The vagus nerve, the tenth cranial nerve, takes its name from the Latin for wandering. This episode traces its route from the medulla oblongata through the jugular foramen, down the neck in the carotid sheath, through the chest and diaphragm, and out over the stomach, liver and colon. The right vagus holds up to 105,000 fibers and the left upwards of 87,000, and 80 to 90 percent of them are sensory, carrying reports from the organs up to the brain. Otto Loewi showed how it speaks: fluid from a frog heart slowed by vagal stimulation slowed a second heart with no nerve attached, revealing acetylcholine, the first neurotransmitter.

As the brake pedal of the parasympathetic system, the vagus explains vasovagal fainting, feelings of fullness, the vomiting reflex triggered by serotonin, and Arnold’s nerve, an ear branch that makes some people cough when cleaning their ears. Doctors now stimulate it to treat epilepsy and drug resistant depression and use the Valsalva maneuver for fast heart rhythms. Cutting it in old ulcer surgery caused B12 deficiency, and a Danish registry study from Aarhus University found full vagotomy cut Parkinson’s risk in half.

• How acetylcholine opens potassium channels to hyperpolarize heart cells
• Why a fear spike followed by an overcorrection makes people pass out at the sight of blood
• Vagal signals telling the liver to stop releasing glucose after a meal
• Intrinsic factor and the slow, potentially fatal B12 deficiency after vagotomy
• Misfolded alpha-synuclein possibly climbing from the gut to the substantia nigra