Lecture Notes: Conduction of the Cardiac System

Overview

• The cardiac conduction system is the electrical current that moves through the heart muscle (myocardium) to initiate contraction.
• Heart muscle cells are specialized and interconnected with gaps allowing for easy transmission of electrical charge.
• At rest, heart muscle cells have a negative charge, which must turn positive for contraction, known as depolarization.

Depolarization

• Depolarization: Change from negative to positive charge; essential for muscle contraction.
• Spread of positive charge from cell to cell facilitates heart muscle contraction.

Sinoatrial Node (SA Node)

• Located in the atria, known as the pacemaker of the heart.
• Sets the sinus rhythm, about 60-100 beats per minute.
• Spontaneously depolarizes, sending positive charge through the atrial muscle.
• Results in the first positive deflection on an ECG or EKG.

Electrocardiogram (ECG/EKG)

• Measures charge changes in the heart muscle using electrodes.
• Positive charge towards a lead results in an upward deflection on the ECG.
• Lead II often used as a reference lead, viewing from the apex towards the base of the heart.

Atrioventricular Node (AV Node)

• Located between atria and ventricles.
• Causes a pause on ECG (~0.2 seconds) before sending positive charge through the bundle of His.

Bundle of His and Purkinje Fibers

• Bundle of His: Specialized fibers conducting charge through ventricles.
• Branch into Purkinje fibers, spreading charge through the ventricular myocardium.
• Results in a large upward deflection on ECG due to thicker left ventricular muscle.

Repolarization

• The resetting of the heart, moving back to a negative charge.
• Results in different wave patterns on ECG.

ECG Waves

• P Wave: Atrial depolarization.
• QRS Complex: Ventricular depolarization.
• T Wave: Ventricular repolarization.
• Atrial repolarization occurs during ventricular depolarization and is not visible as a distinct wave.

Summary

• The heart's electrical activity involves sequential depolarization and repolarization.
• Important structures: SA Node, AV Node, Bundle of His, Purkinje fibers.
• Key ECG components: P wave, QRS complex, T wave.
• Depolarization leads to muscle contraction, while repolarization resets the heart for the next cycle.