Overview

This lecture covers the heart's electrical circuit system, detailing how electrical impulses coordinate rhythmic and synchronous heart contractions.

Electrical Conduction System of the Heart

• The heart's electrical system ensures the atria contract first, fill the ventricles, then triggers ventricular contraction after a delay.
• Sequential contraction maximizes blood ejection and ensures efficient cardiac output.

Components of the Cardiac Conduction System

• The sinus node (SA node) is the heart's pacemaker, initiating the heartbeat.
• Internodal pathways conduct the electrical signal throughout both atria for synchronized contraction.
• The AV node (atrioventricular node) delays the impulse before it reaches the ventricles, allowing ventricular filling.
• Bundle branches and Purkinje fibers distribute the impulse rapidly through the ventricles for unified contraction.

SA Node and Action Potential Generation

• The SA node is located in the right atrium near the superior vena cava and is self-excitable.
• Spontaneous depolarization occurs due to leaky sodium and calcium channels, raising the resting membrane potential.
• When threshold is reached, L-type calcium channels open, causing rapid calcium influx and the action potential.
• Potassium channels open to repolarize the membrane back to about -55 mV.

Impulse Transmission Pathway

• Signal spreads from the SA node through internodal pathways (anterior, middle, posterior) to contract both atria.
• The AV node acts as a "traffic jam" to slow the signal, then resumes rapid transmission through Purkinje fibers to both ventricles.
• AV node typically only allows one-way conduction from atria to ventricles.

Pacemaker Hierarchy and Escape Rhythms

• If the SA node fails, the AV node or Purkinje fibers can initiate heartbeats at slower rates ("escape beats").
• Delay in impulse conduction can cause fainting due to lack of brain perfusion until other pacemakers take over.

Autonomic Regulation of Heart Rhythm

• Parasympathetic (vagus nerve) releases acetylcholine, increases potassium permeability, lowers resting potential, and slows heart rate.
• Sympathetic stimulation (norepinephrine via beta-1 receptors) increases sodium and calcium permeability, raises heart rate, and strengthens contractions.

Key Terms & Definitions

• SA node (sinoatrial node) — natural pacemaker of the heart located in the right atrium.
• AV node (atrioventricular node) — delays electrical impulses before passing them to the ventricles.
• Purkinje fibers — specialized fibers that rapidly conduct impulses through the ventricles.
• Escape beat — a heartbeat initiated by secondary pacemaker sites when the SA node fails.
• Action potential — an electrical impulse that triggers heart muscle contraction.

Action Items / Next Steps

• Review Chapter 10 of Guyton and Hall's Medical Physiology on cardiac conduction.
• Study diagrams of the heart's conduction system for visual understanding.

Certainly! Here's a more detailed and comprehensive summary incorporating all the key points from the lecture:

Detailed Summary: Cardiac Electrical Conduction System (Chapter 10, Guyton and Hall Medical Physiology)

Purpose of the Cardiac Electrical Circuit

The heart’s electrical conduction system functions as a "circuit board" that coordinates the rhythmic and synchronous contraction of the heart muscle. This system ensures that the atria contract first to fill the ventricles with blood, followed by a delayed but coordinated contraction of the ventricles to efficiently eject blood. The delay between atrial and ventricular contraction is critical to allow adequate ventricular filling and maximize cardiac output. Without this precise timing, contractions would be poorly synchronized, reducing the heart’s pumping efficiency.

Components of the Cardiac Conduction System

1. Sinoatrial (SA) Node

• Located in the right atrium near the superior vena cava.
• Acts as the natural pacemaker of the heart.
• Has intrinsic self-excitation ability due to "leaky" sodium and calcium ion channels.
• Resting membrane potential is around -55 mV (less negative than typical cells), due to a slow, gradual influx of sodium through "funny current" channels.
• This slow depolarization continues until threshold is reached.
• At threshold, L-type calcium channels open, causing a rapid influx of calcium ions and generating an action potential.
• After the action potential peak, calcium channels close and potassium channels open, repolarizing the membrane back to about -55 mV.
• This cycle repeats rhythmically, producing regular impulses that initiate heartbeats.

2. Internodal Pathways

• Three main pathways (anterior, middle, posterior) that conduct the electrical impulse from the SA node throughout both atria.
• These pathways act like "highways" to rapidly spread the impulse, ensuring both atria contract simultaneously.

3. Atrioventricular (AV) Node

• Located at the junction between the atria and ventricles.
• Functions as a critical delay point ("traffic jam") in the conduction system.
• The delay is caused by fewer gap junctions and higher resistance, slowing the impulse to allow ventricles to fill with blood.
• The AV node conducts impulses in a one-way direction only—from atria to ventricles.
• In some pathological conditions, abnormal pathways can allow impulses to travel backward, causing arrhythmias.

4. Bundle of His, Bundle Branches, and Purkinje Fibers

• After the AV node, the impulse travels rapidly through the bundle of His, left and right bundle branches, and Purkinje fibers.
• These fibers act as "highways" to distribute the impulse quickly and uniformly throughout the ventricular myocardium.
• This rapid conduction ensures the ventricles contract as a single, coordinated unit.

Pacemaker Hierarchy and Escape Rhythms

• Besides the SA node, other cardiac tissues like the AV node and Purkinje fibers have pacemaker capabilities but at slower intrinsic rates.
• If the SA node fails or its impulse is blocked, the AV node can initiate impulses (escape beats) to maintain heart rhythm, though at a slower rate.
• If both SA and AV nodes fail, Purkinje fibers can generate impulses, but with an even slower rate.
• These escape rhythms prevent cardiac arrest but may cause symptoms like fainting due to delayed ventricular contraction and reduced cerebral perfusion.

Autonomic Nervous System Regulation

Parasympathetic Influence

• Parasympathetic fibers (vagus nerve) innervate mainly the SA and AV nodes.
• Release acetylcholine, which increases potassium permeability.
• This hyperpolarizes the membrane (makes it more negative), making it harder to reach threshold.
• Result: slower heart rate and delayed AV conduction.

Sympathetic Influence

• Sympathetic nerves innervate the entire heart muscle, including nodes.
• Release norepinephrine, which acts on beta-1 adrenergic receptors.
• Increases permeability to sodium and calcium ions.
• This raises the resting membrane potential, allowing threshold to be reached faster.
• Also increases calcium influx during action potentials, strengthening cardiac contractions.
• Result: increased heart rate and contractility.

Additional Notes

• The resting membrane potential of SA node cells is less negative than ventricular muscle cells, facilitating spontaneous depolarization.
• The AV node’s one-way conduction prevents retrograde impulses under normal conditions.
• Abnormal conduction pathways can cause arrhythmias by allowing impulses to re-enter atrial tissue.
• The delay at the AV node is essential for proper timing between atrial and ventricular contractions.

If you want, I can help you create a study guide or highlight key diagrams to visualize this conduction system better! Just let me know.