Basics ICS

• The Intrinsic Conduction System of the Heart:

  • is a number of structural components within the heart which function together with the combined role of generating and conducting action potentials across the myocardium as quickly and orderly as possible
• The Intrinsic Conduction System of the Heart:

  • consists of
  • sinoatrial node
  • specialised conducting highways going from the sinoatrial node to the left atrium
  • more conducting highways going from the sinoatrial node to the atrioventricular node, called the internodal pathways
  • atrioventricular node - acts as a gateway/exit for electrical activity from the atria to the ventricles

  A) sinoatrial node

  B) atrioventricular node

  C) bundle of His

  
• The Intrinsic Conduction System of the Heart:

  • consists of
  • the bundle of His stems from the atrioventricular node and stems into
  • right bundle, which are also high speed conducting pathways, and go to the base of the heart deflects back up the heart, branching off to form purkinje fibres
  • left bundle branch divides into multiple branches which are collectively called the left posterior bundle, which are high speed conducting pathways that take action potentials to the back of the left ventricle
  • left bundle also goes down to base of heart and deflects to form purkinje fibres

  
• • there are more high speed conducting pathways within the ventricles compared to the atria
  • purkinje fibres only exist in the ventricles
  • this is because there is a higher amount of muscle mass within the ventricles, due to them being the main pumps of the heart, so there are a lot of muscle fibres that need to be depolarised quickly (and at the same time) before the heart goes into its contractile phase
  • there are more conducting pathways in the left ventricle than right ventricle as they need to pump blood all around the cardiovascular system, whereas the atria only pump blood to the lungs

  
• Conducting system consists of:

  • sinoatrial node - produces action potential
  • gap junctions - specialised cells within intercalated sacs that allow action potentials to travel from cell to cell that are electrically connected
  • inter-atrial pathway
  • inter-nodal pathway
  • atrioventricular node - exit point for action potentials form atria going to ventricles
  • atrioventricular tract/bundle/bundle of His
  • bundle branches
  • purkinje fibres
• Why the AP pass from atria to ventricles at the AVN only:

  • presence of connective tissue/fibrous skeleton - physically and electrically separates the 2 hemispheres, there are no gap junctions in the border between the atria and ventricles
  • so that electrical excitation and the contractile events of the atria are separated in time to that of the ventricles - the atrioventricular node delays the relay of the action potential by 0.1 seconds
• The AP generating hierarchy:

  • i.e. order in which tissues can most efficiently produce action potentials
  • from most efficient to least:
  • sinoatrial node - 100 action potentials per minute
  • atrioventricular node - 40 to 60 action potentials per minute
  • bundle branch - ~ 40 action potentials per minute
  • Purkinje fibres - ~ 20+ action potentials per minute
  • muscle fibres can also produce action potentials
  • if there is a failure in one component, the next in line can try to take over