The vagus nerves are two very long nerves that originate in the brain stem (one on the right and one on the left of the brain stem), and extend down into the chest and abdomen. The vagus nerves help control the muscles of the throat and the voicebox, regulate the heart rate and keep the gastrointestinal tract in working order. The vagus nerves also carry sensory information from the internal organs back to the brain.
Sudden stimulation of a vagus nerve can produce what is called a "vasovagal reflex," which consists of a sudden drop in blood pressure and a slowing of the heart rate. This reflex can be triggered by gastrointestinal illness or in response to pain, fright or sudden stress. Some people are particularly prone to this reflex, and if their blood pressure and heart rate changes are pronounced they can lose consciousness - a condition called "vasovagal syncope."