According to a 2009 study from the University of Arizona and the University of Maryland, activity in a brain region that regulates emotional reactions called the anterior cingulate cortex helps to explain how an emotional insult can trigger a biological cascade. During a particularly stressful experience, the anterior cingulate cortex may respond by increasing the activity of the vagus nerve—the nerve that starts in the brain stem and connects to the neck, chest and abdomen. When the vagus nerve is overstimulated, it can cause pain and nausea.