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
The action potential of the sinoatrial node 
Select and explain. 
A. Reuslts from the activation of rapid Na+ channels, B. Results from opening of the slow Ca2+ channels, C. Is equivalent in height and conduction velocity to that of the atrial muscle, D. Results from an increase in conductance to K+
B: Results from opening of the slow Ca2+ channels
Answer: B: Results from opening of the slow Ca2+ channels 
Explanation: The action potential of the sinoatrial node and atrioventricular node results from the opening of the slow Ca2+ channels and the influx of Ca2+ (some Na+ may also enter through these slow Ca2+ channels). An action potential is elicited when the membrane potential reaches threshold. Spontaneous depolarization is an intrinsic propey of nodal tissue and can occur in the absence of nerves. The low resting potential results in a small action potential that propagates slowly. The rapid Na+ channels are inactivated in nodal tissue. An increase in the conductance to K+ would hyperpolarize the membrane.