Conducting System and Nerve Supply of Heart

Conducting System of Heart

• The conducting system is formed by specialized cardiac muscle cells.
• These cells exhibit auto rhythmicity, the ability to generate electrical impulses spontaneously without external stimulation.
• The impulses are transmitted rapidly through the conducting system to coordinate contraction of the myocardium.
• The activity of the conducting system is influenced by the autonomic nervous system, which can increase or decrease its rate of activity.

Components of Conducting System

The conducting system includes:

1. Sinoatrial (SA) node
2. Atrioventricular (AV) node
3. Atrioventricular bundle (bundle of His)
4. Right and left bundle branches
5. Subendocardial plexus of Purkinje fibres

• The SA node normally generates impulses at the highest rate, approximately 70–80 impulses/minute, and therefore functions as the pacemaker of the heart.
• The different components of the conducting system conduct electrical impulses at different velocities.

Sinoatrial (SA) Node

• The sinoatrial node is also known as the SA node, Keith–Flack node, or pacemaker of the heart.
• It is an elongated, crescent-shaped collection of specialized cardiac myocytes.
• Location: It lies in the wall of the right atrium, in the upper part of the sulcus terminalis, just below the opening of the superior vena cava.
• Length: 10–20 mm.
• Thickness: Approximately 1 mm.
• Blood supply: Mainly by the SA nodal branch of the right coronary artery.
• Structure:

• It is composed mainly of P cells, also called nodal myocytes.
• These cells are small, fusiform, pale-staining, and highly excitable.
• The SA node receives parasympathetic innervation predominantly from the right vagus nerve.

Atrioventricular (AV) Node

• The AV node is smaller than the SA node.
• Location:

• It lies in the lower part of the interatrial septum, just above the opening of the coronary sinus.
• It is situated subendocardially within the triangle of Koch.
• The triangle of Koch is bounded by the tendon of Todaro, opening of the coronary sinus, and septal leaflet of the tricuspid valve.
• Dimensions: Approximately 8 mm long and 1 mm wide.
• Blood supply: Mainly by the AV nodal branch of the right coronary artery.
• Inferiorly, the AV node continues as the atrioventricular bundle (bundle of His).
• Structure:

• It contains slowly conducting transitional cardiac myocytes.
• Slow conduction through the AV node produces a physiological AV nodal delay, allowing atrial contraction to precede ventricular contraction.
• Rhythmicity: The AV node can generate impulses at approximately 60 beats/minute.

Internodal Tracts

• The SA and AV nodes are connected by three internodal pathways.

Anterior Internodal Tract of Bachmann

• Connects the anterosuperior part of the SA node with the AV node.
• Passes anterior to the opening of the superior vena cava and then continues along the interatrial septum.

Middle Internodal Tract of Wenckebach

• Connects the posteroinferior part of the SA node with the AV node.
• Passes posterior to the opening of the superior vena cava and then descends along the interatrial septum.

Posterior Internodal Tract of Thorel

• Connects the posteroinferior part of the SA node with the AV node.
• Runs along the crista terminalis and the valve of the inferior vena cava.

Atrioventricular Bundle of His

• The atrioventricular bundle, or bundle of His, continues from the anteroinferior part of the AV node.
• Course: It passes through the central fibrous body, including the right fibrous trigone, and reaches the membranous part of the interventricular septum before entering its muscular part.
• At the crest of the muscular interventricular septum, it divides into right and left bundle branches.
• The bundle of His provides the normal electrical connection between the atrial and ventricular myocardium, which are otherwise electrically separated by the fibrous skeleton of the heart.

Right Bundle Branch (Crus Dextrum)

• The right bundle branch is a narrow, rounded fascicle arising from the bundle of His.
• It descends along the right side of the interventricular septum toward the apex of the heart.
• A major part enters the moderator band (septomarginal trabecula) and reaches the anterior papillary muscle.
• It subsequently divides into a subendocardial network of Purkinje fibres.

