The gut-brain axis is a bidirectional communication network between the gastrointestinal tract and the central nervous system. The gut is increasingly called the "second brain."

The enteric nervous system: The gut contains ~500 million neurons — more than the spinal cord — and produces ~95% of the body's serotonin.

Gut-brain communication pathways:

• Vagus nerve — direct neural connection
• Enteric nervous system
• Gut hormones (ghrelin, leptin, GLP-1)
• Short-chain fatty acids from microbiome fermentation
• Immune signaling molecules

Clinical implications:

• Gut dysbiosis is linked to depression and anxiety
• Probiotic trials show modest antidepressant effects
• IBS and anxiety disorders frequently co-occur
• The gut microbiome influences stress response via the HPA axis

Reference:

Wikipedia: Gut–brain axis

https://en.wikipedia.org/wiki/Gut%E2%80%93brain_axis