstriction is a key feature.