The enteric nervous system can be considered part of the autonomic nervous system. It develops long before birth. During embryonic development, part of the tissue that controls nerve formation migrates to the future brain and spinal cord, where it develops into the central nervous system (CNS). Another part of the same initial tissue attaches itself to the gastrointestinal tract.
A complex network of several million nerve cells thus runs through the entire digestive tract. It is of similar structure and complexity as the brain and is therefore also called the "abdominal brain". This ventral – which means located in the abdomen – control centre manages all digestive processes, including blood circulation and intestinal movement as well as some functions of the immune system.
The parasympathetic nervous system is also called the "resting or recovery nerve". It is responsible for the maintenance of the internal balance of the organism. It also activates intestinal movement and stimulates the cells of the intestinal wall which are responsible for the absorption of nutrients. The sympathetic nervous system, on the other hand, is responsible for an increased willingness to perform in exceptional situations: it is activated during physical exertion or in stressful situations and puts the body into a state of readiness to fight or escape. In return, it slows down the digestive functions.
Stress factors can thus directly influence the intestinal activity: in stressful situations, digestive processes are reduced, which can lead to complaints such as constipation or diarrhoea. In the long term, this can be accompanied by reduced nutrient uptake and a shift in the bacterial balance in the intestine.