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Explain how heart contraction triggered

Explain how heart contraction triggered?

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Heart contraction is independent from neuronal stimulus (although it can be modulated by autonomous nervous system). In heart, there are pacemaker cells which trigger by themselves the action potentials which start the muscle contraction. Such cells are concentrated at two special points of the heart: the sinoatrial node (that is, SA node) positioned in the superior part of the right atrium and the atrioventricular node (that is, AV node) positioned near the interatrial septum.

The action potentials produced by depolarization of the SA node cells propagate cell to cell all through the atria generating the atrial contraction. The atrial depolarization as well propagates to the AV node which then transmits the electric impulse to the ventricles via specialized conduction bundles of the interventricular septum (that is, the bundle of His) and then to the Purkinje fibers of the ventricle walls causing the ventricular contraction. (Atrial contraction leads the ventricular contraction for blood to fill the ventricles prior to the ventricular contraction.)The re-polarization of the SA node forms the atria relax and then the ventricles relax as well.

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