What is the state of the heart when the parasympathetic nervous system is in control?

When the parasympathetic nervous system is in control, the state of the heart is characterized by an increase in vagal tone. The parasympathetic nervous system, primarily through the vagus nerve, serves to lower the heart rate and promote a state of rest and digestion. This is achieved by releasing acetylcholine, which acts on the heart's pacemaker cells to decrease the rate of electrical impulses, thus slowing the heart rate.

Increased vagal tone indicates a stronger influence of the parasympathetic system, which not only slows down the heart rate but also can enhance the heart's efficiency by facilitating better filling of the ventricles. This contrasts with situations where the sympathetic nervous system is more active, which would typically raise heart rate and increase cardiac output.

The other options represent states that occur when the sympathetic nervous system is more dominant or indicate conditions that do not describe the typical response of the parasympathetic system. Therefore, vagal tone is the most accurate representation of the heart's condition under parasympathetic control.