This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Which of the following normally has a slowly depolarizing “prepotential” 
Select and explain. 
A. Sinoatrial node, B. Atrial muscle cells, C. Bundle of His, D. Purkinje fibers
A: Sinoatrial node
Answer: A: Sinoatrial node 
Explanation: Rhythmically discharging cells have a membrane potential that, after each impulse, declines to the firing level. Thus, this prepotential or pacemaker potential triggers the next impulse.