In the gut there is a large network of nerve endings originating from the sympathetic and parasympathetic autonomic nervous system, especially the vagus nerve and spinal cord nerves, as well as an intrinsic neural network, called the enteric nervous system. These networks are interconnected. The vagus nerve is a cerebral nerve that runs down the neck and extends (or "wanders") virtually throughout the body, hence its name (not that it is a "lazy" nerve, as many people think). It is the main nerve of the parasympathetic system. At the intestinal level, it promotes the secretions of the digestive organs and the bowel movements that promote digestion and move the food bolus and faeces forward. It has about 20% of efferent fibres (carrying nerve messages from the brain to the gut) and 80% of afferent fibres (transmitting information from the gut to the brain). Thanks to this nerve, there is constant two-way communication between the brain and the gut. This communication takes place by means of chemicals (neurotransmitters, short-chain fatty acids, peptides, hormones, cytokines, etc.) that are released by the vagus nerve into the gut in the case of efferent information, or by the gut to the vagus nerve for the information to travel to the brain in the case of afferent messages. These chemicals can be produced both by nerve endings, intestinal or immune cells in the gut wall, and, to a large extent, by the microbiota. Thus, the microbiota plays a crucial role in gut-brain communication. I would like to clarify that the vagus nerve, while being the main communication pathway between the gut and the brain, is by no means the only one. Another important communication pathway is the blood pathway (hormones, cytokines or other chemicals produced in the brain travel to the gut via the blood, and vice versa). I won't go much further into the exciting world of the gut-brain axis, which would be enough to write several books, but, by way of example, I could say that it is more than proven that people suffering from psychiatric illnesses such as depression or anxiety, neurodegenerative diseases such as Alzheimer's or Parkinson's or neurodevelopmental disorders (autism spectrum disorders for example) often have a profound alteration of their gut microbiota, known as "dysbiosis". If you want to know more about gut microbiota, I recommend you read this article: The role of the gut and its microbiota. Gut-brain axis.
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