2.5 Conducting System of the Heart

The conducting system of the heart is a network of specialized cardiac muscle cells that generate and transmit electrical impulses, coordinating the contraction of the heart chambers. This system ensures that the atria contract before the ventricles, allowing efficient pumping of blood.

Definition

Conducting System of the Heart: The conducting system of the heart is a group of specialized cardiac muscle tissues that initiate and conduct electrical impulses through the heart. It coordinates the contraction of the atria and ventricles, ensuring a rhythmic and efficient heartbeat. The components include the sinoatrial (SA) node, atrioventricular (AV) node, atrioventricular bundle (bundle of His), bundle branches, and Purkinje fibers.

Main Idea

• Generates electrical impulses (SA node)
• Conducts impulses through the heart
• Coordinates atrial and ventricular contraction
• Ensures atria contract before ventricles
• Maintains a rhythmic heartbeat

The SA node is the pacemaker of the heart because it initiates the heartbeat.

Core Concept

• Myogenic = heartbeat originates within the heart
• SA node = pacemaker; initiates impulse
• AV node = relay station; delays impulse
• Bundle of His = conducts impulse to ventricles
• Bundle branches = right and left branches
• Purkinje fibers = conduct impulse to ventricular muscle
• Delay = allows atria to contract before ventricles
• **Pacemaker** = SA node
• Artificial pacemaker = used for arrhythmias

Detailed Explanation

Components of the Conducting System

The components of the conduction system are:

Sinoatrial (SA) Node

• Consists of a specialized plexus of cardiac muscles.
• Embedded in the upper wall of the right atrium.
• Close to where the vena cavae enter the atrium.
• Developed from the sinus venosus.
• Has become a part of the atrium.
• Called the sinoatrial node.

• The SA node has the capacity to generate spontaneous action potentials.
• Cells of the SA node do so at a greater frequency than other cardiac muscle cells.
• As a result, the SA node is called the pacemaker of the heart.
• It initiates each heartbeat.

Atrioventricular (AV) Node

• Another specialized group of cardiac muscle fibers.
• Present near the junction of the right atrium and right ventricle.
• Connected to a strand of specialized muscles in the ventricular septum.

• Receives impulse from the SA node.
• Within the AV node, action potentials are propagated slowly compared with the remainder of the conducting system.
• As a consequence, there is a delay of 0.11 second from the time action potentials reach the AV node until they pass to the AV bundle.
• The total delay of 0.15 second allows completion of the atrial contraction before ventricular contraction begins.

Atrioventricular Bundle (Bundle of His)

• AV node is connected to a strand of specialized muscles in the ventricular septum.
• Known as the atrioventricular bundle or bundle of His.
• Passes through a small opening in the fibrous skeleton.
• Reaches the interventricular septum.
• Divides to form right and left bundle branches.

Bundle Branches

• Extend beneath the endocardium on either side of the interventricular septum.
• Go to the apices of the right and left ventricles respectively.
• The inferior, terminal branches of the bundle branches are called Purkinje fibers.

Purkinje Fibers

• Large-diameter cardiac muscle fibers.
• Have fewer myofibrils than most cardiac muscle cells.
• Do not contract forcefully.
• Intercalated disks are well developed between Purkinje fibers.
• Contain numerous gap junctions.
• As a result of these structural modifications, action potentials travel along the Purkinje fibers much more rapidly than through other cardiac muscle tissue.

Sequence of Impulse Conduction

Timing of Impulse Conduction

Pacemaker

• The SA node is called the pacemaker of the heart.
• It initiates the heartbeat.
• It sets the rhythm of the heart.
• If the SA node fails, the AV node can take over as pacemaker.
• The AV node has a slower intrinsic rate.

Artificial Pacemaker

• A disturbance in the electrical rhythm of the heart.
• May be bradycardia (heartbeat less than 40 beats per minute).
• May be tachycardia (heartbeat more than 100 beats per minute).

• Supplies electrical initiation to myocardial contraction.
• Put surgically under the skin.
• May be programmed.
• Generates electrical rhythm at a set rate.
• Controls arrhythmias.

