• Allison Fahrenbach

The Gut-Brain Connection & Pain: Part 2

In last weeks blog I detailed the importance of knowing what the gut is, the role the microbiome plays in your health and wellbeing, the links between the gut microbiome and pain, and the role your gut lining plays in your immune function, inflammation and pain.


One of the ways in which the gut is linked to pain is via the gut-brain connection, which is what I wanted to get into further this week.


In this blog I’ll explain what the gut-brain connection is, what the enteric nervous system is, the role of the vagus nerve, and how this all relates to pain.



What is the gut-brain connection?

Scientists have been researching the role nutrition plays in mental health and chronic pain, and they believe one of- if not “the”- key(s) lies in the gut-brain connection.

Our gut is intricately linked to our brain, and the communication is bidirectional, meaning our brain communicates with our gut and vice versa.

In other words it’s a two way street.

The communication between the central nervous system (brain and spinal cord) and the enteric nervous system (nervous system found in the gut) occurs through a complex communication network known as the gut-brain axis, which involves the:

  • Hypothalamic-pituitary-adrenal (HPA) axis – primarily involved in adaptive responses to stress including our limbic systems in our brain

  • Immune cells (cytokine and chemokines)

  • Vagus nerve – thought to be the main driver behind the mind-body connection, and an entire blog post could be devoted to just this nerve!

  • Short chain fatty acids (brain food) – produced when commensal gut bacteria ferment fibers in the colon

  • Neurotransmitters (including serotonin our mood regulator, acetylcholine and GABA)

  • Gut permeability – transportation, absorption and balance of nutrients, immunity and tolerance of foreign substances

  • Gut microbiome

  • Autonomic nervous system (ANS) – driving both afferent (sensory) and efferent (motor) signals

What is the enteric nervous system?

The enteric nervous system (ENS) is a part of, or division of the autonomic nervous system and consists of a mesh-like system of neurons that governs the function of the gastrointestinal tract.

It’s because of this intricate 50-100 million nerve cell network that the gut has been affectionately labeled the “second brain.”


The complex network of nerve cells allows us to monitor, communicate and integrate information from our gut to the rest of our bodies.

It also controls:

  • movement throughout the gut (peristalsis)

  • exchange of fluids to assist with digestion (digestive juices)

  • maintains blood flow


The ENS is the heart of the gut-brain axis.

The link between mood and our gut

There is an enormous volume of literature supporting how gut-health is linked to mood, with the key lying in the gut-brain axis.

The millions of nerve cells that line the gut, do so much more then just help us digest the food we eat. They also guide our emotions.


Serotonin is a chemical (neurotransmitter) produced in the brain and in the gut that makes us feel happy, sleep better, modulates pain perception and helps us feel a general sense of wellbeing. In fact, over 90% of the serotonin in your body is produced in the gut. Researchers have found that an imbalance of serotonin and other neurotransmitters can significantly impact mood, sleep and our daily behavior.

  • Researchers are looking at the impact of the gut-brain axis on neurotransmitters, and vice versa. They believe the relationship between the two may hold the key to decreasing pain, symptomology and/or disease progression.

  • Research done on the microbiome of animals has produced evidence suggesting that neurotransmitters may be manipulated by introducing bacteria into the gut. There is also ongoing research into how neurotransmitters play a role in our perception of pain. For example in individuals with IBS, there’s been notably lower levels of the neurotransmitters dopamine and serotonin which may explain the heightened pain response to discomfort felt in the gastrointestinal system. Serotonin is thought to be one of the most important signaling molecules involved in the peristaltic reflex. This is the reflex that controls the movement of food through the gut. A shortage of serotonin in the neurons of the gut can cause sluggish digestion and constipation, just as a serotonin shortage in the brain can lead to depression. Interestingly enough, the serotonin receptors 5HT3 and 5HT4 are not just key mediators of motility- they also mediate pain sensation. In other words if serotonin production in the gut is limited, pain sensations are magnified. This indicates that the same gas pain in a healthy gut may be the EXACT same gas pain in the gut of someone with a GI condition such as IBS, but the individual with IBS reacts with a significantly stronger pain response. This also helps explain why SSRIS (selective serotonin reuptake inhibitors) have been shown to have a positive impact in individuals who struggle with IBS and gastrointestinal pain. SSRIS are a classification of antidepressants.

