A study in Cell says breathing increased levels of carbon dioxide increases acid levels in the brain, and that acid-sensing channels, especially in the amygdala, invoke fear behaviors. This is why people freak out when they are drowning or suffocating. This is almost like using the brain as a sense organ.
This suggests a place to look for causes and treatments of panic disorders. It also suggests that genetic variations in this acid sensing might cause people to be more or less susceptible to PTSD or other anxieties.