The vagus nerve (tenth cranial nerve) is the longest cranial nerve. In Latin, the word “vagus” means wandering, an appropriate way to describe the path of this nerve.
It runs from the brain stem to the transverse colon. Along the way, it innervates the larynx, esophagus, lungs, trachea, heart, and most of the digestive tract. The brain and gut feed-back and feed-forward loops are linked by the vagus nerve.
The long, convoluted pathway means the vagus is the main nerve of the parasympathetic nervous system, often called the rest and digest system. This is the autonomic system that helps the body conserve energy by slowing the heart rate (rest).
The vagus nerve modulates digestion through the migrating motor complex and relaxes the ileocecal valve and other sphincter muscles in the gut (digest).
The messages the vagus nerve transmits to the brain from the gut microbiome also impact the hypothalamus-pituitary axis, which in turn controls the neuroendocrine system, mood, and the immune response.
The vagus nerve link is bidirectional. About 80 percent of the messages sent along the vagus superhighway go from the gut to the brain; about 20 percent go in the other direction. What are they talking about?
Signals from the gut microbiome and from the gut itself tell the brain what’s going on down there; signals from the brain tell the gut what changes to make in response.
The gut tells the brain about the production, expression, and turnover of neurotransmitters such as serotonin and GABA and growth factors such as BDNF (brain-derived neurotrophic factor).
The gut also reports on the production of intestinal barrier chemicals and tight junction integrity, the modulation of enteric sensory afferents, bacterial metabolites, and mucosal immune regulation.
The brain responds with messages that tell the gut to alter mucus and biofilm production, motility, intestinal permeability, and immune function.
For example, when vagus afferent nerve fibers within the gut detect inflammatory cytokines and other indicators, such as tumor necrosis factor, created by the gut bacteria, they pass the message to the brain.
The brain responds by stimulating the production of anti-inflammatory neurotransmitters that regulate the immune response.
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