## Sinoatrial Node: Anatomy and Physiology ### Correct Statements (Options 0, 1, 2) **Key Point:** The SA node is the heart's primary pacemaker, located at the junction of the superior vena cava (SVC) and right atrium in the terminal groove (sulcus terminalis). | Feature | Details | |---------|----------| | **Location** | SVC-RA junction; terminal groove (sulcus terminalis) | | **Blood Supply** | Right coronary artery (60–70% of people); left circumflex in remainder | | **Intrinsic Rate** | 60–100 bpm (fastest automaticity in the heart) | | **Innervation** | Sympathetic (increases rate); Parasympathetic (decreases rate) | | **Histology** | P cells (pacemaker cells) with sparse myofibrils | ### Why Option 3 Is Incorrect **High-Yield:** Parasympathetic (vagal) stimulation **decreases** SA node firing rate and AV nodal conduction, NOT increases it. Acetylcholine (ACh) hyperpolarizes nodal cells, slowing spontaneous depolarization. **Clinical Pearl:** Vagal maneuvers (Valsalva, carotid massage) and adenosine slow the SA node and AV node by increasing parasympathetic tone — this is the basis of their use in terminating supraventricular tachycardia (SVT). **Mnemonic:** **SAND** — Sympathetic Accelerates, parasympathetic (vagal) decelerates Node and AV node. ### Autonomic Effects on Cardiac Conduction | System | SA Node | AV Node | Ventricular Muscle | |--------|---------|---------|--------------------| | **Sympathetic** | ↑ Rate | ↑ Conduction | ↑ Force, ↑ Rate | | **Parasympathetic** | ↓ Rate | ↓ Conduction | ↓ Force (atria mainly) | [cite:Gray's Anatomy 42e Ch 3]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.