This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
Prepotential SA node is due to all except 
Select and explain. 
A. Ca2+ spark, B. Fast sodium channels opening, C. K+ decay, D. Transient Ca channel opening
B: Fast sodium channels opening
Answer: B: Fast sodium channels opening 
Explanation: The action potentials in the SA and AV nodes are largely due to Ca2+, with no contribution by Na+ influxWhen the cholinergic vagal fibers to nodal tissue are stimulated, the membrane becomes hyperpolarized and the slope of the prepotentials is decreased because the acetylcholine released at the nerve endings increases the K+ conductance of nodal tissue.The action potentials in the SA and AV nodes are largely due to Ca2+Ref: Ganong's Review of Medical Physiology Twenty-Third Edition Page No:491