Glossary /
Vagus Nerve
The tenth cranial nerve — a long, wandering nerve linking brain, heart, lungs, and gut, and the main highway of the parasympathetic system.
What it is
The vagus is the longest cranial nerve, running from the brainstem through the neck and into the chest and abdomen. Roughly 80 percent of its fibers carry signals from the body up to the brain, not the other way around. It slows the heart, regulates digestion, modulates inflammation, and is the chief anatomical correlate of what people loosely call “the parasympathetic nervous system.”
Why it matters
Vagal tone — how responsive this nerve is — predicts cardiovascular health, emotional regulation, and recovery from stress. You cannot will it stronger, but you can train it indirectly. Slow nasal breathing, especially with exhales longer than inhales, humming, gargling, cold water on the face, and meditation all increase vagal activity. HRV is the most accessible proxy for measuring it over time.