Enteric Nervous System: The First Brain a Pain Doctor Forgets

Pain medicine is taught from the skull down. The brain gives orders, the spinal cord relays them, and the gut hangs at the bottom of the chart as plumbing that sometimes complains. The enteric nervous system, the network of neurons built into your gut wall, does not fit that chart, and the way it breaks the chart matters to anyone whose pain has outlasted every scan.

In the video You Have the Stomach of a Hyena, I, Dr. Gurpreet Singh Padda, MD, MBA, MHP, walk through the evolutionary argument. Here I want to stay with what it means for a nervous system that hurts: how much wiring the gut really carries, which direction its traffic runs, and which part of the signal you can change.

How big the enteric nervous system really is

For years the figure in circulation was 400 to 600 million enteric neurons. It was repeated in reviews, but it was a citation handed from paper to paper rather than a count. When a team finally counted with a standardized whole-mount method in human, mouse and guinea pig tissue, the human total came to 168 million (Michel et al., 2022). That is well below the folklore, and it still sits in the same range as the neurons of your spinal cord.

It is not simple wiring, either. The enteric system uses at least 26 definable neuron types, it is the largest division of the peripheral nervous system, and strands inside the gut wall carry axons as far as 13 centimeters (Spencer & Hu, 2020). It senses and acts without waiting for permission from the head.

The vagus nerve is built to report upward

You may have read that the vagus is 80 or 90 percent sensory. That tidy ratio rests on weak ground, including a rat study whose authors found far more motor fibers than the textbooks assumed, and no modern study has counted sensory against motor fibers in a human vagus. What has been measured in human tissue is how the cable is built. In the abdominal vagus, the segment that carries gut traffic, 94 percent of fibers are unmyelinated, while the myelinated share in the neck is 54 percent (Stakenborg et al., 2020).

And the cable is not one cable. In ten human vagus nerves studied after death, the number of fiber bundles doubled over a few centimeters, from 8.30 at one centimeter below the upper neck to 16.20 at nine (Thompson et al., 2026). Organ-specific bundles stayed grouped for only about one to one and a half centimeters after a branch joined. A nerve that rebraids itself that often is precision territory, not a single dial.

Why a pain physician should care what the gut reports

The head reads the gut far more than it runs it, and the reports arrive on more than one clock. Some lining cells, called neuropod cells, synapse directly onto nerves and pass a signal to vagal neurons within 60 to 800 milliseconds in mice (Kaelberer et al., 2018). Yet only about one hormone-making cell in five of that kind touches a nerve fiber, and whole-nerve firing in the same work peaked on average 72.7 seconds after the gut was stimulated. One synapse is quick. The nerve as a whole takes about a minute to settle on its message.

Then there is what the brain does with the report. Healthy volunteers had a bag inflated in the stomach while different solutions ran into the small intestine. Maltodextrin and fat turned simple fullness into something that felt like a meal, and only the fat made people feel sick (Feinle et al., 1997). The investigators wrote that the pressure changes could not account for what the subjects felt.

Read that with pain in mind. The same stretch of stomach wall produced different sensations depending on what the gut chemistry reported alongside it. Sensation is a report that gets interpreted, not a measurement that gets delivered. That is one reason a scan can disagree with the pain you feel. When the reporting organ is inflamed or underfed, the interpreter upstairs is working from a corrupted feed.

Most pain patients never hear any of this, and the reason is structural rather than personal. The gut belongs to one specialty and one billing code, the pain to another, and nobody is paid to ask whether the report and the pain are one problem. It is the same siloed thinking behind why we treat insulin before we treat the joint, and behind the metabolic thread that connects pain, opioids and diabetes.

The acid gate and the drugs that lower it

The second fact that matters for pain care sits at the top of the tube. Human stomach acid runs at a scavenger’s setting. In 66 normal subjects who wore a radio capsule for up to two days, gastric pH stayed between 1.0 and 2.5 in every one of them (Evans et al., 1988). That acid is a sterilizing gate. In adults who make acid, 99.9 percent of a test bacterium died within thirty minutes below pH 4.0, while stomachs that make no acid had not reduced the load at all an hour later (Giannella et al., 1972).

