Q. Name the three branches off the aortic arch. This structure is known as the ligamentum arteriosum. It was at one time a duct that allowed passage of blood between the great vessels. When and why would this normally occur? Explain the cardiac cycle and the role of the cardiac conduction system in coordinating this action. How are autorhythmic and myocardial (contractile) cells similar? How are they different? What intercellular junctions are located at intercalated disks? How do they function? Calcium is exchanged for sodium through an antiporter, using the potential energy in the sodium gradient. What would the effect be on intracellular calcium if the sodium gradient were disturbed? What would be the effect on contraction? Clarify. Why is the action potential prolonged in contractile cells? Is summation of muscle twitches possible for cardiac muscle? Explain. What is the maximum theoretical heart rate based on this information? Cardiac muscle cells are capable of grade.