Local irritation of the stomach, such as bacteria or food poisoning, activates the emetic center of the brain which in turn signals an imminent vomiting reflex. Retroperistalsis begins in the small intestine and pyloric sphincter.
When local stomach irritation occurs, like from bacteria or food poisoning, it stimulates the vomiting center in the brain, leading to a reflex that causes vomiting. Simultaneously, retroperistalsis begins in the small intestine and the pyloric sphincter.