What is the enteric nervous system?

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The enteric nervous system is a network of approximately 500 million neurons embedded in the lining of the gastrointestinal tract, extending from the esophagus to the anus. It is capable of functioning independently of the central nervous system and coordinates the entire process of digestion autonomously. It has been called the "second brain" because of its size, complexity, and capacity for independent action. Dr. Michael Gershon at Columbia University is credited with popularizing the concept of the enteric nervous system as a semi-independent nervous system and the "second brain" label, through his book of the same name. The enteric nervous system contains more neurons than the spinal cord, produces about 95% of the body's serotonin, and communicates bidirectionally with the central nervous system through the vagus nerve and through the bloodstream. The enteric nervous system produces and responds to many of the same neurotransmitters as the brain, including serotonin, dopamine, glutamate, and norepinephrine. The fact that about 95% of serotonin is produced in the gut, rather than the brain, has significant implications for understanding mood regulation: serotonin produced in the gut affects gut motility and is not the same serotonin that crosses the blood-brain barrier, but the gut's serotonergic system communicates with the brain through vagal signaling and other pathways. The enteric nervous system developed early in evolution, before the central nervous system in its current form. Some researchers theorize that the brain evolved partly as a system to regulate the gut's activities, not the other way around. Whether or not this evolutionary story is exactly right, the bidirectionality of the gut-brain communication means that gut health affects brain function and brain states affect gut function in ways that are profound and only beginning to be understood. For trauma survivors, the gut-brain axis is particularly relevant. Stress and trauma have well-documented effects on gut function: IBS, nausea, constipation, bloating, and changes in gut microbiome composition are all associated with trauma and chronic stress. Conversely, gut health interventions including dietary changes, probiotic supplementation, and practices that support the microbiome may influence mood, anxiety, and stress reactivity through the bidirectional gut-brain axis.