Question

which of the following statements is true regarding parasympathetic tone?
multiple choice
it holds the resting heart rate below its intrinsic rate.
it accelerates the resting heart rate above its intrinsic rate.
it might increase or decrease heart rate depending on the situation.
it affects heart rate only after myocardial infarction.
it does not affect heart rate.

Explanation:

Parasympathetic tone is a constant activity of the parasympathetic nervous system. The parasympathetic nervous system (via the vagus nerve) has a tonic effect on the heart. The intrinsic rate of the sino - atrial node (SA node, the heart's natural pacemaker) is about 100 beats per minute. But the resting heart rate is about 70 beats per minute. This is because of the parasympathetic (vagal) tone which slows down the heart rate below its intrinsic rate.
The sympathetic nervous system accelerates the heart rate. Parasympathetic tone has a consistent effect (slowing) on the heart rate at rest, not a variable one depending on the situation (except in specific physiological changes, but at rest it's mainly inhibitory). It affects the heart rate at rest, not just after myocardial infarction. And it clearly has an effect (slowing) on the heart rate.

Answer:

It holds the resting heart rate below its intrinsic rate.