The Second Brain
28 May 2019
Many of us may be familiar with the term “It’s just a gut feeling”, but why has the term evolved?
The gut-brain axis sounds a little like a new sci-fi movie but actually, it’s probably one of the most important aspects of our overall health and staying well. 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.
The microbiota, the gut, and the brain communicate through the gut-brain axis in a bidirectional way that involves the autonomic nervous system via this nerve called The Vagus. It is the tenth cranial nerve, extending from its origin in the brainstem through the neck and the thorax down to the abdomen. Because of its long path through the human body, it has also been described as the “wanderer nerve”. The autonomic nervous system is a control system that acts (largely unconsciously) and regulates many of our bodily functions such as the heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal and is in itself, subdivided into 2 branches: the sympathetic branch or the parasymathetic branch. These two branches work in a yin- yang way:
Sympathetic Nervous System
Meet Jim - Jim is hiking in the wilderness all by himself and enjoying the tranquillity of the great outdoors when he turns a corner and finds himself only feet away from a full-grown bear. In a fraction of a second, the brain recognises the animal in front of him as a bear and classifies it as a very big threat. Immediately, the sympathetic nervous system is activated and without any conscious control by Jim at all, several body functions are affected all at once:
The Parasympathetic Nervous System
By contrast, the parasympathetic nervous system is responsible for the body's rest and digestion response when the body is relaxed, resting, or feeding. It basically undoes the work of sympathetic division after a stressful situation. The parasympathetic nervous system decreases respiration and heart rate and increases digestion. Stimulation of the parasympathetic nervous system results in:
The enteric nervous system (ENS) is another part of the autonomic nervous system that controls the functioning specifically of the gastrointestinal tract. The ENS is made up of two thin layers with more than 100 million nerve cells in them — more than the spinal cord! These cells line the gastrointestinal tract, controlling blood flow and secretions to help the GI tract digest food. They also help us “feel” what’s happening inside the gut.
As such we see that high levels of cortisol and adrenaline from stress the sympathetic nervous system dominates, inhibits the Vagus Nerve and thus has detrimental effects on the enteric nervous system and gastrointestinal tract overall. Here are some symptoms related to just the gut from the impacts of stress:
Another concept to grasp is that “stress” as far as the body is concerned is mental, emotional and physical: be that running late for a meeting, waiting for a cancelled train (Southern Rail!!), having crossed words with our children or partners to fighting an infection, we must evoke a stress response.
The vagus nerve, could be referred to as the captain the the parasympathetic nervous system - it’s activation leads to the release of acetylcholine and stimulates muscle contractions in the parasympathetic nervous system, as such an anti-inflammatory pathway has been described through Vagus Nerve fibres, which is able to dampen peripheral inflammation and to decrease leaky gut, thus very probably modulating microbiota composition.
That Gut Feeling
The most important function of the vagus nerve is afferent, bringing information of the inner organs, such as gut, liver, heart, and lungs to the brain. In this direction - gut to brain: The Vagus Nerve is able to sense the microbiota metabolites through its fibres, to transfer this gut information to the central nervous system where it is integrated in the central autonomic network, and then to generate a response. For instance, fibres sense inflammatory bacterial byproducts, called LPS or endotoxin. LPS activate vagal afferent fibres and have been documented in depression and mood disorders.
In a different study healthy mice chronically treated with the probiotic strain Lactobacillus rhamnosus had reduced stress-induced corticosterone and anxiety - and depression-related behaviour - now that’s a gut feeling if ever I head one!
The other direction of travel (efferent) is brain to gut. Historically, the vagus has been studied as an efferent nerve and as an antagonist of the sympathetic nervous system. In the gastrointestinal tract, the activation of the parasympathetic nervous system increases bowel motility and glandular secretion. In contrast to it, the sympathetic activity leads to a reduction of intestinal activity and a reduction of blood flow to the gut, allowing a higher blood flow to the heart and the muscles, when the individual faces existential stress.
How to work on the Vagus
Working on the Vagus Nerve (VN) which in itself has anti-inflammatory properties, is of key interest to restore homeostasis in the microbiota-gut-brain axis.
You can indirectly stimulate your vagus nerve to relieve keyed up or shut down nervous system states. Remember, your vagus nerve passes through your belly, diaphragm, lungs, throat, inner ear, and facial muscles. Therefore, practices that change or control the actions of these areas of the body can influence the functioning of the vagus nerve through the mind-body feedback loop:
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I hope this blog, albeit a little long has been interesting for you.
In health, Tanya x