Cardiac contraction. Animation of the sequence of electrical impulses that occur during cardiac contractions. The normal sequence of events is triggered by the sinoatrial node (yellow oval, upper left). The impulse from this node is conducted rapidly (red arrows) over both atria (the upper chambers of the heart) which contract together. This impulse is blocked by the fibrous ring between the atria and the ventricles (the lower chambers of the heart) except at one point, the atrioventricular node (yellow, centre). Here, the impulse is picked up by the atrioventricular bundle (yellow lines) and distributed to the ventricles which contract shortly after the atria. The two nodes are also supplied by motor nerve fibres from the autonomic nervous system (not shown here).
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