Which receptor type mediates the increase in heart rate when the sympathetic nervous system is activated?

Explanation

## Sympathetic Control of Heart Rate: β1-Adrenergic Mechanism **Key Point:** β1-adrenergic receptors on cardiac pacemaker cells (SA node) mediate the positive chronotropic effect of sympathetic stimulation. Noradrenaline (norepinephrine) released from postganglionic sympathetic terminals binds these receptors. ### Mechanism of β1 Activation at the SA Node 1. **Receptor activation:** Noradrenaline binds β1-adrenergic receptors 2. **G-protein coupling:** Gs protein activation → ↑ adenylyl cyclase 3. **cAMP cascade:** ↑ cAMP → ↑ PKA activity 4. **Ion channel effects:** - Increased L-type Ca²⁺ channel opening (faster diastolic depolarization) - Increased K⁺ channel activity (less hyperpolarization) 5. **Result:** Faster spontaneous depolarization → increased heart rate (positive chronotropic effect) ### Sympathetic vs. Parasympathetic Effects on Heart Rate | Parameter | Sympathetic (β1) | Parasympathetic (M2) | |-----------|------------------|----------------------| | Neurotransmitter | Noradrenaline | Acetylcholine | | Receptor | β1-adrenergic | M2 muscarinic | | 2nd messenger | ↑ cAMP | ↓ cAMP | | Heart rate effect | ↑ (positive chronotropic) | ↓ (negative chronotropic) | | Mechanism | ↑ Ca²⁺ influx, ↑ diastolic slope | ↑ K⁺ efflux, ↓ Ca²⁺ influx | **High-Yield:** β1-adrenergic receptors are the **primary cardiac sympathetic receptors**. They are found on: - Sinoatrial node pacemaker cells - Atrioventricular node - Atrial muscle - Ventricular muscle (inotropic effect) **Mnemonic:** **"β1 = Heart (1 heart)"** — β1 receptors are the main sympathetic receptors in the heart. β2 receptors are found in bronchial and vascular smooth muscle. **Clinical Pearl:** β-blockers (e.g., metoprolol, propranolol) antagonize β1 receptors and reduce heart rate and contractility — commonly used in hypertension, angina, and post-MI management.