The Nodal (Conducting) System of the Heart

Overview Diagram

Overview of the cardiac conducting system - Junqueira's Basic Histology, 17e

Components of the Nodal/Conducting System

1. Sinoatrial (SA) Node - The Pacemaker

• Location: Beneath the epicardium on the posterior surface of the right atrium, near the opening of the superior vena cava, in the sulcus terminalis.
• Also called: Keith-Flack node, or simply the pacemaker.
• Size: Spindle-shaped, about 10 mm long.
• Function: Spontaneously generates electrical impulses at a rate of 60-80 (up to 100) per minute at rest. This automaticity (self-firing ability) makes it the primary pacemaker of the heart.
• All other parts of the conducting system can generate impulses too, but at slower rates - so the SA node normally dominates.

"The sinoatrial node...lies beneath the epicardium on the posterior surface of the right atrium, near the opening of the superior vena cava...referred to as the cardiac pacemaker, as it generates 60-80 impulses per minute." - Color Atlas of Human Anatomy Vol. 2

2. Internodal Pathways (Atrial Conduction)

• After the SA node fires, the impulse spreads through the right atrial myocardium cell to cell via gap junctions.
• There are three proposed internodal pathways that carry the signal to the AV node (anterior, middle, and posterior). The existence of discrete internodal tracts is still debated among physiologists.
• Bachmann's bundle (interatrial tract) carries the signal across to the left atrium, so both atria contract nearly simultaneously.

Conduction pathways through the heart - Medical Physiology (Boron & Boulpaep)

3. Atrioventricular (AV) Node

• Location: At the atrioventricular septum in the interatrial septum, between the opening of the coronary sinus and the septal cusp of the tricuspid valve.
• Also called: Aschoff-Tawara node.
• Size: About 5 mm long.
• Key function - the AV delay: The AV node slows the impulse (by about 0.1 seconds) before passing it to the ventricles. This delay is absolutely critical - it allows the atria to finish contracting and push blood into the ventricles before the ventricles contract.
• Electrical insulation: The fibrous atrioventricular ring electrically insulates the atria from the ventricles. The AV node is the only normal electrical connection between atria and ventricles.
• The AV node acts as a backup pacemaker at 40-60 beats/min if the SA node fails.

"Normally, the atrioventricular (AV) node, where conduction delay is physiologic, serves as the only electrical connection linking the atria and ventricles; the AV valve rings are insulated." - Goldman-Cecil Medicine

4. Bundle of His (Atrioventricular Bundle)

• Location: Arises from the AV node, penetrates the cardiac skeleton (the fibrous ring), and reaches the superior margin of the muscular interventricular septum.
• Structure: A bundle of specialized conducting cells (Purkinje-type fibers).
• Function: Carries the impulse rapidly from the AV node into the ventricular septum.
• At the top of the interventricular septum, the Bundle of His divides into two branches.

5. Right and Left Bundle Branches

• The Bundle of His splits into the right bundle branch and left bundle branch at the interventricular septum.
• Both branches run beneath the endocardium (subendocardially) down the septum toward the apex of the heart.

• Runs down the right side of the septum.
• Enters the septomarginal trabecula (moderator band) to reach the anterior papillary muscle.
• Peripheral branches form a subendocardial plexus.

• Fans out in flat bundles along the left side of the interventricular septum.
• Divides into two major fascicles: the anterosuperior and posteroinferior left bundle branches.
• Branches spread to the bases of both papillary muscles and then fan out across the ventricular wall.

6. Purkinje Fibers (Subendocardial Conducting Network)

• The terminal branches of the bundle system are the Purkinje fibers.
• They fan out throughout the subendocardial layer of both ventricles, reaching from the apex toward the base.
• Speed: Purkinje fibers conduct impulses at the fastest speed in the heart (up to 4 m/s) - much faster than ordinary myocardium (0.5 m/s). This ensures the entire ventricular wall is activated almost simultaneously.
• Histology: Purkinje fibers are modified cardiac myocytes. They are larger than regular myocardial cells, stain paler (because they are rich in glycogen and have relatively few myofibrils and organelles), and are joined by intercalated disks.