  • Researchers are looking at the role our gut microbiome plays in mood. They believe that our gut microbiota influences brain function via the neuroimmune and neuroendocrine pathways, as well as the nervous system. Early findings show marked changes to microbial diversity in people with mood disorders and it is believed that with more research, the brain-gut-microbiotia axis could be a promising target for disease diagnosis and therapeutic intervention.


The link between the vagus nerve and pain

You cannot discuss the gut-brain connection and not discuss the vagus nerve. The vagus nerve—Latin for “wandering” nerve—is the tenth of the twelve cranial nerves.


Much like root system of a tree, the vagus nerve extends from the brain all the way down through the intestines.

It is the longest, largest, and most complex of the cranial nerves.


The vagus nerve influences symptom changes in all sorts of painful mental and physical problems like depression, migraines, epilepsy, inflammation, and arthritis, among others.


In addition, stimulating the nerve has proven to be effective in treating many conditions and mitigating painful symptoms. For example, stimulating the vagus nerve can help to treat symptoms of depression.


Further down, the system reaches the gut, and individuals with painful digestive issues are often found to have insufficient vagal activity. On the flipside, stimulating the vagus nerve can help alleviate painful gastrointestinal spasms in individuals with diseases like IBD or issues like IBS. Painful symptoms such as abdominal bloating, cramps, and spasms, all decrease with activation of the vagus nerve. Additionally symptoms like reflux or GERD, and motility issues like constipation and diarrhea improve. This is because the vagus nerve stimulates the muscles of the small and large intestine to push food and waste through the digestive tract.


Even conditions like SIBO have been connected the vagus nerve. This is because the vagus nerve helps with HCL production. If the vagus nerve is damaged or not functioning optimally, it can result in low stomach acid, and low enzyme production (which needs stomach acid to be activated). This, in turn, results in poor digestion, gut pain, and can even open the doorway to bacterial overgrowth, viruses, and parasites.

The nerve’s activation or deactivation also connects to the well-being of the heart, lungs, and immune system . This is why the nerve has been implicated in shallow breathing, rapid heart rate and headaches. The vagus nerve acts as the brakes for the heart. So irritation or inhibition of the nerve removes the brakes for heart rate, which means a faster heart.


Much of the role the vagus nerve plays in pain mediation is because it’s (what I call) the “Zen mediator” of the parasympathetic nervous system—the laid-back rest and digest counterpart to the sympathetic nervous system and its fight-or-flight response.


Meaning that if the vagus nerve is working well, we can relax and become meditative and our response to, and perception of pain lessens. But if it’s not working well then we become anxious and overstimulated and as a result our pain response is magnified. This is because the vagus nerve controls our relaxation levels by slowing our heart rate, switching off the body’s inflammatory responses, and initiating the release of calming chemicals.


In fact with enough activity the vagus nerve has the ability to soothe an asthma attack or even an epileptic seizure. It’s even been shown to help treat fibromyalgia, a medical condition that causes systemic muscle pain.


The Bottom Line:

1. The gut and the brain are intricately connected via the gut brain connection

2. The ENS (located in the gut) and neurotransmitters (produced in the gut) impact our mood and pain response

3. The vagus nerve is the bi directional line of communication between the brain and the gut

4. Understanding the role of the vagus nerve in either upregulation of the nervous system (fight or flight) or relaxation and pain relief (a parasympathetic state) help you minimize pain and decrease inflammation.

5. When you feel pain, the gut brain connection can either cause the brain to say, “Hey that’s interesting. Turn up the volume on this pain information that’s coming in”, or it can say, “Oh no – let’s turn down the volume on that and pay less attention to it.”


In next weeks blog I will get into stress and the gut, and the role both play in pain!

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