Intermediate Level

Components of the Conducting System

Impulse Conduction Pathway

Timing of Events

Role of Gap Junctions

• Intercalated disks contain gap junctions.
• Gap junctions allow ions to flow between cells.
• This allows action potentials to pass rapidly.
• Purkinje fibers have numerous gap junctions.
• This makes them conduct impulses rapidly.

Artificial Pacemaker

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Conducting system initiates and conducts electrical impulses.
• 2. Heart is myogenic.
• 3. SA node is the pacemaker.
• 4. SA node is in the upper wall of the right atrium.
• 5. AV node is near the junction of right atrium and right ventricle.
• 6. AV node delays the impulse.
• 7. Delay is 0.11 sec in AV node.
• 8. Total delay is 0.15 sec.
• 9. Bundle of His conducts impulse to ventricles.
• 10. Bundle branches conduct impulse to apex.
• 11. Purkinje fibers conduct impulse to ventricular muscle.
• 12. Purkinje fibers have many gap junctions.
• 13. Action potentials travel rapidly along Purkinje fibers.
• 14. Artificial pacemaker controls arrhythmias.
• 15. Bradycardia = heart rate < 40 beats/min.
• 16. Tachycardia = heart rate > 100 beats/min.

Remember

• - Conducting system = SA node, AV node, bundle of His, bundle branches, Purkinje fibers
• - **SA node** = pacemaker
• - AV node = relay station; delays impulse
• - Bundle of His = conducts impulse to ventricles
• - Bundle branches = right and left
• - Purkinje fibers = conduct impulse to ventricular muscle
• - Delay = 0.11 sec in AV node; 0.15 sec total
• - **Pacemaker** = SA node
• - Artificial pacemaker = controls arrhythmias
• - Bradycardia = heart rate < 40 beats/min
• - Tachycardia = heart rate > 100 beats/min

Common Mistakes

• 1. Confusing SA node and AV node — SA node is the pacemaker; AV node is the relay station.
• 2. Thinking the AV node initiates the heartbeat — The SA node initiates the heartbeat.
• 3. Forgetting the delay in the AV node — The delay allows atria to contract before ventricles.
• 4. Mixing up the bundle branches — Right and left bundle branches.
• 5. Thinking Purkinje fibers contract forcefully — They conduct impulses rapidly; they do not contract forcefully.
• 6. Confusing bradycardia and tachycardia — Bradycardia is slow; tachycardia is fast.
• 7. Forgetting the role of gap junctions — Gap junctions allow rapid impulse conduction.
• 8. Believing the heart is neurogenic — The heart is myogenic.
• 9. Mixing up the components of the conducting system — SA node, AV node, bundle of His, bundle branches, Purkinje fibers.
• 10. Thinking the conducting system is only in the atria — It is in both atria and ventricles.

Quick Revision

The conducting system of the heart is a network of specialized cardiac muscle cells that generate and transmit electrical impulses. The components are the SA node, AV node, bundle of His, bundle branches, and Purkinje fibers. The SA node is the pacemaker; it initiates the heartbeat. The AV node relays the impulse and delays it by 0.11 seconds, allowing the atria to contract before the ventricles. The total delay is 0.15 seconds. The bundle of His conducts the impulse to the ventricles. The bundle branches conduct the impulse to the apex. The Purkinje fibers conduct the impulse to the ventricular muscle. Purkinje fibers have many gap junctions, allowing rapid conduction. The heart is myogenic. Artificial pacemakers are used to control arrhythmias. Bradycardia is a heart rate below 40 beats per minute; tachycardia is above 100 beats per minute.

60-Second Revision

• Conducting system = SA node, AV node, bundle of His, bundle branches, Purkinje fibers
• SA node = pacemaker
• AV node = relay station; delays impulse
• Delay = 0.11 sec in AV node; 0.15 sec total
• Bundle of His = conducts impulse to ventricles
• Purkinje fibers = conduct impulse to ventricular muscle
• Heart is myogenic
• Artificial pacemaker = controls arrhythmias
• Bradycardia = heart rate < 40 beats/min
• Tachycardia = heart rate > 100 beats/min
• Warning: SA node is the pacemaker; AV node delays the impulse

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• 2.6 ELECTROCARDIOGRAM

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