Parasympathetic (vagal) nerves leave the vagal nuclei within the medulla as preganglionic efferent vagal fibers. These preganglionic efferent fibers are relatively long and do not synapse until they reach their target organ (e.g., the heart).
medical Definition of isovolumetric : of, relating to, or characterized by unchanging volume; especially: relating to or being an early phase of ventricular systole in which the cardiac muscle exerts increasing pressure on the contents of the ventricle without significant change in the muscle fiber length and the ventricular volume remains constant
Isovolumetric Contraction. In the second phase of the cardiac cycle both the atrioventricular and semilunar valves of the heart remain closed. At this point, the ventricles begin to contract and although the ventricular myocardial muscle fibers only shorten a little, the intraventricular pressure rises rapidly.