Describe the mechanism by which the human heart beat is initiated and controlled.

Explanation

The rhythmic contraction and expansion of heart is called heart beat. The contraction of the heart is called systole and the relaxation of the heart is called diastole. The human heart is myogenic. The pacemaker cells are located in the right sinoatrial (SA) node. On the left side of the right atrium, there is a mode called auriculo ventricular node (AV). Two special cardiac muscle fibres which originate from the AV node are called the bundle of His. It runs down into the interventricular spectrum and the fibres spread into the ventricle as the Purkinje fibres. The pacemaker cells produce excitation through depolarization of their cell membrane. Early depolarization is slow and takes place by sodium influx and reduction in potassium effiux. Minimum potential is required to activate voltage gated calcium (Cat) channels that cause rapid depolarization which results in action potential. The pace maker cells repolarise slowly via Kt efflux.