Conduction System of the Heart

Overview

• Understand how electrical signals pass through the heart.
• Atria contract first, followed by ventricles.

The Journey of the Electrical Signal

Sinoatrial Node (SA Node)

• Located in the right atrium.
• Known as the pacemaker of the heart.
• Cells are autorhythmic, generating action potentials without nervous input.
• Generates approximately 75 beats per minute.
• Action potentials spread through the right and left atria, leading to atrial depolarization and contraction.

Atrioventricular Node (AV Node)

• Located at the bottom of the atria, beginning of the ventricles.
• Autorhythmic but slower than the SA node.
• Acts as a backup if the SA node fails.

AV Bundle

• Also known as the Bundle of His.
• Transmits signals from the AV node to interventricular septum.
• Autorhythmic, can function independently at a slower rate.

Bundle Branches

• Transmit signals down sides of the interventricular septum.
• Depolarizes the ventricles.

Purkinje Fibers

• Transmit signals up the sides of the ventricles.
• Ensures contraction starts at the bottom and moves upward.

Modulation of Heart Rate

Parasympathetic Input

• From vagus nerve (cranial nerve 10).
• Neurotransmitter: Acetylcholine.
• Decreases heart rate and strength of contraction.

Sympathetic Input

• From thoracolumbar branch (T1-L2).
• Neurotransmitter: Norepinephrine.
• Increases heart rate and contractility by increasing calcium availability.

Electrocardiogram (EKG/ECG)

• Measures electrical activity of the heart.
• X-axis: Time, Y-axis: Voltage.

Key Waves

• P-wave: Atrial depolarization.
• QRS Complex: Ventricular depolarization.
• T-wave: Ventricular repolarization.

Disorders

Wolf-Parkinson-White Disorder (WPW)

• Aberrant pathways in atria.
• Causes tachycardia and atrial fibrillation.
• Treated by ablating autorhythmic cells.

Ectopic Focus

• Renegade myocardial cells setting their own rhythm.
• Causes irregular heartbeats.
• Usually benign but can become dangerous.
• Influenced by stress, fatigue, caffeine, and dehydration.