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Although cardiac muscle has a slow spontaneous rate of depolarization and contraction, all normal excitation originates from the pacemaker in the sinoatrial node (SAN). The SAN is located in the antero-medial wall of the right atrium. A wave of depolarisation and consequent contraction rapidly spreads across both atria. It is unable to cross the annulus fibrosus which electrically insulates the atria from the ventricles.
Continuation of wave of depolarisation occurs via the atrio- ventricular node (AVN) which lies within the interatrial septum. Electrical activity in the AVN is carried very rapidly through the annulus fibrosis and into the interventricular septum via the bundle of His. The bundle of His soon divides into the right bundle branch and the left main bundle branch.
The conducting system is a prime example of structure being adapted to function so that: