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Function of myocardial cells

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Physiology 3120
Tom Stavraky

Human Physiology Wednesday, January 13, 2010 “CV V” Function of Myocardial Cells • Looking at microscopic level (cellular, molecular, etc.) • Heart beat = most important life-sustaining event; heart is first organ formed during embryogenesis • Three types of muscle fibres 1. Atrial** 2. Ventricular** 3. Excitatory & conductive  Not much contractile capacity  Rhythmicity (excitatory system to generate impulses)  Rapid conduction (conduct impulses) • In skeletal muscle, muscle fibres function independently of each other; cardiac muscle fibres are interconnected & form a system (i.e. syncytium)  Where two fibres meet, membrane folds in  region called intercalated disc  Two functional syncytium in heart 1. Atrial syncytium • Contract before the ventricles because of the impulse generated in the SA node (must conduct down into ventricles) 2. Ventricular syncytium **have contractile properties like skeletal muscle (i.e. sliding filament theory; however, there are major differences [i.e. troponin])** • Impulse generated at sinus node (located just under vena cava in right atrium)  The “pacemaker” of the heart  Impulse generation in this area occurs faster than elsewhere in the heart  Special properties (all of which work to depolarize the SA node cells)  Greater Na permeability  K permeability declines during diastole (more positive charges inside cell)  Slow inward Ca current  No “STABLE” resting membrane potential  During diastole, the SA node slowly depolarizes (called the pre-potential); doesn’t hyperpolarize as much as in other cells (-55 to -65 mV)  Threshold is about -40mV; opens slow sodium & calcium channels (different from V.G. channels); once memb
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