Abigail Marsh, associate professor of psychology at Georgetown University, said that fear is the expectation or the anticipation of possible harm and that people know that their body is highly sensitive to possibility of threat, so there are multiple pathways that bring that fear data into the brain.
She said that the nerves in their ears that transduce that sound are the first part of the nervous system.
The signal is relayed to the thalamus, a telephone switching station in brain, and then to the amygdala that releases neurotransmitters throughout the body - especially glutamate, essentially the chemical behind fear, Fox News reported.
The response to it comes from an area of the brain called "periaqueductal gray," - a region that's deep within the ancient brain that controls two classic responses to fear: jumping and freezing.
A signal sent to the adrenal glands in the person's torso causes them to send out cortisol and adrenaline; fear also releases glucose into the bloodstream.