A sense of butterflies in your stomach can result from a surge of stress hormones released during a fight-or-flight event. According to Dr. Gershon, author of The Second Brain, you feel like there are butterflies in your stomach when brain neurons send a message of anxiety to gut neurons. The gut neurons then send messages back up to the brain that it’s unhappy, too. But gut neurons can work on their own, initiating messages that go up to brain neurons when the GI system isn’t happy. Serotonin also acts as a go-between, keeping brain neurons up to date with what is happening in gut neurons—with perhaps 90% of the messages traveling from gut neurons up to brain neurons. Conditions such as anxiety, depression, irritable bowel syndrome, ulcers, and Parkinson's disease evidence symptoms both at brain neuron and gut neuron levels.