Vagus nerve

The vagus nerve is the tenth cranial nerve. It leaves the brainstem and branches to the organs of the chest and abdomen, including the heart. It carries about 75% of all parasympathetic parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system fibers, which makes it the main route for the body’s “rest and digest” signals.

At the heart, vagal signals slow the pacemaker. They act fast: a vagal signal changes the heart rate within the same beat or the next one, and the effect fades almost as quickly when the signal stops. Sympathetic Sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system signals take up to 5 seconds to start speeding the heart up. So when the gap between two beats changes sharply from one beat to the next, the vagus nerve is the main reason. This is why beat-to-beat measures such as RMSSD RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD are read as markers of vagal activity vagal activity Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone .

The vagus nerve also carries information the other way. Most of its fibers, roughly 85 to 90%, send signals from the body to the brain, not from the brain to the organs.

The fast, breathing-linked swings that make up much of resting HRV are largely the vagus nerve at work. The lesson The nervous system behind HRV covers how it shares control of the heart with the sympathetic system.