Control of the cardiac cycle

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The sinoatrial node (SAN): The sinoatrial node is the heart's pacemaker. It is a small patch of tissue that sends out waves of electrical excitation at regular intervals to initiate contractions.

Purkyne tissue: Purkyne tissue is specially adapted muscle fibres that conduct the wave of excitation from the AVN down the septum to the ventricles.

The need for coordination

  • Cardiac muscle- can initiate its own contractions- myogenic 
  • The muscles in the atria & ventricles have their own natural frequency of contraction 
  • Atria muscle tends to contract at a higher frequency than the ventricles 
  • Contractions of chambers need to be synchronised to prevent fibrillation 

How the heartbeat starts

  • At the top…


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