Systole (medicine) - Physiological Mechanism

Physiological Mechanism

Systole of the heart is initiated by the electrically excitable cells of the sinoatrial node. These cells are activated spontaneously by depolarization of their membranes beyond a given threshold for excitation. At this point, voltage-gated calcium channels on the cell membrane open and allow calcium ions to pass through, into the sarcoplasm of muscle cell. Calcium ions bind to ryanodine receptors on the sarcoplasmic reticulum causing a flux of calcium ions to the sarcoplasm.

Calcium ions bind to troponin C, causing a conformational change in the troponin-tropomyosin complex, and thus allowing myosin head binding sites on F-Actin to be exposed. This transition allows cross bridge cycling to occur. The cardiac action potential spreads distally to the small branches of the Purkinje tree via the flux of cations through gap junctions that connect the sarcoplasm of adjacent myocytes. The electrical activity of ventricular systole is coordinated by the atrioventricular node, this discrete collection of cells receives electrical stimulation from the atrium, but also has a slower intrinsic pacemaker activity. The cardiac action potential is propagated down the bundle of His to Purkinje fibres which rapidly causes coordinated depolarisation, and excitation-contraction coupling from the apex of the heart up to the roots of the great vessels.