Left Bundle Branch (Crus Sinistrum)

• The left bundle branch arises from the bundle of His as several fine fascicles.
• These fascicles descend along the left side of the muscular interventricular septum toward the apex.
• They then spread through the left ventricle and terminate as Purkinje fibres.

Purkinje Fibers

• Purkinje fibres are terminal branches of the right and left bundle branches.
• They form a widespread subendocardial plexus over the interventricular septum and ventricular walls.
• This network rapidly distributes electrical impulses throughout the ventricular myocardium.

Blood Supply of Conducting System

• The conducting system receives its blood supply from branches of both coronary arteries, although the right coronary artery (RCA) supplies most of its components in the majority of individuals.
• The SA nodal artery usually arises from the RCA; in about 40% of individuals, it originates from the circumflex branch of the left coronary artery.
• The AV node is usually supplied by the AV nodal branch of the RCA.
• The bundle branches receive blood from septal branches of the coronary arteries, particularly the anterior interventricular (left anterior descending) artery.

Nerve Supply of Heart

• Knowledge of the cardiac nerve supply helps explain the anatomical basis of angina pectoris and pain associated with myocardial infarction.
• The heart receives both sympathetic and parasympathetic innervation.

Sympathetic Fibres

• Preganglionic sympathetic neurons arise from the lateral horn of the T1–T5 segments of the spinal cord.
• They synapse in the cervical and upper thoracic sympathetic ganglia.
• Postganglionic fibres reach the heart through the cardiac nerves.
• Sympathetic stimulation:
  • Increases heart rate
  • Increases the force of myocardial contraction
  • Increases conduction through the cardiac conducting system
  • Increases cardiac output
• Sympathetic activity also contributes to regulation of coronary blood flow.
• Visceral pain fibres from the heart generally travel with sympathetic fibres and enter the spinal cord mainly through T1–T5 spinal segments.

Parasympathetic Fibres

• Parasympathetic innervation is provided by the vagus nerves.
• Preganglionic fibres arise mainly from the nucleus ambiguus and dorsal motor nucleus of the vagus in the medulla.
• These fibres synapse in small ganglia located within the cardiac plexus and close to the cardiac wall.
• Parasympathetic stimulation:
  • Decreases heart rate
  • Reduces conduction through the AV node
  • Produces a mild reduction in atrial contractility
• Parasympathetic influence on ventricular myocardium is relatively limited.
• Sympathetic and parasympathetic fibres reach the heart through the superficial and deep cardiac plexuses.

Superficial Cardiac Plexus

• Location: Lies below the arch of the aorta, anterior to the right pulmonary artery, and close to the ligamentum arteriosum.
• A small cardiac ganglion may be present near the ligamentum arteriosum.

Formation

The superficial cardiac plexus is formed mainly by:

1. Sympathetic fibres
   • Cardiac branch of the left superior cervical sympathetic ganglion
2. Parasympathetic fibres
   • Inferior cervical cardiac branch of the left vagus nerve

Branches

The superficial cardiac plexus gives:

1. Branches accompanying the right coronary artery
2. Communicating branches to the deep cardiac plexus
3. Communicating branches to the left anterior pulmonary plexus

Deep Cardiac Plexus

• Location: Lies anterior to the bifurcation of the trachea and posterior to the arch of the aorta.

Formation

The deep cardiac plexus is formed by:

1. Sympathetic fibres
   • Cardiac branches from the cervical sympathetic ganglia
   • Cardiac branches from the upper thoracic sympathetic ganglia
2. Parasympathetic fibres
   • Cardiac branches of the vagus nerves
   • Cardiac branches associated with the recurrent laryngeal nerves

• The branches that contribute to the superficial cardiac plexus are excluded from the corresponding contribution to the deep plexus.

Branches

The deep cardiac plexus divides into right and left portions and gives branches to:

1. Right and left atria
2. Right and left coronary arteries through the coronary plexuses
3. Right and left anterior pulmonary plexuses

Click to see: Medical Disclaimer and Copyright Notice