Conducting system of the heart viewed from right (A) and left (B) - Color Atlas of Human Anatomy

The Full Sequence of Impulse Conduction

SA Node (fires spontaneously)
    ↓
Internodal pathways → Right atrium depolarizes
    ↓ (also via Bachmann's bundle → Left atrium)
BOTH ATRIA CONTRACT
    ↓
AV Node (0.1 second delay - atria finish pumping)
    ↓
Bundle of His
    ↓ (splits into two)
Right Bundle Branch          Left Bundle Branch
    ↓                              ↓ (anterosuperior + posteroinferior fascicles)
    ↓                              ↓
Purkinje Fibers ←←←←←←←←←←←←←←←←←←
    ↓
BOTH VENTRICLES CONTRACT (from apex upward - efficient ejection)

Why Does the Impulse Spread from Cell to Cell?

Hierarchy of Pacemakers (Safety Net)

Autonomic Modulation

• Sympathetic stimulation (e.g., exercise, stress): increases heart rate, increases force of contraction, speeds up conduction through the AV node.
• Parasympathetic stimulation (via vagus nerve, e.g., rest, sleep): slows heart rate, decreases force, slows AV node conduction.

Clinical Significance

Summary Table

• Junqueira's Basic Histology, 17e, p. 542-543
• Color Atlas of Human Anatomy, Vol. 2, p. 60-62
• Medical Physiology (Boron & Boulpaep), p. 710-711
• Goldman-Cecil Medicine

• The nodal/conducting system = the electrical trigger. It generates and routes the electrical impulse through the heart in the right order and timing.
• The cardiac muscle (myocardium) = the mechanical response. It physically contracts when it receives that electrical signal, which is what actually pumps the blood.

Nodal System (electrical) → triggers → Myocardium (mechanical) → produces → Cardiac Cycle

• The nodal system is the ignition and timing system
• The myocardium is the engine
• The cardiac cycle (blood moving in and out) is the car moving

Nodal System vs. Cardiac Cycle - The Actual Difference

The Core Difference in One Line

1. The Nodal (Conducting) System

Components (already covered):

• SA Node - fires 60-100/min (pacemaker)
• Internodal pathways - carry impulse through atria
• AV Node - delays impulse (0.1 sec)
• Bundle of His - carries impulse to ventricles
• Right & Left Bundle Branches - distribute to each ventricle
• Purkinje Fibers - spread impulse through ventricular walls

What it does:

• Generates the electrical signal
• Routes it in the correct order (atria first, ventricles second)
• Controls the timing and rate of the heartbeat
• Ensures both ventricles contract simultaneously

Nature:

• Electrical in nature
• Always present - doesn't start and stop
• Works at the cellular / microscopic level
• Measured on an ECG (electrocardiogram)

2. The Cardiac Cycle

Phases of the Cardiac Cycle:

• AV valves (mitral + tricuspid) are open
• Aortic + pulmonary valves are closed
• Blood flows passively from atria into ventricles (~70% of filling is passive)
• At the end, atria contract (atrial systole) to push in the remaining ~30%

• All four valves are closed
• Ventricles start contracting - pressure builds rapidly
• No blood moves yet (volume stays the same - hence "isovolumetric")
• Lasts ~0.05 seconds

• Ventricular pressure exceeds aortic/pulmonary artery pressure
• Aortic + pulmonary valves open
• Blood is ejected into the aorta and pulmonary artery
• Ventricles shrink in size

• Ventricles stop contracting and begin relaxing
• All four valves are closed again
• Pressure in ventricles falls rapidly
• No blood moves yet

Phases of Systole - Ganong's Review of Medical Physiology

Phases of Diastole - Ganong's Review of Medical Physiology

Timing (at 75 beats/min, one cycle = 800 ms):

• Systole (contraction) = ~300 ms
• Diastole (relaxation) = ~500 ms

Nature:

• Mechanical in nature
• Repeating event - starts and ends with each beat
• Works at the organ / pressure-volume level
• Measured by blood pressure, heart sounds, echocardiography

Side-by-Side Differences Table

How They Relate to Each Other

NODAL SYSTEM                    CARDIAC CYCLE
(Electrical)         →          (Mechanical)

SA node fires        →   Atria contract  (atrial systole)
       ↓
AV node delays       →   AV valves close
       ↓
Bundle of His        →   Ventricles build pressure
       ↓
Purkinje fibers      →   Ventricles eject blood (ventricular systole)
       ↓
Next SA node fire    →   Ventricles relax, fill again (diastole)

The Nodal System ≠ The Nervous System

The Key Point

• Sympathetic (fight-or-flight) - speeds the heart up
• Parasympathetic (vagus nerve, rest) - slows the heart down

So Your Statement, Corrected:

"The nodal/conducting system is a structural system within the heart muscle that performs the electrical sequence of events that causes contraction of the heart and drives the pumping of blood."