Select the correct statement about the function of myocardial cells. the entire heart contracts as a unit or it does not contract at all. cardiac muscle cells are innervated by sympathetic, parasympathetic, and somatic nerve fibers so that the nervous system can increase heart rate. the refractory period in skeletal muscle is much longer than that in cardiac muscle. the influx of potassium ions from extracellular sources is the initiating event in cardiac muscle contraction.