Glossary: vagus nerve & nervous system terms

Plain-English definitions of the terms used across this site. Many of these words sound intimidating — here is what they actually mean, and why they matter for vagus nerve stimulation.

Vagus nerve

The longest cranial nerve; it connects the brain to the heart, lungs and gut and is the main pathway of the "rest-and-digest" (parasympathetic) system.

Vagal tone

A measure of how active the vagus nerve is. Higher vagal tone is associated with better recovery, calm and regulation, and is often estimated via HRV.

VNS (vagus nerve stimulation)

Any technique that stimulates the vagus nerve, either implanted (surgical) or non-invasive (through the skin, ear or neck).

taVNS

Transcutaneous auricular vagus nerve stimulation: non-invasive stimulation of the auricular branch of the vagus nerve through the ear (tragus or concha). This is what Nurosym does.

TENS

Transcutaneous electrical nerve stimulation. A generic electrical-stimulation device; with an ear clip it can be used for DIY taVNS.

HRV (heart rate variability)

The variation in time between heartbeats. An indirect indicator of autonomic balance and physiological stress; many wearables (Oura, Whoop, Garmin) report it.

Autonomic nervous system

The part of the nervous system that regulates automatic functions (heart, breathing, digestion). It has two branches: sympathetic and parasympathetic.

Fight-or-flight

The sympathetic system response to a perceived threat: raised heart rate, tension and alertness. Chronic activation is what many users try to reduce.

POTS

Postural orthostatic tachycardia syndrome: the heart rate spikes on standing, with dizziness and fatigue. A form of dysautonomia.

Dysautonomia

Dysfunction of the autonomic nervous system; an umbrella that includes POTS and other disorders of automatic regulation.

ME/CFS

Myalgic encephalomyelitis / chronic fatigue syndrome: a chronic illness with severe fatigue, post-exertional malaise (PEM) and other symptoms.

PEM (post-exertional malaise)

A worsening of symptoms after physical or mental exertion, characteristic of ME/CFS and linked to the risk of a "crash".

Long Covid

Persistent symptoms after a COVID-19 infection, which can include fatigue, dysautonomia and POTS.

Fibromyalgia

A chronic widespread pain disorder, with fatigue and disturbed sleep.

MCAS

Mast cell activation syndrome: excessive allergic/inflammatory responses; a profile that can be sensitive to stimulation.

Crash

A marked, prolonged worsening of symptoms after a trigger (exertion, stimulation). A key risk to watch for in ME/CFS and POTS.