Whyat triggers my gag reflex when the dentist takes x rays in my mouth?

Stimuli to brain: A gag reflex is normal to prevent anything, which is not part of the natural eating and swallowing process, from entering the throat in order to keep the person from choking by obstruction of the respiratory tract, the upper part of which is connected to the upper part of the digestive tract. The glossopharyngeal nerve fibers transmit impulses to your brain. Some people are more sensitive.

Can't breathe: I believe it is a reaction to your tongue being pushed back into your airway, cutting off your airway. If you were going to gag yourself, you would put your finger down the back of your throat, the xray goes nowhere near there, but if your have a small oral cavity something extra is just too much for you to handle.