Abstract
As the heart's contraction is in response to a single action potential transmitted to all fibers, the cardiac muscle cells have evolved a sophisticated system by which the force of contraction can be modulated from beat to beat. Electrical activation of the heart is conducted from cell to cell through gap junctions. The SA node serves as the pacemaker and the AV node controls conduction from the atria to the ventricles. Contraction is initiated by a rise in cytosolic calcium coming from both the sarcoplasmic reticulum and the interstitial fluid with each beat. Relaxation is effected by removing calcium from the cytosol by both a sarcolemmal calcium/sodium exchanger and by a sarcoplasmic calcium ATPase. As insufficient calcium is present during systole for occupying all calcium binding sites on the troponin C molecules, the heart controls the heart's contractility by varying the cytosolic calcium concentration during systole. The force of contraction is determined by the contractility and by the length of the muscle fibers. A cardiac cycle can be described by an ejection loop, the analysis of which reveals that stroke volume can be varied by changing preload, afterload, or contractility.
Published Version
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