Question #149
Which of the following is true of cardiac muscle?
The prolonged plateau phase seen in ventricular muscle is attributable to Ca2+ influx via L-type channels. Repolarization in largely attributable to K+ efflux, as in other excitable cells. A current carried on the Na+/Ca2+ exchanger may contribute to depolarization and be involved in the initiation of cardiac arrhythmia.
The electrocardiographic R wave, representing ventricular depolarization, is usually positive in the limb leads. While one might expect the T wave, representing repolarization, to be negative it is in fact positive as a consequence of interregional variations in action potential duration (depolarization propagates from endocardium to epicardium, while repolarization propagates from epicardium to endocardium).
Transverse tubules are absent from atrial muscle cells.