Medicine lowers that gate on a huge scale, often for good reasons. In 1,815 people across three cohorts, the 211 taking a proton pump inhibitor had lower gut microbial diversity and shifts in 20 percent of detected bacterial groups. The newcomers were mouth bacteria, with Rothia strongly over-represented (Imhann et al., 2016). It was observational, so sicker people may simply be the ones given the drug.

For a pain patient the lesson is about the whole medication list, not one drug. Anything that changes what gets past the gate changes what the first brain has to report. If you take an acid-suppressing drug, the productive conversation with your physician is about what it is treating, whether that reason still applies, and what the plan is. The drug has a real job before it has a cost, and our post on acid suppression weighs that tradeoff.

A lining rebuilt from what you ate this week

The surface where all of this happens is renewed constantly. The intestinal crypt turns over roughly every 4 to 5 days, and absorptive cells climb from the crypt to the villus tip in three to five days. The raw material for each rebuild is your recent food. For most households that material is whatever an industrial food system could grow and ship most cheaply, which mostly means grain and what is made from grain.

Diet does not remodel that surface the way supplement ads imply. In a randomized crossover, 15 healthy volunteers ate two weeks of high-fat and two weeks of high-carbohydrate food at matched calories, with jejunal biopsies after each (Casselbrant et al., 2022). The villi did not change their enlargement factor, which was the primary outcome. The microvilli at the base of the villus did: they were longer after the carbohydrate diet, and crypt cell division rose.

I did not always read the gut this way. For years I was a strict vegetarian who taught the dietary guidelines I now spend much of my time taking apart, until a plate of barbecue in Kansas City ended it. The lesson was not about meat. It was that the tube has expectations of its own, and it keeps rebuilding itself from whatever I hand it.

What to do with this if you live with pain

• Describe your gut to your pain physician in plain terms. Which foods pass without comment, and which leave hours of gas and pain? That pattern is information about your equipment, and the gas a meal produces is worth measuring.
• Bring your full medication list, including acid reducers and anything taken for heartburn, and ask what each one is treating.
• Be skeptical of any single number about your gut, including the ones printed on supplement labels, and read what stomach acid does to a probiotic capsule before buying the next one.

If you want the evidence itself, the Chapter 0 Deep Dive holds each paper behind these figures, its full numbers and its limits, plus questions for your physician. It accompanies The Angry Gut, the book I wrote with Ami Michelle Grimes. The next step is what happens to mood and pain when the first brain becomes inflamed, covered in gut health and anxiety.

Frequently asked questions

What does the enteric nervous system do?

It runs the gut locally. The human enteric nervous system holds about 168 million neurons of at least 26 types. It senses what arrives, coordinates muscle movement and controls fluid movement across the lining without needing the brain, which is why isolated bowel still moves its contents. It also reports upward through the vagus nerve, so its condition shapes what the brain receives. See how an inflamed gut changes mood and pain.

Is the gut really a second brain?

We call the gut the first brain, not the second. Nervous systems in the gut wall appear in animals that have no central nervous system at all, including hydra, so they arose early and before any head existed. The brain in your skull came later and reads what the gut reports. Calling the gut the second brain gets the order of construction backward. Learn why pain signals and scans often disagree.

Does the vagus nerve carry signals from the gut to the brain?

Yes, and its structure suggests reporting is a major job. In human tissue, 94 percent of fibers in the abdominal vagus are unmyelinated, the thin, slow type typical of sensory cable. The popular claim that 80 or 90 percent of vagal fibers are sensory has never been confirmed by a direct human count, so treat that exact ratio as unproven. Follow the wire between the two brains in more detail.

Do acid-reducing medications affect gut bacteria?

They appear to. In a study of 1,815 people, those taking a proton pump inhibitor had lower microbial diversity and more mouth-type bacteria in the gut. A small randomized trial also found that acid suppression helped a swallowed bacterium colonize. These drugs have real uses, so any change belongs in a conversation with the physician who prescribed yours. Read when acid suppression is worth its cost.

