The enteric nervous system is a network of neurons, which are nerve cells; chemical messengers called neurotransmitters; and special proteins located throughout the gastrointestinal system. It is sometimes referred to as the nervous system of the gut, or the "brain" or "mind" of the gut, but because it actually runs from the beginning to the end of the gastrointestinal system, it is not really confined to the gut area. Neurons and the neurotransmitters by which neurons are influenced are located not only in the brain, which belongs to the central nervous system, but also in the lining of the esophagus, stomach, small intestine and large intestine. Observations of the effects of treatments and drugs on the gastrointestinal system have prompted some neurologists and other healthcare providers to think of the enteric nervous system as a second "brain" that actually communicates with and affects the primary brain.
During embryonic development, the enteric nervous system is formed from the same chunk of tissue from which the central nervous system is formed. That tissue is called the neural crest. The fact that these two systems share the same origin makes it less surprising to find that they contain some of the same type of cells, neurotransmitters, brain proteins — and that one affects the other. For example, certain substances that affect the brain or the mental state of a person also affect the enteric nervous system. Drugs such as antidepressants exert an influence on the neurotransmitter called serotonin that is located in the central nervous system as well as in the enteric one.
Although an antidepressant works on the psychic aspect of the person, it also can work in the gastrointestinal system, causing such disturbances as diarrhea or nausea. Heroin and morphine figure among the drugs that can significantly interfere with normal digestion. It is evident that what takes place in the central nervous system is somewhat mirrored in the enteric nervous system, causing some neurologists to suspect that drug addiction might be a matter of a dependency in both systems. This connection is easily realized when speaking of the enteric nervous system in the figurative sense.
When an individual is faced with a frightening and dangerous situation, the brain of the central nervous system responds by releasing hormones to help the person react to the sudden stress. This is how the "fight or flight" decision is made and is evidence that even when drugs are not involved, the person's mental state affects the enteric nervous system. Sensory nerves located in the stomach are affected by the release of stress hormones, which is why "butterflies" in the stomach can be produced at such times.