The Gut-Brain Axis

 The Gut-Brain or “Second Brain”

“Technically known as the enteric nervous system, the second brain consists of sheaths of neurons embedded in the walls of the long tube of our gut, or alimentary canal, which measures about nine meters end to end from the esophagus to the anus. The second brain contains some 100 million neurons, more than in either the spinal cord or the peripheral nervous system.”(4)

The enteric nervous system (ENS) or intrinsic nervous system is part of the autonomic nervous system which includes all of the nerves that branch out from the brain and spinal cord and extend to other parts of the body including muscles and organs. It is the control system that acts largely unconsciously to regulate bodily functions such as heart rate, digestion, respiratory rate, pupillary response etc. The ENS consists of a mesh-like system of neurons which governs the function of the gastrointestinal tract. 

Governing the Messy Business of Digestion

The gut is lined with hundreds of millions of neurons which are in constant communication with the brain, yet can function independently from it. It’s likely we evolved this intricate web of nerves equipped with its own reflexes and senses, to perform the complex business of digestion and to expel waste "on site", without having to involve the brain - which should be free to ponder philosophy and how to survive - and cutting out the middleman (the spinal cord) also. So the messy and complex task of digestion is delegated to this neural network which is so extensive, it has been termed the “Gut-Brain” or “Second Brain’”. But it’s complexity and influence goes far beyond mere digestion and absorption(4). 

The gut-brain axis appears to be bidirectional - the brain acts on GI and immune functions which help to shape the gut's microbial communities, and our gut microbes create neuroactive compounds - including neurotransmitters and metabolites - which impact the brain. Given the sheer size of the second brain and the fact that 90% of the information carried via the vagus nerve “highway” directly linking the gut with the brain, carries information directly from the gut to the brain (and not the other way round), you may not be surprised to learn that the gut can influence and inform our state of mind and emotions(5). The enteric nervous system (ENS) uses more than 30 neurotransmitters, just like the brain, and 95% of the body's serotonin and melatonin is found in the small and large intestines(4), (6). It’s likely you will have experienced this direct gut-brain connection before in the form of “butterflies” in the stomach or getting a “gut feeling”. 

Little Love (Gut) Bugs

The gut’s microbiota inhabitants can also influence the health of both the gut and the brain, impacting mental clarity and affecting our emotions. Stress can alter our gut bacteria populations, which can in turn can influence our mood. It is widely recognised that depression and anxiety are influenced and governed by the health of our gut microbiota. Each of our unique gut microbiome populations and their interactions with our ENS, gut, brain and impact on our overall health is extensive(5). 

Wanderer

"Vagus" means “wanderer” in Latin, accurately representing how the nerve wanders all over the body and influencing the function of numerous internal organs. The vagus nerve plays a key part of your parasympathetic “rest and digest” nervous system, influencing breathing, digestive function and heart rate. Thus, it has a huge impact on your mental and physical health. Stimulating the vagus nerve can activate vagal tone and thus, the parasympathetic nervous system. Having higher vagal tone means that your body can relax faster after stress, while also provide other numerous health benefits, including improved digestion(8). We list a few ways to stimulate the vagus nerve on the Health-Hacks page.

“90% of our brain’s output goes into something called the pontomedullary area, it’s the lower two-thirds of the brain stem, and that goes into the vagus, or the pneumogastric nerve, which innervates the digestive tract.”(3) 

Brain > Vagus Nerve > Gut

The brain and vagus nerve and inextricably linked - when the brain isn’t functioning or firing optimally, (if its inflamed for example) this can lead to poor vagal activity, expressed by decreased pancreatic enzyme secretion, poor gallbladder function and poor overall gut function. This leads to decreased intestinal immune activity and intestinal blood flow and thus, an opportunity for an overgrowth of pathogenic bacteria and yeast arises(3). Vice-versa, when something dysfunctions with the gut, particularly in the case of leaky gut, this can create negative consequences throughout the rest of the body.

How the Brain Influences the Gut 

The frontal cortex stimulates the vagal nuclei to activate gut motility (intestinal peristalsis), which moves the contents of our gut through the digestive tract and out the other end. It also stimulates enzyme secretion, digestive enzyme secretion, the release of hydrochloric acid (HCL) in the stomach and modulates blood flow to the gut. The brain primarily communicates with the gut via neuronal projections and also hormones which are secreted by the hypothalamus. In the gut, the main players are the ENS, the intestinal immune system, (the gut-associated lymphoid tissue and the gut flora), and then the intestinal microglia. Read about the Gut Immune System on the Nature + the Microbiome page.

As we discuss in length in the SIBO + Autoimmunity section, a pathogenic overgrowth of bacteria and/or yeast causes intestinal permeability or leaky gut, and leaky gut causes a state of chronic low grade inflammation. The increased permeability of the intestinal mucosa allows bacteria, bacterial toxins, potentially harmful microorganisms and small molecules to ‘leak’ into the bloodstream. The inflammatory response thus, can become systemic, travelling via cytokines and crossing the blood-brain barrier (BBB). Although one of the BBB’s primary functions is to separate the CNS from soluble inflammatory mediators and effector immune cells from peripheral circulation, when inflamed the BBB can also become ‘leaky’ and inflammation can occur in the brain as a result. So, leaky gut and systemic inflammation can lead to the BBB being compromised, which leads to an inflammation of the brain (aka Leaky Brain)(3), (9). 

Why a “Leaky Brain” + Brain Inflammation is Bad

As we mull over here, any inflammation, especially chronic inflammation should be avoided as it’s hugely damaging. 

Recognising the signs and symptoms of Leaky Brain (which we list below) can be tricky as many overlap with autoimmune systems. But be wary of them- this condition can lead to an increased risk of developing neurological or psychiatric conditions, as well as also being caused by them. It can also cause a host of other neurological diseases and complications which can be read about here. 

 Read more about the gut-brain axis here

• Brain fog
• Fatigue
• Chronic digestive issues
• Progressive cognitive decline
• Memory loss
• Mental disorders - anxiety, depression, ADHD, autism spectrum disorder, behavioral problems
• Cold hands + feet

The below listed causes all have one uniting factor - inflammation. 

1. Gut Dysbiosis/SIBO
2. Leaky Gut
3. Autoimmune Disease
4. Mental Health Conditions
5. Stress
6. Vagus Nerve Dysfunction

Symptoms + Signs

A leaky brain can manifest in numerous ways(4) - 

• ADD/ADHD/Autism spectrum disorders
• Brain fog or difficulty concentrating
• Chronic fatigue that does not improve with rest or sleep
• Headaches or migraines that come on suddenly become worse with standing, and grow more severe as the day progresses
• Memory loss or another cognitive decline which may lead to dementia and Alzheimer’s disease
• Mood disorders - anxiety, depression, and schizophrenia
• Peripheral neuropathy or muscular sclerosis
• Seizures

Treating a leaky brain starts by healing a leaky gut which we provide help for on our Healing the Gut + Autoimmunity page. 

There is a lot more to this which we won’t go into on this page, but you can read more here.