Control of the cardiac cycle

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…