Which is the most common neurotransmitter receptor type responsible for parasympathetic effects on cardiac rate and contractility?

Explanation

## Muscarinic Receptor Subtypes and Cardiac Function **Key Point:** M2 muscarinic receptors are the predominant parasympathetic receptors on cardiac tissue, mediating negative chronotropic and inotropic effects via G~i~ protein coupling. ### Muscarinic Receptor Distribution and Function | Receptor | Primary Location | G-protein | Main Effect | Cardiac Role | | --- | --- | --- | --- | --- | | **M1** | CNS, ganglia, salivary glands | G~q~ | Excitation, secretion | Minimal | | **M2** | Heart (SA/AV node, atria), lungs, smooth muscle | G~i~ | **Inhibition, ↓ cAMP** | **Primary: ↓ HR, ↓ contractility** | | **M3** | Smooth muscle, glands, endothelium | G~q~ | Contraction, secretion | Minimal | | **M4** | CNS | G~i~ | Inhibition | Absent in heart | | **M5** | CNS | G~q~ | Excitation | Absent in heart | **High-Yield:** M2 receptors couple to **G~i~ proteins**, which: 1. **Decrease cAMP** → reduced PKA activity → ↓ L-type Ca²⁺ channel opening 2. **Activate K⁺ channels** (G~i~βγ subunits) → hyperpolarization of SA/AV nodal cells 3. Result: **negative chronotropic effect** (↓ HR) and **negative inotropic effect** (↓ contractility, especially atria) ### Mechanism of M2-Mediated Cardiac Inhibition ```mermaid flowchart TD A[Acetylcholine released from vagal nerve terminals]:::action --> B[Binds M2 muscarinic receptors on cardiac myocytes]:::action B --> C[Activates Gi protein]:::action C --> D{Two parallel pathways}:::decision D -->|Pathway 1| E[↓ Adenylyl cyclase activity]:::action D -->|Pathway 2| F[Activate G-protein-coupled K+ channels]:::action E --> G[↓ cAMP levels]:::outcome F --> H[Hyperpolarization of nodal cells]:::outcome G --> I[↓ L-type Ca2+ influx]:::outcome I --> J[Decreased SA/AV nodal automaticity]:::outcome H --> J J --> K[Negative chronotropic effect]:::outcome ``` **Clinical Pearl:** Atropine (M1/M2/M3 antagonist) blocks these M2 effects, causing tachycardia — a classic exam finding and clinical sign of anticholinergic toxicity. Selective M2 antagonists would theoretically preserve parasympathetic effects on other organs while sparing the heart. **Mnemonic:** **"M2 = Minus"** — M2 mediates the *minus* (inhibitory) effects of parasympathetic stimulation on the heart (↓ rate, ↓ force).