The communication path between gut and brain
How do the gut and the brain talk with each other?
In the first week, we mentioned that the gut-brain axis is not clearly defined as the neuroendocrine axes.
Multiple communication channels link these two parts of our body.
We will discuss them starting from the vagus nerve.
This nerve starts from the medulla oblongata, a part of the brainstem, and reaches all the major internal organs of the body: the heart, via the cardiac plexus; the lungs; via the pulmonary plexus, the oesophagus and continues all the way down to the colon.
The vagus nerve is part of the parasympathetic nervous system, the one associated with feeding and resting behaviour (while its opposite one, the sympathetic nervous system, is associated with sudden activation, fighting or fleeing). But it also carries information in the other sense, from the inner organs to the brain, giving it feedback on the state of the body.
This nerve is one of the pathways through which the microbiota can affect the brain: the vagus nerve can “sense” the metabolic products of the bacteria and transmit this information to where it can be integrated.
On the other hand, studies found that the vagus nerve is also composed of nervous fibres that carry anti-inflammatory signals. Decreasing inflammation in the body, and especially in the gut, and decreasing the permeability of the gut membranes, are two ways through which the brain can directly influence the composition of our gut microbiota.
Another communication pathway that connects the gut microbiome and the brain is formed by the short-chain fatty acids (SCFA). We already know that fatty acids can be formed of longer or shorter building blocks. SCFAs, as the name suggests, are formed by only a few atoms of carbon. They are produced by the bacteria when metabolizing the carbohydrates that were not able to be digested. SCFAs are used as an energy substrate by the digestive system, and they improve our gut health by lowering inflammation, by increasing the secretion of mucus and by maintaining the integrity of the intestinal barrier.
A part of the SCFA is absorbed into the bloodstream and can reach the brain, passing the blood-brain barrier, thanks to the monocarboxylate transporters. In the brain, SCFA can bind to specific receptors and activate the hypothalamus, one area that we already know for its role in the regulation of metabolism. They can also modulate neurotransmission in the brain, and promote the synthesis of serotonin, a major neurotransmitter used by the brain.
A third channel connecting the brain and the gut is constituted by the hormonal signalling pathways. We already described in the second week some hormones that are produced by the digestive system and that can influence our feeding behaviour. What is interesting, is that the production of these hormones, such as peptide YY, can be modulated by the amount of SCFAs circulating in our body. SCFAs can also modulate the levels of insulin, ghrelin and leptin, hormones that have an effect on our metabolism, as well as on our mood and cognition.
In conclusion, the brain and the gut can communicate in several ways, some of which would be unthinkable only a few decades ago.
The new knowledge we have recently gained helps us understand not only how the gut and the brain work in normal conditions, but can also shed some light on long-term effects of stress and other psychological conditions on the good functioning of our digestive system.