Conducting system (heart, anatomy)

Although cardiac muscle has a slow spontaneous rate of depolarization and contraction, all normal excitation originates from the pacemaker in the sinoatrial node (SAN). The SAN is located in the antero-medial wall of the right atrium. A wave of depolarisation and consequent contraction rapidly spreads across both atria. It is unable to cross the annulus fibrosus which electrically insulates the atria from the ventricles.

Continuation of wave of depolarisation occurs via the atrio- ventricular node (AVN) which lies within the interatrial septum. Electrical activity in the AVN is carried very rapidly through the annulus fibrosis and into the interventricular septum via the bundle of His. The bundle of His soon divides into the right bundle branch and the left main bundle branch.

• the right main branch supplies the Purkinje network of the right ventricle.
• the left main branch splits again after coursing only 1–2 cm
  • this results in a left anterior fascicle, which runs toward the anterior-superior papillary muscle, and a left posterior fascicle, which courses toward the posterior- inferior papillary muscle
• pathology, from whatever cause, affecting these bundle branches or fascicles may manifest themselves on the surface ECG as bundle branch blocks (right or left), or fascicular blocks (left anterior or left posterior)
  • if there are multiple sites of conduction delay result in multifascicular block (e.g., bifascicular or trifascicular block)

The conducting system is a prime example of structure being adapted to function so that:

• there is a delay between atrial and ventricular contraction to permit efficient ventricular filling
• the papillary muscles are increasingly taut prior to ventricular contraction to reinforce the AV valves
• the ventricular contraction wave spreads rapidly from the apex to the base of the ventricles