Have you ever experienced butterflies in the stomach before an important meeting? If so, then you are already acquainted with the existence of a connection between our moods and our gut. Indeed, the brain and the digestive system are linked by complex pathways where information flows back and forth on a continual basis: certain feelings and thoughts can stimulate an exaggerated gut response, while sensitized nerves in the gut can trigger changes in the brain.
The nervous system and the "second brain"
The enteric nervous system (ENS) is a semi-independent part of the autonomic nervous system (ANS) whose function is to control the gastrointestinal (GI) system. It has been coined “the second brain” by Dr. Michael D. Gershon in 1996 in reference to the complexity of its functions. Since the 80’s, the concept of the enteric nervous system and the role of neurotransmitters in the gut have been accepted by the scientific community. The connection between the "two brains" is accountable for the direct relationship between emotional stress and physical distress and it explains why conditions such as anxiety, depression, irritable bowel syndrome, ulcers and Parkinson's disease manifest symptoms both at the brain and at the gut level.
The ENS comes complete with a network of more than 100 million neurons, neurotransmitters and proteins, combined to a complex neuronal circuitry that enables it to control the bowel and produce "gut feelings" separate from the brain's impulses. In each situation, the gut must assess conditions including progress of digestion, presence of nutrients, acidity level, etc., decide on a course of action and initiate a reflex. This amazing piece of work continues to function even when the vagus nerve - the primary neural conduit between the gut and the brain - is severed! It’s location in the GI tract, right next to the systems that requires control, makes perfect sense from an evolutionary standpoint.
Psychology or physiology, which one comes first?
Psychology clearly plays an important role in gut disorders. According to Dr. Emeran Mayer, professor of medicine, physiology and psychiatry at U.C.L.A., the majority of patients with anxiety and depression present alterations of their GI function. Dr. Mayer also reports that up to 70 % of the patients he treats for chronic gut disorders had experienced early childhood traumas. This observation is corroborated by recent studies in animal models which demonstrate that early life stress is associated with chronic GI diseases. 
Stress affects the gut in several ways. In response to a perceived stressor, the brain triggers a response along two major bodily paths: the hypothalamic-pituitary-adrenal axis and the autonomic nervous system. The resulting increased secretion of cortisol, adrenaline and noradrenaline directly affects the ENS. Corticotropin-releasing-factor (CRF), a peptide found in both the brain and the gut, is another substance which appears to have major significance in the stress response. Another interesting experiment on rodents demonstrates the correlation between stress and a leaky gut. 
On the flip side, several factors lend credence to physiology as the source of intestinal dysfunctions. For example, when the mast cells activate an immune response  resulting in mucosal inflammation, the release of inflammatory cytokines generates an acute stimulation of the hypothalamic-pituitary-adrenal (HPA) axis. In other words, GI inflammation triggers an increased firing of the gut’s sensory neurons culminating in a kind of sensory hyperactivity.
Serotonin provides another interesting argument supporting the gut-over-brain theory. This key neurotransmitter essential to our well-being is stored at 95 percent in the ENS where it is synthesized. Among other things, serotonin acts as a go-between, keeping the brain up to date with what is happening in the gut. Contrary to earlier assumptions, it has been found that 90 percent of the fibers in the vagus nerve carry information from the gut to the brain, and not the other way around.
Finally, the emerging concept that bacteria teeming in the gut - collectively known as the microbiome - can affect not only the gut, but also the mind may shed additional light on our understanding of the gut-brain axis.
Commensal gut microbiota
We are all born with a sterile gut but over time, it gets colonized by a diverse and distinct brew of bacterial species determined by genetics and by bacteria surrounding us. The incredible 100 trillion microbes - more than ten times the amount of cells in our entire body! – which make the GI tract their playground are absolutely critical to our health. Over the last few years, increasing evidence from studies in rodents are pointing to an effect of commensal gut microbiota on the CNS. Researchers have found that the gut microbiome can influence neural development, brain chemistry and a wide range of behavioral phenomena including emotional behavior, pain perception and the stress response.
In a 2011 study, Bienenstock and colleagues fed a broth enhanced with the probiotic Lactobacillus rhamnosus to a group of mice and plain broth to the control group. After 28 days, the researchers subjected the mice to a battery of tests to detect signs of anxiety and depression. What they discovered is that mice who had been fed the probiotic solution demonstrated less fear-response behaviors and anxiety compared to the control group. Interestingly, when the vagus nerve was severed, the effects of gut bacteria on brain biochemistry, stress response and behavior evaporated. The researchers concluded that: "These findings highlight the important role of bacteria in the bidirectional communication of the gut-brain axis and suggest that certain organisms may prove to be useful therapeutic adjuncts in stress-related disorders such as anxiety and depression".
Some studies also suggest that gut bacteria are closely tied to early brain development and subsequent behavior. According to Dr. Rochellys Diaz Heijtz, a researcher specialized in the neurobiology of common neurodevelopment psychiatric disorders such as Attention-Deficit/Hyperactivity Disorder (ADHD) and autism: "The data suggests that there is a critical period early in life when gut microorganisms affect the brain and change the behavior in later life."
Happy gut, happy mind
Given the intimate feedback loop between the gut and the brain, we must consider addressing GI dysfunctions when treating mood imbalances and behavioral or developmental issues. The use of high quality probiotics including Lactobacillus rhamnosus and Bifidobacterium longum strains, along with digestive support such as DPP-lV enzyme and L-glutamine are proven strategies. Restoring mood balance is also crucial in achieving GI health. Adaptogenic herbs including Siberian ginseng, ashwagandha, rhodiola or holy basil combined with adrenal tissue extracts can support a healthy stress response, while substances such as GABA, L-theanine, inositol, SAMe and B complex vitamins can help achieve a balanced mood.