7.1c Cardiac Conduction Pathway

Overview:

• The heart’s rhythm is coordinated by a specialized conduction system comprised of cardiomyocytes, not nerves.
• This system starts with the SA node (sinoatrial node), the primary pacemaker.

SA Node (Sinoatrial Node):

• Located in the roof of the right atrium.
• Composed of specialized cardiomyocytes that spontaneously depolarize due to leak channels.
• Pacemaker of the heart: depolarizes at 80-100 beats per minute naturally.
• Under parasympathetic influence (“rest and digest”), the heart rate slows to 60-80 bpm.

Electrical Wave Propagation:

• Depolarization spreads rapidly through gap junctions in the atrial myocardium.
• Wave travels via internodal pathways to the AV node (located at the floor of the right atrium).
• The delay at the AV node (pause) allows the atria to contract and ventricles to fill before ventricular depolarization.

Ventricular Conduction:

• From the AV node, the wave moves down the atrioventricular (AV) bundle (bundle of His).
• It divides into left and right bundle branches.
• The wave then spreads via the Purkinje fibers throughout the ventricular myocardium.
• Synchronization: All ventricular cells depolarize together via gap junctions, leading to coordinated contraction.

Intrinsic Pacemaker Rates:

• SA node: 80-100 bpm (normal resting HR 60-80 bpm due to parasympathetic tone).
• AV node: 60-80 bpm (if SA node fails).
• Bundle of His and Purkinje fibers: 20-40 bpm (can support life temporarily if primary nodes are damaged but is insufficient long-term).

Heart Innervation & Control:

• Parasympathetic fibers from the vagus nerve modulate the SA node, slowing HR during rest.
• Sympathetic stimulation increases HR during stress or activity.

Heart Transplant Consideration:

• Post-transplant, the heart lacks parasympathetic innervation.
• Heart rate is typically higher (~80-100 bpm at rest).
• Surgeons cannot reattach parasympathetic nerves; they “reconnect” major vessels, not nerves.

Pathophysiology & Clinical Implications:

• If the SA node or internodal pathways are damaged or fail, the AV node becomes the secondary pacemaker.
• Pacemaker implantation: Used if intrinsic conduction is insufficient, especially if damage is extensive.

Its rate supports life but may require intervention for optimal heart function.

Summary:

The heart’s electrical activity begins with the SA node, setting a regular rhythm. The conduction system ensures synchronized contraction of atria and ventricles. Damage to primary nodes can be compensated temporarily by secondary pacemakers, but long-term support often requires a pacemaker. Understanding this pathway is crucial in diagnosing arrhythmias and designing appropriate treatments.