Bronchial calibre is reduced by:
**Core Concept**
The regulation of bronchial calibre is a complex process involving various neural and non-neural mechanisms. The airway calibre is influenced by the balance between bronchodilatory and bronchoconstrictive forces, which are mediated by different receptors and neurotransmitters.
**Why the Correct Answer is Right**
The correct answer involves the action of a specific neurotransmitter that causes bronchoconstriction. Acetylcholine (ACh) is released from the parasympathetic nerve terminals in the airway smooth muscle and acts on muscarinic receptors to cause bronchoconstriction. This results in a decrease in bronchial calibre, making it more difficult to breathe. The M3 muscarinic receptors are primarily responsible for this response. When activated, they trigger a series of intracellular signaling events that lead to muscle contraction and bronchoconstriction.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is not directly related to the reduction of bronchial calibre. While histamine can cause bronchoconstriction, it is not the primary mechanism involved in reducing bronchial calibre.
* **Option B:** This option is incorrect because it is the opposite of what is being asked. Beta-2 adrenergic agonists cause bronchodilation, not bronchoconstriction.
* **Option C:** This option is incorrect because it is a receptor subtype that is primarily involved in the regulation of the heart rate, not bronchial calibre.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the regulation of bronchial calibre is a complex process that involves multiple receptors and neurotransmitters. Understanding the role of acetylcholine and muscarinic receptors in bronchoconstriction is crucial for the management of respiratory diseases such as asthma.
**Correct Answer: C. Acetylcholine.