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Wednesday, 19 July 2017

Mechanism of heartbeat

Most muscles contract as a result of impulses they receive from nerves. The cardiac muscles are however, myogenic because the rhythmic contractions arise from within the muscles tissues themselves. The heart can therefore, continue to beat rhythmically even after its nerves are severed or the heart is cut out of the body completely.

Heartbeat is initiated by the sino-atrial node (SAN) or pacemaker, which is a plexus of fine cardiac muscles. It is embedded in the wall of the right atrium near the venae cavae. Contraction of the heart is preceded by waves electrical excitation which starts at the SAN or pacemaker. The electrical excitation accompanied by contractions, spreads over the two atria and causes the atria to contract in unison. When the wave reaches the junction between the atria and ventricles, it excites the atrio-ventricular node (AVN). The AVN continues down the interventricular septum as a modified bundle of cardiac muscles fibres called Purkinje tissue. The Purkinje tissue breaks into a sheet-like reticulum over the walls of the ventricles just beneath the endothelial lining. When the AVN receives the excitation from the atria, it sends impulses down the Purkinje tissues. The Purkinje tissues then spread the waves to the cardiac muscle tissues in the wall of the ventricles, which causes the ventricles to contract in unison. Thus, the SAN sends out rhythmical waves of electrical excitation, which are transmitted first over the atria to the AVN then through the Purkinje tissue to the ventricles.

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