This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Conducting System of Hea, Subject name: Physiology
Major current for depolarization in Purkinje fiber is of: 
Select and explain. 
A. Na+, B. Ca++, C. K+, D. Cl-
A: Na+
Answer: A: Na+ 
Explanation: Purkinje fiber is a fast-response type or rapidly depolarizing fiber. The rapid depolarization is due a rapid influx of Na+ through the fast Na+ channels. Major current for depolarization in pacemaker cells is due to Ca++