BIO 004 · Human Anatomy
The Cardiac Conduction System
The Cardiac Conduction System and Disorders
A reference for the cardiac conduction video and lab. This page covers the conduction pathway and its components, the nerve supply to the heart, what an electrocardiogram records, and common conduction disorders.
How to use this sheet
Notes prints the full reference for review.
Study prints as a fill-in-the-blank worksheet to complete while you watch the video or read.
Quiz me is on-screen practice: type the term, or click Reveal to check yourself.
The tables and sequences respond to Study and Quiz too, with a Reveal button on each row.
- Trace the conduction pathway from the SA node to the Purkinje fibers.
- Locate each component of the conduction system and state its role.
- Describe the nerve supply to the heart and what an ECG records.
- Name common conduction disorders.
Your pre-work
Work through these the evening before class. None of it is turned in. It is how you learn the material and build your spaced recall.
This is more than a checklist. Ticking these boxes is the start, not the finish. Committing this material to memory and being able to apply it takes considerable time and repeated effort. You are not done when the boxes are checked. Put in the real hours, and keep coming back for frequent recall and review until the material is genuinely yours.
The Conduction System, an Overview
The heart sets its own beat. A network of specialized cardiac muscle cells, the conduction system, generates each heartbeat and spreads it so the chambers contract in the right order.
- Cardiac conduction systemthe network of specialized cardiac muscle cells that generates and spreads the signal for each heartbeat
- Autorhythmicitythe heart's built-in ability to generate its own rhythm without a signal from the nervous system
- Conducting cellsspecialized non-contractile cardiac cells that generate and carry the electrical signal
- Contractile cellsthe ordinary working cardiomyocytes that contract when the signal reaches them
- Why the order mattersthe conduction system makes the atria contract first and the ventricles second, so each chamber fills before it empties
The Conduction Pathway
Each heartbeat travels a fixed route through the conduction system. Follow the signal in order, from the pacemaker to the ventricle walls.
- SA nodethe sinoatrial node, the pacemaker; it fires first and sets the pace of the whole heart
- Across the atriathe signal spreads through both atria, and the atria contract
- AV nodethe atrioventricular node; the signal pauses here briefly, giving the atria time to finish emptying
- AV bundlethe atrioventricular bundle, or bundle of His; it carries the signal into the interventricular septum, the only electrical link between the atria and the ventricles
- Bundle branchesthe right and left bundle branches carry the signal down both sides of the interventricular septum toward the apex
- Purkinje fibersspread the signal through the walls of both ventricles, and the ventricles contract from the apex upward
The Components of the Conduction System
The pathway is built from five named structures. Compare each by where it sits and what it does.
| Structure | Location | Role |
|---|---|---|
| SA node | the wall of the right atrium, near the opening of the superior vena cava | the natural pacemaker; it fires fastest and sets the heart rate |
| AV node | the floor of the right atrium, near the interatrial septum | delays the signal briefly, then passes it on to the ventricles |
| AV bundle | the superior part of the interventricular septum | the only electrical bridge from the atria to the ventricles |
| Bundle branches | the right and left sides of the interventricular septum | carry the signal down toward the apex of the heart |
| Purkinje fibers | throughout the walls of both ventricles | deliver the signal to the contractile cells of the ventricle walls |
Nerve Supply to the Heart
The conduction system runs on its own, but the nervous system can speed it up or slow it down. Autonomic nerves reach the SA and AV nodes and adjust the rate to the body's needs.
- Cardiac centersregions of the medulla oblongata in the brainstem that adjust the heart rate
- Cardiac plexusthe network of autonomic nerve fibers near the base of the heart through which these nerves reach the nodes
- Sympathetic nervesautonomic nerves that reach the heart through the cardiac plexus and speed the heart rate
- Parasympathetic nervesautonomic fibers carried in the vagus nerve that slow the heart rate
- A modulating rolethese nerves do not start the heartbeat; they only adjust the pace the SA node already sets
The Electrocardiogram
An electrocardiogram, an ECG or EKG, is a tracing of the heart's electrical activity recorded from the body surface. Each deflection of one heartbeat lines up with an event in the conduction system. Compare the three.
| Deflection | Conduction event | What follows |
|---|---|---|
| P wave | the signal spreads across the atria | the atria contract |
| QRS complex | the signal spreads across the ventricles | the ventricles contract |
| T wave | the ventricles electrically recover | the ventricles relax |
Conduction Disorders
When the conduction system fails, the rhythm goes wrong. Compare the common disorders by what happens and why it matters.
| Disorder | What happens | Why it matters |
|---|---|---|
| Arrhythmia | any deviation from the normal heart rhythm | the umbrella term for the conduction disorders below |
| Tachycardia | an abnormally fast resting heart rate | the chambers may not have time to fill between beats |
| Bradycardia | an abnormally slow resting heart rate | the body may not receive enough blood flow |
| Heart block | the signal is delayed or stopped at the AV node | the atria and ventricles can beat out of step |
| Atrial fibrillation | rapid, disorganized signals in the atria | the atria quiver instead of contracting; common and treatable |
| Ventricular fibrillation | rapid, disorganized signals in the ventricles | the ventricles cannot pump; a life-threatening emergency |
| Ectopic pacemaker | a site outside the SA node takes over the rhythm | the normal pacing order of the heart is lost |
See also: The Heart for the chambers the conduction system drives, and Blood Vessels, Structure and Types, the next page in this block.
Study questions
Work on answering these in writing, in your own words. They are the questions to bring to class, and good practice for the reasoning the exams ask for.
- Trace an impulse through the cardiac conduction system in order, naming each structure.
- Why does the impulse pause at the AV node, and what does that delay allow?
- Explain why the SA node sets the heart rate even though other structures can also fire.
- Connect the conduction pathway to the order in which the heart chambers contract.
Step 2 . Retrieval check
Now explain it back, in your own words.
In 60 words or more, pull together what the video just taught you. Include the key concepts. This is the point where the learning actually sticks. After you submit, your spaced-recall cards for this topic unlock.