Cardiac Conduction

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•  Signals from the sinoatrial node that cause contraction cannot pass directly from atria to ventricles

•  There is a delay between the arrival and passing on of a stimulus at the atrioventricular node

Cardiac muscle cells are not fused together but are instead connected via gap junctions at intercalated discs

  • This means that while electrical signals can pass between cells, each cell is capable of independent contraction
  • The coordinated contraction of cardiac muscle cells is controlled by specialised autorhythmic cells (‘pace makers’)

Atrial Contraction

Within the wall of the right atrium is a specialised cluster of cardiomyocytes which directs the contraction of heart tissue

  • This cluster of cells is collectively called the sinoatrial node (SA node or SAN)
  • The sinoatrial node acts as a primary pacemaker, controlling the rate at which the heart beats (i.e. pace 'making’)
  • It sends out electrical signals which are propagated throughout the entire atria via gap junctions in the intercalated discs
  • In response, the cardiac muscle within the atrial walls contract simultaneously (atrial systole)

The atria and ventricles of the heart are separated by a fibrous cardiac skeleton composed of connective tissue

  • This connective tissue functions to anchor the heart valves in place and cannot conduct electrical signals
  • The signals from the sinoatrial node must instead be relayed through a second node located within this cardiac skeleton
  • This second node is called the atrioventricular node (or AV node) and separates atrial and ventricular contractions
  • The AV node propagates electrical signals more slowly than the SA node, creating a delay in the passing on of the signal

Overview of Atrial Contraction / Systole

atrial systole

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•  This delay allows time for atrial systole before the atrioventricular valves close

•  Conducting fibres ensure coordinated contraction of the entire ventricular wall

The separation of atrial and ventricular contraction is important as it optimises the flow of blood between the heart chambers

  • The delay in time following atrial systole allows for blood to fill the ventricles before the atrioventricular valves close

Ventricular Contraction

Ventricular contraction occurs following excitation of the atrioventricular node (located at the atrial and ventricular junction)

  • The AV node sends signals down the septum via a specialised bundle of cardiomyocytes called the Bundle of His
  • The Bundle of His innervates Purkinje fibres in the ventricular wall, which causes the cardiac muscle to contract
  • This sequence of events ensures contractions begin at the apex (bottom), forcing blood up towards the arteries

Overview of Ventricular Contraction / Systole

ventricular systole

Heart Relaxation / Diastole

After every contraction of the heart, there is a period of insensitivity to stimulation (i.e. a refractory period)

  • This recovery period (diastole) is relatively long, and allows the heart to passively refill with blood between beats
  • This long recovery period also helps prevent heart tissue becoming fatigued, allowing contractions to continue for life

Overview of Electrical Conduction within the Heart