It is a vital human response to sense danger and to respond with fear. We have what is called a 'flight or fight' reaction to threat. In the amygdala, sited in the temporal lobe region of the brain, a frightening stimulus causes excitation.
This causes release of stress hormones, and various bodily changes such as pupil dilation, breathing rate, a rush of blood flow and glucose to the skeletal regions, all of these preparing us to react with a swift escape or an angry attack. Also, the hippocampus - related to the amygdala - is alerted. This area of the brain is involved in interpreting the threat. The fight or flight process is vital to our well-being and appears to be hard-wired into our neural mechanism.