What is Vagus Nerve?
The vagus nerve (cranial nerve X) runs from the brainstem through the neck and chest into the abdomen. It is the main nerve of the parasympathetic nervous system, but most of its fibres are sensory: they carry information from the organs to the brain, while a minority carry commands from the brain to the organs.
Key Functions
- •Reporting to the brain — signals about the state of the gut, heart, lungs and other organs
- •Heart — slowing the heart rate
- •Throat and voice — the muscles of the pharynx and larynx for swallowing and speaking
- •Gut — taking part in digestion and gut movement
- •Reflexes — coughing, swallowing, vomiting
Breathing muscles are a separate matter: the diaphragm is driven by the phrenic nerve, not the vagus. The vagus carries sensory signals from the lungs and airways.
Vagal Tone
Vagal tone cannot be measured directly; HRV measures such as RMSSD reflect vagally mediated changes in heart rate. Heart rate variability is an indirect window, and breathing itself can distort it. Slow breathing is associated with higher vagally mediated HRV; whether a longer exhale adds anything beyond slowing the breath is still debated. Cold water on the face triggers the diving response, which slows the heart through the vagus — an ordinary reflex, not proof that the nerve is being "toned" or "reset".
Polyvagal Theory
Polyvagal theory proposes "ventral vagal", "sympathetic" and "dorsal vagal" states, and a "social engagement system". These are terms of the theory, which is debated: a critical review argues its basic premises are untenable. ONDA uses the terms only as the theory's language, not as established physiology.
Learn more
The Vagus Nerve: What It Does, and What Is Myth and The Autonomic Nervous System give the full picture with sources.