Q. How is heart contraction triggered?

Heart contraction is independent from neuronal stimulus although it can be modulated by the autonomous nervous system in the heart there are pacemaker cells that trigger by themselves the action potentials that begin the muscle contraction. These cells are concentrated at two special points of the heart that are 1) the sinoatrial node (SA node) located in the superior portion of the right atrium and 2) the atrioventricular node (AV node) located near the interatrial septum.

The action potentials generated by depolarization of the SA node cells propagate cell to cell throughout the atria producing the atrial contraction, The atrial depolarization also propagates to the AV node that then transmits the electric impulse to the ventricles through specialized conduction bundles of the interventricular septum the bundle of His and then to the Purkinje fibers of the ventricle walls causing ventricular contraction. The atrial contraction precedes the ventricular contraction for blood to fill the ventricles before the ventricular contraction.

The repolarization of the SA node makes the ventricles relax and then the atria relax too.