What stimulates the vagus nerve?

The vagus nerve is a critical component of the parasympathetic nervous system, and its stimulation has a profound effect on heart rate. When the vagus nerve is activated, it releases acetylcholine, which directly influences the heart by decreasing the heart rate. This process is known as negative chronotropic effect, where the activity of the vagus nerve slows down the heartbeat by acting on the sinoatrial node, the natural pacemaker of the heart.

This physiological response plays an essential role in regulating heart function, especially during times of rest or relaxation. In contrast, increased heart rate or contractility would typically involve the activation of the sympathetic nervous system, which functions to prepare the body for 'fight or flight' responses. Fluid retention is more associated with the regulation of blood volume and pressure by the kidneys and hormones, rather than direct stimulation of the vagus nerve's action on heart rate. Thus, decreased heart rate is the direct response that occurs when the vagus nerve is stimulated.