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 cardiovascular parameter which is the best indicator of vagal tone? 
Select and explain. 
A. Hea rate, B. Stroke volume, C. Ejection fraction, D. Diastolic blood pressure
A: Hea rate
Answer: A: Hea rate 
Explanation: There are central influences for hea rate. Medullary centers are cardioinhibitory center and vasomotor centre. There is a constant discharge of impulses from CIC through the vagus which influences the activity of the hea.Continuous stream of impulses from CIC to hea is responsible for vagal tone which is very prominent in humans. Ref: Textbook of medical physiology by N Geetha, 2nd edition, page no. 161,162