The individual fibers are separated by membranes, but depolarization spreads radially (syncytium) because of the presence of gap junctions.

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Atrial systole 

ventricle is relaxed, & because the AV valve is open, the ventricle is filling with blood from the atrium, even prior to atrial systole.

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When blood is actively ejected from the atrium to the ventricle through the open AV valves. The corresponding “blip” in ventricular pressure reflects this additional volume added to the ventricle from atrial systole.

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Fourth heart sound (S4)

Not audible in normal adults

Heard in ventricular hypertrophy, where ventricular compliance is decreased

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Isovolumetric Ventricular Contraction

Initiates contraction of the ventricles

Intraventricular pressure rises sharply as the myocardium presses on the blood in the ventricle

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Isovolumetric Ventricular Contraction As soon as ventricular pressure exceeds atrial pressure, the AV valve closes. 

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Closure of the AV valves produces the first heart sound (S1).