1.3 Cardiac Conduction System and Electrical Activity

Cardiac Conduction System Components

The cardiac conduction system is a network of specialized cells that initiates and coordinates each heartbeat. Without it, the heart's chambers would contract randomly instead of in the precise sequence needed to pump blood. Understanding this system also sets the foundation for reading electrocardiograms (ECGs), which you'll use to connect electrical events to mechanical ones.

Specialized Cardiac Muscle Cells

The conduction system is made of specialized cardiac muscle cells distinct from the contractile cells that do the actual squeezing. These conduction cells have two key properties:

• Automaticity: they can spontaneously generate electrical impulses without input from the nervous system
• Conductivity: they transmit those impulses rapidly along defined pathways to coordinate contraction timing

Anatomical Structures

The conduction pathway follows a top-to-bottom route through the heart:

• Sinoatrial (SA) node: located in the posterior wall of the right atrium, near the entrance of the superior vena cava. This is where each normal heartbeat originates.
• Atrioventricular (AV) node: located in the interatrial septum near the opening of the coronary sinus. It's the only electrical connection between the atria and ventricles.
• Bundle of His (AV bundle): a tract of conduction fibers that originates from the AV node and penetrates the fibrous skeleton of the heart to enter the interventricular septum.
• Left and right bundle branches: continuations of the Bundle of His that descend along either side of the interventricular septum toward the apex.
• Purkinje fibers: fine, rapidly conducting fibers that branch off the bundle branches and spread throughout the ventricular walls, delivering the impulse to the contractile myocardium.

The fibrous skeleton that separates the atria from the ventricles acts as an electrical insulator. The Bundle of His is the only pathway through it, which forces all impulses to travel through the AV node before reaching the ventricles. This is what makes the AV delay possible.

SA Node as Pacemaker

Primary Pacemaker Function

The SA node is the heart's primary pacemaker, firing at an intrinsic rate of about 60–100 impulses per minute. Its cells depolarize spontaneously because they have unstable resting membrane potentials. Instead of sitting at a fixed voltage between beats, SA node cells slowly drift toward threshold through a mechanism called the pacemaker potential (also called the prepotential). This drift is driven by a gradual influx of sodium through "funny channels" ($I_f$), followed by calcium entry.

The autonomic nervous system modulates this rate:

• Sympathetic stimulation (norepinephrine) increases the slope of the pacemaker potential, so threshold is reached faster and heart rate rises.
• Parasympathetic stimulation (acetylcholine via the vagus nerve) decreases the slope and hyperpolarizes the cell, slowing heart rate.

At rest, parasympathetic tone dominates, which is why resting heart rate sits around 75 bpm rather than the SA node's unmodified rate of ~100 bpm.

Backup Pacemakers

The conduction system has built-in redundancy. If a higher pacemaker fails, the next one down takes over:

Each level fires more slowly than the one above it. Under normal conditions, the SA node's faster rate overrides the others before they can reach threshold on their own. This is called overdrive suppression. A backup pacemaker only "escapes" and sets the rhythm when the faster pacemaker above it fails.

Electrical Impulse Propagation Sequence

Here's the step-by-step path an impulse follows during a single heartbeat:

1. SA node fires: the impulse originates in the SA node.
2. Atrial depolarization: the impulse spreads through both atria via gap junctions between atrial muscle cells (and internodal pathways to the AV node), causing atrial contraction.
3. AV node delay: the impulse arrives at the AV node and is delayed for approximately 100 milliseconds. This pause is critical because it gives the atria time to finish contracting and push the last of their blood into the ventricles before the ventricles contract.
4. Bundle of His: the impulse exits the AV node and travels through the Bundle of His into the interventricular septum.
5. Bundle branches: the impulse splits into the left and right bundle branches, carrying it toward the apex of the heart.
6. Purkinje fibers: the impulse fans out through the Purkinje fibers, which conduct very rapidly (~4 m/s). This ensures the ventricular myocardium depolarizes almost simultaneously from apex to base.
7. Ventricular contraction: the contractile cells depolarize and the ventricles squeeze, ejecting blood into the pulmonary trunk and aorta.
8. Ventricular repolarization: the ventricles repolarize, relax, and refill for the next cycle.

The apex-to-base contraction pattern matters. It pushes blood upward toward the outflow tracts (pulmonary trunk and aorta), which exit from the top of the ventricles. If the ventricles contracted top-to-bottom, blood would be pushed the wrong way.

Electrical Activity and the ECG

The ECG records the electrical activity of the heart from the body surface. Each wave and interval on the ECG corresponds to a specific electrical event. Connecting these to the conduction pathway is one of the most testable parts of this unit.

Atrial Events

• P wave: represents atrial depolarization. It begins when the SA node fires and the impulse spreads across both atria. Atrial contraction (atrial systole) follows the P wave.
• PR interval: measured from the start of the P wave to the start of the QRS complex. It includes atrial depolarization plus the AV node delay. A normal PR interval is 0.12–0.20 seconds. A prolonged PR interval suggests the impulse is being delayed too long at the AV node (a condition called heart block).

Ventricular Events

• QRS complex: represents ventricular depolarization as the impulse travels through the Bundle of His, bundle branches, and Purkinje fibers. Despite involving a large muscle mass, the QRS complex is narrow (normally 0.06–0.10 seconds) because the Purkinje fibers conduct so quickly. Ventricular contraction (ventricular systole) begins during the QRS complex.
• ST segment: the brief period between the end of the QRS complex and the beginning of the T wave. During this time, the ventricles are fully depolarized and actively contracting. The ST segment is normally flat at baseline; elevation or depression can indicate ischemia or infarction.
• T wave: represents ventricular repolarization. The ventricles relax (ventricular diastole) after repolarization is complete.
• QT interval: measured from the start of the QRS complex to the end of the T wave. It represents the total time for ventricular depolarization and repolarization. A prolonged QT interval can increase the risk of dangerous arrhythmias.

Note that atrial repolarization does occur, but it's hidden within the QRS complex on the ECG because the much larger ventricular depolarization signal masks it.