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.

The myocardial cells form the cardiac muscle present in the heart. The heart receives its signal from the autonomic nervous system but not all the cells receive the signals.

The SV and AV node present in the upper heart produces the potential for the contraction. The action potential gets transferred through the cell and finally reach the lower ventricle through Purkinje fibres.

This allows the contraction of both the ventricles simultaneously and sows that the heart cells contract simultaneously.

Thus, Option-D is the correct answer.