Select the correct statement about the function of myocardial cells. A. Each cardiac muscle cell is innervated by a sympathetic nerve ending so that the nervous system can increase heart rate. B. The influx of potassium ions from extracellular sources is the initiating event in cardiac muscle contraction. C. The refractory period in skeletal muscle is much longer than that in cardiac muscle. D. The entire heart contracts as a unit or it does not contract at all.