What is the effect on heart rate if the vagus nerve is cut?

When the vagus nerve is cut, the primary effect on heart rate is an increase. The vagus nerve is part of the parasympathetic nervous system, which plays a crucial role in slowing down the heart rate under resting conditions. It exerts a braking effect on the heart, releasing acetylcholine that acts on the heart's pacemaker cells to reduce heart rate.

When the vagus nerve is severed, this inhibitory tone is removed, leading to an unopposed influence of the sympathetic nervous system, which is responsible for increasing heart rate. Therefore, without the vagal signal to slow it down, the heart rate increases due to increased sympathetic activity.

This understanding is fundamental in cardiovascular physiology, as it highlights the balance between the sympathetic and parasympathetic branches of the autonomic nervous system in regulating heart function.