Heart - Conducting System

Question 1

What components make up the cardiac conduction system?

Answer 1

1) Sino-atrial node
2) Atrioventricular node
3) Atrioventricular bundle (bundle of His)
4) Purkinje fibres

Question 2

Overview of heart contraction

Answer 2

1) An excitation signal (an action potential) is created by the sinoatrial node (SAN)
2) The wave of excitation spreads across the atria, causing them to contract.
3) Upon reaching the atrioventricular node (node), the signal is delayed.
4) It is then conducted into the bundle of His, down the interventricular septum.
5) The bundle of His and the Purkinje fibres spread the wave impulses along the ventricle, causing them to contract.

Question 3

SAN

Answer 3

This is a collection of specialised cells (pacemaker), and is located in the upper wall of the right atrium, at the junction where the superior vena cava enters.

These pacemaker cells can spontaenously generate electrical impulses. The wave of excitation created by the SA node spreads via gap junctions across both atria, resulting in atrial contraction (atrial systole) - with blood moving from the atria into the ventricles.

The rate at which the SA node generates impulses is influenced by the autonomic nervous system:
- Sympathetic nervous system - increases firing rate of the SAN and thus increases the heart rate.

• Parasympathetic nervous system - decreases firing of the SAN and thus decreases the heart rate.

Question 4

AVN

Answer 4

The electrical impulses then converge onto the AVN - located within the AV septum, near the opening of the coronary sinus.

The AV node acts to delay the impulses by approximately 120ms, t ensure the atria have enough time to fully eject blood into the ventricles before ventricular systole.

The wave of excitation then passes from the AVN into the atrioventricular bundle.

Question 5

Bundle of His

Answer 5

This is a continuation of the AV node, and serves to transmit the electrical impulse from the AVN to the Purkinje fibre of the ventricles.

It descends down the membranous part of the interventricular septum, before dividing into two main bundles:

• Right bundle branch - conducts the impulse to the Purkinje fibres of the right ventricle.
• Left bundle branch - conducts the impulse to the purkinje fibres of the left ventricle.

Question 6

Purkinje fibres?

Answer 6

These are network of specialised cells. They are abundant with glycogen and have extensive gap junctions.

These cells are located in the subendocardial surface of the ventricular walls, and are able to rapidly transmit cardiac action potentials from the Bundle of His to the myocardium of the ventricles.

This rapid conduction allows coordinate ventricular contraction (ventricular systole) and blood is moved from the right and left ventricles to the pulmonary artery and aorta respectively.

Question 7

Clinical relevance - artificial pacemaker?

Answer 7

An artificial pacemaker is a small electrical device commonly fitted to monitor and correct heart rate and rhythm. It is inserted into the chest under the left clavicle, with wires connected to the heart via the venous system.

The most common indication for a pacemaker is bradycardia. Once inserted, the pacemaker monitors the heart rate, and only fires if the rate becomes too slow. Pacemakers can also be used to treat some tachycardias, certain types of heart block and other rhythm abnormalities.