The 'second brain' of the enteric nervous system, the vagus nerve, gut hormones and the microbiome: what's known about two-way traffic between gut and brain — and what's still hype.
Spinal Cord & Body Wiring
Intermediate
The Gut-Brain Axis
The Editors
November 10, 2025
2 min read
In plain English
Your gut has its own network of hundreds of millions of neurons and talks constantly with your brain, mainly through the vagus nerve and hormones. Gut bacteria may influence the brain too — but most of that evidence comes from animals, and many popular claims go beyond the science.
Intermediate level: Builds on the basics. Introduces the real vocabulary and the mechanisms behind it.
Concepts to know first
The phrase gut-brain axis describes the many routes of communication between the digestive system and the brain. Some are well established; others are the subject of exciting but early research.
The second brain
The wall of the digestive tract, from oesophagus to colon, contains the enteric nervous system: an estimated 400 to 600 million neurons, more than in the spinal cord, arranged in two meshes (plexuses). It can control the rhythmic contractions that move food along, secretions and local blood flow largely on its own — even if its connections to the brain are cut. The physician Michael Gershon popularised the term "second brain".
The routes
Nerves: the vagus nerve links brainstem and gut, and around 80% of its fibres carry information from the gut to the brain — about stretch, nutrients and inflammation. Hormones: gut cells release hormones such as ghrelin (hunger), GLP-1, CCK and PYY (fullness), which act on the brain. Immune signals: the gut houses a large part of the immune system, and inflammatory molecules can influence brain function and mood. Microbial products: gut bacteria produce substances, such as short-chain fatty acids, that may affect the body and brain.
The microbiome
The gut holds trillions of bacteria. In mice, the microbiome affects stress responses, behaviour and brain development — germ-free mice behave differently, and transferring microbes between animals can transfer some traits. In humans, the microbiome differs on average in people with depression, autism and Parkinson's disease, but it is hard to tell cause from effect (illness changes diet, medicines change bacteria). Early trials of "psychobiotics" have had mixed results.
What's solid
Clearly established: the gut and brain communicate constantly; stress affects digestion (and conditions like irritable bowel syndrome involve gut-brain signalling); gut hormones control appetite — drugs that mimic GLP-1 act partly on the brain to reduce hunger. Promising but unproven: that specific probiotics reliably improve mood or treat brain disorders.
Everyday examples & things to try
Real world
Butterflies in your stomach
Before a big presentation your stomach churns. That's the brain-to-gut direction: the sympathetic nervous system and stress hormones change gut movement and blood flow. Most of the vagus nerve, though, carries traffic the other way — gut to brain.
Key takeaways
- The enteric nervous system has hundreds of millions of neurons in the gut wall.
- Gut and brain communicate via the vagus nerve, hormones, immune signals and microbial products.
- Microbiome effects on the human brain are plausible but not yet well proven.
Explore further
Search YouTube for…
Tap a phrase to copy it, then paste it into YouTube's search box. This site doesn't link to other websites — pick channels from universities, hospitals and established science educators.
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