Final answer:
The statement regarding high heart rates not being possible without decreasing vagal tone is true. Vagal tone suppresses the intrinsic heart rate through parasympathetic stimulation, and a reduction in this tone is required for the heart rate to increase.
Step-by-step explanation:
High heart rates are not possible without decreasing vagal tone. The statement given is True. Vagal tone refers to the influence of the vagus nerve, which is a component of the parasympathetic nervous system, on heart rate. During a state of rest, vagal tone predominates and keeps the heart rate lower than its intrinsic rate. The autonomic nervous system controls heart rate (HR) through the cardiovascular centers in the medulla oblongata. For the heart rate to significantly increase above its baseline rate, which would be about 100 bpm without autonomic influence, there must be a reduction in parasympathetic stimulation, which means decreasing vagal tone, or an increase in sympathetic stimulation. Therefore, to achieve a high heart rate, vagal tone must be decreased to diminish the parasympathetic restraining effect on the heart rate, allowing the sympathetic effects to prevail and increase the heart rate.