How often does the gut lining replace itself?

Quickly. The intestinal crypt renews roughly every 4 to 5 days, and absorptive cells travel from the crypt to the villus tip in three to five days. The surface that meets your food is rebuilt again and again from what you recently ate, which makes diet a steady input to the gut’s own nervous system. See the mechanism behind sugar, refined grain and seed oil.

Sources

1. Michel, K., Kuch, B., Dengler, S., Demir, I. E., Zeller, F., & Schemann, M. (2022). How big is the little brain in the gut? Neuronal numbers in the enteric nervous system of mice, Guinea pig, and human. Neurogastroenterology and Motility, 34(12), e14440. https://doi.org/10.1111/nmo.14440
2. Stakenborg, N., Gomez-Pinilla, P. J., Verlinden, T. J. M., Wolthuis, A. M., D’Hoore, A., Farré, R., Herijgers, P., Matteoli, G., & Boeckxstaens, G. E. (2020). Comparison between the cervical and abdominal vagus nerves in mice, pigs, and humans. Neurogastroenterology and Motility, 32(9), e13889. https://doi.org/10.1111/nmo.13889
3. Kaelberer, M. M., Buchanan, K. L., Klein, M. E., Barth, B. B., Montoya, M. M., Shen, X., & Bohórquez, D. V. (2018). A gut-brain neural circuit for nutrient sensory transduction. Science, 361(6408), eaat5236. https://doi.org/10.1126/science.aat5236
4. Giannella, R. A., Broitman, S. A., & Zamcheck, N. (1972). Gastric acid barrier to ingested microorganisms in man: studies in vivo and in vitro. Gut, 13(4), 251–256. https://doi.org/10.1136/gut.13.4.251
5. Spencer, N. J., & Hu, H. (2020). Enteric nervous system: sensory transduction, neural circuits and gastrointestinal motility. Nature Reviews Gastroenterology & Hepatology, 17(6), 338–351. https://doi.org/10.1038/s41575-020-0271-2
6. Casselbrant, A., Wallenius, V., Elebring, E., Marschall, H.-U., Johansson, B. R., Helander, H. F., & Fandriks, L. (2022). Morphological adaptation in the jejunal mucosa after iso-caloric high-fat versus high-carbohydrate diets in healthy volunteers: Data from a randomized crossover study. Nutrients, 14(19), 4123. https://doi.org/10.3390/nu14194123
7. Imhann, F., Bonder, M. J., Vich Vila, A., Fu, J., Mujagic, Z., Vork, L., Tigchelaar, E. F., Jankipersadsing, S. A., Cenit, M. C., Harmsen, H. J. M., Dijkstra, G., Franke, L., Xavier, R. J., Jonkers, D., Wijmenga, C., Weersma, R. K., & Zhernakova, A. (2016). Proton pump inhibitors affect the gut microbiome. Gut, 65(5), 740-748. https://doi.org/10.1136/gutjnl-2015-310376
8. Feinle, C., Grundy, D., & Read, N. W. (1997). Effects of duodenal nutrients on sensory and motor responses of the human stomach to distension. The American Journal of Physiology, 273(3 Pt 1), G721-G726. https://doi.org/10.1152/ajpgi.1997.273.3.G721
9. Thompson, N., Mastitskaya, S., Iacoviello, F., Turhani, F., Shearing, P. R., Aristovich, K., & Holder, D. (2026). Human vagus nerve fascicular anatomy and its implications for targeted cardiac stimulation: a microCT segmentation and histological pilot anatomical study. Frontiers in Neuroscience, 20, 1731234. https://doi.org/10.3389/fnins.2026.1731234

Medically reviewed by Dr. Gurpreet Singh Padda, MD, MBA, MHP, board certified in anesthesiology, pain medicine, interventional pain management, addiction medicine and obesity medicine. Adapted from Chapter 0 of The Angry Gut by Dr. Padda and Ami Michelle Grimes. Last reviewed September 2026.

Dr. Gurpreet Singh Padda, MD, MBA, MHP