Heart Beat

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Understanding:

•  The heart beat is initiated by a group of specialised muscle cells in the right atrium called the sinoatrial node

•  The sinoatrial node acts as a pacemaker

    
The contraction of the heart is myogenic – meaning that the signal for cardiac compression arises within the heart tissue itself

  • In other words, the signal for a heart beat is initiated by the heart muscle cells (cardiomyocytes) rather than from brain signals


Within the wall of the right atrium are a specialised cluster of cardiomyocytes which direct the contraction of heart muscle tissue

  • This cluster of cells are collectively called the sinoatrial node (SA node or SAN)


The sinoatrial node acts as the primary pacemaker – controlling the rate at which the heart beats (i.e. pace ‘making’)

  • The SA node triggers roughly 60 – 100 cardiac contractions per minute (normal sinus rhythm)
  • If the SA node fails, a secondary pacemaker (AV node) may maintain cardiac contractions at roughly 40 – 60 bpm
  • If both fail, a final tertiary pacemaker (Bundle of His) may coordinate contractions at a constant rate of roughly 30 – 40 ppm


The interference of the pacemakers will lead to the irregular and uncoordinated contraction of the heart muscle (fibrillation)

  • When fibrillation occurs, normal sinus rhythm may be re-established with a controlled electrical current (defibrillation)


The Role of a Pacemaker


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Understanding:

•  The sinoatrial node sends out an electrical signal that stimulates contraction as it is propagated through the 

    walls of the atria and then the walls of the ventricles

    
The electrical conduction of a heart beat occurs according to the following events:

  • The sinoatrial node sends out an electrical impulse that stimulates contraction of the myocardium (heart muscle tissue)
  • This impulse directly causes the atria to contract and stimulates another node at the junction between the atrium and ventricle
  • This second node – the atrioventricular node (AV node) – sends signals down the septum via a nerve bundle (Bundle of His)
  • The Bundle of His innervates nerve fibres (Purkinje fibres) in the ventricular wall, causing ventricular contraction


This sequence of events ensures there is a delay between atrial and ventricular contractions, resulting in two heart sounds

  • This delay allows time for the ventricles to fill with blood following atrial contractions so as to maximise blood flow


Myogenic Control of the Heart Beat