Gut basics
What is the enteric nervous system? Your body’s second brain explained
The enteric nervous system is a network of several hundred million nerve cells embedded in the wall of the gut, from the food pipe to the rectum. It controls the movement of the gut, the release of digestive juices and local blood flow, largely by itself. Because it can work independently of the brain, it is often called the second brain.
In short
- The gut has its own nervous system, with 200 to 600 million nerve cells.
- It runs digestion without conscious control and keeps working even if its connection to the brain is cut.
- It communicates constantly with the brain, mainly through the vagus nerve.
- When its signalling is disturbed, the result is pain, bloating and altered bowel habit, as in irritable bowel syndrome.
Where is it, and how is it organised?
The nerve cells are arranged in two layers that run the length of the gut:
| Layer | Where it lies | What it controls |
|---|---|---|
| Myenteric plexus | Between the muscle layers | Movement: the waves that mix and propel food |
| Submucosal plexus | Just beneath the lining | Secretion of fluid and enzymes, and blood flow |
Sensory nerve cells detect stretch and the chemical make-up of the food. Other nerve cells process that information and instruct the muscles and glands. It is a complete circuit, which is why the gut can respond to a meal without waiting for the brain.
What does it do?
- Sets the pattern of movement: propulsion after a meal, and housekeeping waves during fasting
- Adjusts the secretion of acid, enzymes and fluid to what has been eaten
- Regulates blood flow to the gut during digestion
- Exchanges signals with immune cells and with gut bacteria
How does it talk to the brain?
Through the vagus nerve and nerves of the spinal cord. Most of the fibres in the vagus nerve carry information upward, from gut to brain. The brain, in turn, can speed, slow or quieten the gut through the “rest and digest” and “fight or flight” branches of the nervous system.
The gut’s cells also produce chemical messengers. Most of the body’s serotonin, for example, is made in the gut, where it helps to regulate movement and sensation. It acts locally and does not travel to the brain.
Read more about the vagus nerve.
What happens when it goes wrong?
Irritable bowel syndrome
The gut’s nerves become over-sensitive and its movement irregular. See IBS.
Functional indigestion
The stomach does not relax properly to receive a meal, or senses normal filling as discomfort.
Slow stomach emptying
Nerve damage, most often from diabetes, weakens the stomach’s contractions.
Achalasia
Loss of nerve cells in the lower food pipe prevents its valve from relaxing.
After a gut infection
Inflammation can leave the nerves over-responsive for months.
What affects the enteric nervous system day to day?
- Stress: stress hormones change gut movement and heighten sensation
- Sleep: the gut’s nerves follow a daily rhythm that poor sleep disturbs
- Meal pattern: regular meals give the system a predictable routine
- Gut bacteria: their products act on nerve endings in the gut wall
- Medicines: strong painkillers and several other medicines act directly on these nerves
How can I support it?
Slow breathing, with the out-breath longer than the in-breath, activates the vagus nerve and shifts the gut toward “rest and digest”. Regular sleep and meal times, daily physical activity and a fibre-rich diet all help. Where symptoms persist, medicines that act on the gut’s nerves and gut-directed psychological therapies are part of standard treatment.
When should I see a doctor?
Pain, bloating or altered bowel habit lasting more than a few weeks deserves an assessment. Blood in the stool, weight loss, difficulty swallowing and night-time symptoms are not explained by nerve sensitivity and need investigation.
Questions patients ask me
Is the second brain able to think or feel emotions?
No. It processes information about digestion. Emotions arise in the brain, but they are shaped by signals coming from the gut, which is why the two feel so closely linked.
Is IBS a disease of the enteric nervous system?
In part. IBS is now described as a disorder of gut-brain interaction, involving the gut’s nerves, the brain’s handling of their signals, gut bacteria and the immune system.
Does serotonin made in the gut affect my mood?
Not directly, because it does not cross into the brain. It does affect gut movement and sensation, and the gut influences mood by other routes, mainly the vagus nerve.
Sources
This page is written for patients and is based on the guidelines and studies below. It is for education and does not replace a consultation.
- Review of the enteric nervous system. Furness JB. Nat Rev Gastroenterol Hepatol. 2012. PubMed 22392290
- Review of the microbiota-gut-brain axis. Cryan JF et al. Physiol Rev. 2019. PubMed 31460832
- Review of the vagus nerve in the microbiota-gut-brain axis. Bonaz B et al. Front Neurosci. 2018. PubMed 29467611
- Rome IV criteria for bowel disorders. Lacy BE et al. Gastroenterology. 2016. PubMed 27144627