Chapter 1: Basic Anatomy and Physiology of the Cardiac Conduction System

Chapter 1: Basic Anatomy and Physiology of the Cardiac Conduction System

# 1. Location of the Major Components of the Conduction System:

• Sinoatrial (SA) node: Located near the opening of the superior vena cava into the right atrium.

• Atrioventricular (AV) node: Located in the floor of the right atrium, near the interventricular septum, opposite the opening of the coronary sinus into the right atrium.

• Bundle of His: A short structure that starts from the junction of the AV node, then immediately divides into two branches:

• Right branch: A long, thin branch, running along the right side of the interventricular septum, ending in the Purkinje network in the endocardium of the right ventricle.

• Left branch: Divides into 3 separate branches like a fan:

• Left anterior: The anterior wall of the left ventricle.

• Left middle: The interventricular septum, combining with the anterior and posterior branches.

• Left posterior: Runs along the posterior wall before ending in the Purkinje network.

# 2. Types of Electrically Active Cells in the Heart:

• Myocardial cells: Present in the atria and ventricles, responsible for contraction.

• Conduction cells: Capable of rapid conduction, found in the Purkinje network.

• Pacemaker cells: Have the ability to spontaneously depolarize, present in the SA node and throughout the His-Purkinje system.

# 3. Action Potential of Myocardial Cells:

• The action potential of myocardial cells has 5 phases:

• Phase 4: Resting potential = -90mV.

• Phase 0: Depolarization of the cell, potential from -90 to +20mV due to the opening of fast Na+ channels.

• Phase 1: Gradual decrease in potential from the peak to 0mV.

• Phase 2: Plateau phase, potential maintained around 0mV.

• Phase 3: Rapid repolarization brings the cell back to its initial polarized state (-90mV).

# 4. Comparison of Action Potentials of the SA Node and Ventricular Muscle:

• SA Node:

• Shows spontaneous depolarization in phase 4 (Na+ enters the cell, increasing the potential).

• Resting potential is only -60mV, depolarization reaches only +10mV, threshold is -40mV.

• Ventricular Muscle:

• No spontaneous depolarization in phase 4.

• Resting potential is -90mV, depolarization reaches +20mV, threshold is -70mV.

# 5. Repolarization of Heart Cells:

• Absolute refractory period: Cells do not respond to any stimulus (includes phases 1 and 2).

• Effective refractory period: Cells generate a potential if stimulated, but it is very small and does not propagate (includes a small part of phase 3).

• Relative refractory period: Cells are partially repolarized, capable of responding to stronger than normal stimuli (includes part of phase 3 to the threshold of -70mV).

• Supernormal period: Cells respond to weaker than normal stimuli (includes the end of phase 3 when the potential is below the threshold of -70mV).

# 6. Structure and Function of the SA Node:

• Structure: The head originates mostly from the epicardium, at the junction of the superior vena cava and right atrium, the tail mostly under the endocardium.

• Function: Composed of pacemaker cells that have the fastest firing rate, making it the pacemaker of the heart.

• Blood supply: By the SA nodal artery: 65% from the right coronary artery, 35% from the left coronary artery.

# 7. Internodal Pathways:

• Structure connecting the SA node to the AV node, consisting of 3 parts: anterior, middle, posterior.

# 8. Structure of the AV Node:

• Consists of 3 parts:

• Upper part (AN): Has automaticity.

• Middle part (N): Responsible for signal delay, is the site of acetylcholine release.

• Lower part (NH): Has automaticity.

• Blood supply: By the right coronary artery in 90% of cases.

# 9. Characteristics of Conduction Through the His-Purkinje System:

• Significantly affected by the parasympathetic system and not significantly by the sympathetic system.

# 10. Blood Supply to the Bundle of His:

• By the anterior descending coronary artery and the posterior descending coronary artery (from both the right and left coronary arteries).

# 11. Mechanism of Digitalis:

• Inhibits Na+-K+ ATPase, Na+ must exit through the Na+-Ca2+ antiporter, increasing Ca2+ accumulation in myocardial cells.

# 12. Correlation of Action Potential with ECG Waves:

• Phase 0: QRS complex.

• Phase 1 and beginning of phase 2: J point (end of QRS).

• Phase 2: ST segment.

• Phase 3: T wave.

• Phase 4: T-Q segment.

# 13. Reasons for Faster Conduction Through the His-Purkinje System:

• Due to a lower resting potential of -95mV compared to other cells, depolarization is faster, phase 0 is steeper, higher, and the action potential lasts longer.

• Conduction is 5 times faster than myocardial cells.

# 14. Pacemaker Ability of Cells:

• SA node cells: Have the ability to generate rhythm independent of stimulation, the fastest spontaneous firing rate due to the greatest rate of depolarization.

• Scattered cells in the His-Purkinje system: Have the ability to generate rhythm slower than the SA node.

• Atrial and ventricular myocardial cells: Do not have the ability to generate rhythm, but if damaged or ischemic, they have this ability.

• Note: Cells closer to the AV node will have a faster rate of spontaneous depolarization, so if the SA node fails, the cells in the AV node at the junction with the Bundle of His are often the next pacemaker.