Cardiac Conduction System and ECG

Cardiac Conduction System Function

Coordinates the timing of atrial and ventricular contractions.

Primary Pacemaker of the Heart

SA node (located in the right atrium; has the fastest spontaneous rate of depolarization).

Atrial Pathways Function

Spread electrical excitation through both atria, leading to atrial contraction.

AV Node Function

Briefly delays the electrical signal to allow ventricles to fill with blood.

AV Bundle (Bundle of His) Function

Conducts the electrical signal through the fibrous skeleton.

Right and Left Bundle Branches Function

Carry the electrical signal down the interventricular septum.

Purkinje Fibers Function

Rapidly distribute excitation through ventricular muscle to produce coordinated contraction.

Pacemaker vs. Contractile Cells (Main Function)

Pacemaker cells generate and conduct electrical signals; contractile cells produce mechanical force.

Pacemaker vs. Contractile Cells (Resting Potential)

Pacemaker cells have an unstable resting potential that slowly depolarizes; contractile cells have a relatively stable resting membrane potential.

Pacemaker Cell Action Potential: Depolarization

Voltage-gated calcium channels open, allowing calcium to enter the cell.

Pacemaker Cell Action Potential: Repolarization

Calcium channels close and potassium leaves the cell, making the membrane more negative.

Contractile Cell Action Potential: Rapid Depolarization

Voltage-gated sodium channels open, allowing sodium to enter the cell.

Contractile Cell Action Potential: Plateau Phase

Calcium enters through L-type calcium channels while potassium leaves, sustaining the plateau.

Electrical Role of Calcium

Calcium current contributes to the plateau phase of a contractile cell's action potential.

Mechanical Role of Calcium

Triggers calcium release from the sarcoplasmic reticulum and binds to troponin to allow actin-myosin interaction.

Calcium-Induced Calcium Release

Calcium entering through L-type channels triggers further calcium release from the sarcoplasmic reticulum.

Electrocardiogram (ECG) Measurement

Records the electrical activity of the heart from the body's surface, not actual contraction or pressure.

P Wave (ECG)

Represents atrial depolarization, followed by atrial contraction.

PR Segment (ECG)

Represents the signal traveling through the AV node, providing a delay for ventricular filling.

QRS Complex (ECG)

Represents ventricular depolarization, followed shortly by ventricular contraction.

ST Segment (ECG)

Represents the period when ventricles are largely depolarized in their plateau phase, contracting and ejecting blood.

T Wave (ECG)

Represents ventricular repolarization, followed by ventricular relaxation.

Hidden ECG Event

Atrial repolarization occurs during the QRS complex and is hidden by the larger ventricular signal.

Depolarization vs. Systole

Depolarization is an electrical event, whereas systole is mechanical contraction.