### **Core Concept**
Muscarinic receptors are a subtype of cholinergic receptors that are activated by acetylcholine. They are G-protein coupled receptors that can modulate various physiological processes, including smooth muscle contraction, glandular secretion, and heart rate regulation.

### **Why the Correct Answer is Right**
The correct answer involves a statement related to the muscarinic receptor's role in the parasympathetic nervous system. Muscarinic receptors, particularly M2 subtype, are known to decrease heart rate by inhibiting the L-type calcium channels in the sinoatrial (SA) node. This action is mediated through the Gi protein, which reduces the intracellular cAMP levels, leading to a decrease in heart rate.

### **Why Each Wrong Option is Incorrect**
**Option A:** This statement is incorrect as it does not accurately describe the role of muscarinic receptors in the parasympathetic nervous system. Muscarinic receptors are not primarily involved in the regulation of blood pressure through vasodilation.

**Option B:** This statement is incorrect as it misrepresents the effect of muscarinic receptor activation on heart rate. While muscarinic receptors can cause bradycardia, they do not have a direct effect on cardiac contractility.

### **Clinical Pearl / High-Yield Fact**
It is essential to remember that muscarinic receptors, particularly M2 subtype, play a crucial role in the regulation of heart rate and cardiac function. Understanding the mechanism of action of these receptors is critical in managing conditions such as atrioventricular block and heart failure.

### **Correct Answer: C.**