<prompt> Single most impoant factor in control of automatic contractility of hea is:Myocordial wall thicknessRight atrial volumeSA node pacemaker potentialSympathetic stimulation  <response>: D i.e. Sympathetic stimulation Intrinsic factor Length of activations - Most impoant factor Frank- starling law: Energy of contraction is propoional to initial length of cardiac muscle fiber. Length of muscle fiber is propoionate to end diastolic blood volume preloadQ. Extrinsic factor Adrenergc neural stimulation Sympathetic stimulationQ - Inotropic effect is smaller, Maximum in atria - It becomes impoant when the hea is increased as it permits adequate diastolic filling. 