The latest and perhaps most remarkable finding is the ability that gut flora have to moderate our brain and behavior.

Facts to link between gut and brain

The goings on in our guts is a matter of life or death. If the gut is empty, our brain must be told; if there is a problem with our gut that will hinder food processing and therefore nutrition absorption, the brain will need to be informed. If our gut is facing a pathogen attack, our brain should be kept in the loop.

Generally relation between our gut and brain are hormonal, immunological, and neural, via the central nervous system and the enteric nervous system, which governs the function of the gut. Collectively, they are termed the gut-brain axis.

Although,at first glance, the connections between the gut and brain might seem surprising, we have all experienced it in action. The relationship between stress, anxiety, and a swift bowel movement are no stranger to anyone.

The brain-gut-enteric microbiota axis

These gut-brain conversations have been studied for some time. However, a new level to this partnership has recently been glimpsed; researchers are now considering the influence of our microbiome on the gut-brain axis. In other words,researchers are asking: do the bacteria in our gut affect our psychology and behavior?

Termed,rather clumsily, the brain-gut-enteric microbiota axis or microbiome-gut-brainaxis, researchers are only beginning to scratch its surface.

The gut and stress

In humans, the hypothalami-pituitary-adrenal (HPA) axis is the primary respondent stresses of any kind. It is one of the major players in the limbic system and is heavily involved in emotions and memory.

Stress activates the HPA axis and eventually results in the release of cortisol - the"stress hormone" - which has a variety of effects on many organs,including the brain and gut.

In this way, the brain's response to stress has a direct influence on the cells of the gut, including epithelial and immune cells, enteric neurons, interstitial cells of Cajal (the pacemakers of the bowels), and enterochromaffin cells(serotonin synthesizing cells).

Conversely,these cell types are also under the influence of our resident army of bacteria.Although the mechanisms by which the microbiota regulates the brain are less clear, evidence is mounting that there is, indeed, a two-way dialogue.

Microbes control our mental activity

The first clues that microbes might have some control over our mental activity came more than 20 years ago. Patients with hepatic encephalopathy - a decline in brain function due to severe liver disease - were found to improve substantially after taking oral antibiotics.

Later studies provided further hints that the microbiome had more than a passing influence on states of mind; it was found to impact anxiety and depressive-like behaviors.

Figure is showing how microbes control our mental activity

Another key observation linked dysbiosis (microbial imbalance) with autism. Children with autism often have abnormal and less diverse communities of bacteria in their gut. One researcher concluded:

"We suspect that gut microbes may alter levels of neurotransmitter-related metabolites, affecting gut-to-brain communication and/or altering brain function. [...] Correlations between gut bacteria and neurotransmitter-related metabolites are stepping stones for a better understanding of the crosstalk between gut bacteria and autism."

Researchers in 2004 noted that mice bred to have no gut bacteria had an exaggerated HPA axis response to stress. Further investigations using similar germ-free mice have demonstrated that their lack of gut bacteria alters memory function.

Germ-free mice have been a useful tool to study the micrometer-gut-brain axis. They have helped prove that something is going on, but the results are impossible to extrapolate into humans. They replicate no natural situation known to man -there is no such thing as a germ-free human.

Other studies have used different approaches; some investigated the effects of the neuroactive compounds that gut flora produce; others still have looked at the differences in the gut flora of individuals with psychiatric or neurological differences.

Research,in general, has not been conclusive. Even if changes in gut flora are seen, the eternal chicken or egg question persists: was the psychiatric condition caused by the change in gut flora, or did the psychiatric condition and its altered behavior patterns cause the gut flora to change? Or, is there a two-way interaction?

The major communication paths between gut microbes and the brain

The bidirectional communication between gut microbes and the brain occurs via a number of routes (Dinan and Cryan, 2013). The vagus nerve (cranial nerve X) has both efferent and afferent divisions, and plays a fundamental role in enabling signals from brain to gut and vice versa. Activation of the vagus nerve has also been shown to have a marked anti-inflammatory capacity, protecting against microbial-induced sepsis, an impact mediated by the acetylcholine nicotinic receptor α7 sub unit (Boeckxstaens, 2013). Many of the effects of the gut microbiota or potential probiotics (live bacteria with a health benefit) on brain function are dependent on vagal activation (Bercik et al., 2012). However, vagus-independent mechanisms are also at play in microbiota–brain interactions, as vagotomy fails to influence certain aspects of communication.

The multiple bidirectional routes of communication between the brain and the gut microbiota. These routes include the vagus nerve, the hypothalami-pituitary-adrenal axis (HPA), cytokines produced by the immune system, tryptophan metabolism and production of short chain fatty acids.


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