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
Single most important factor in control of automatic contractility of the heart is 
Select and explain. 
A. Myocardial wall thickness, B. Right atrial volume, C. SA node pacemaker potential, D. Sympathetic stimulation
D: Sympathetic stimulation
Answer: D: Sympathetic stimulation 
Explanation: Cardiac contraction is controlled by sympathetic nerves as parasympathetic nerves do not innervate myocardial cells of the ventricle in sufficient numbers.