When you perceive something as dangerous, whether it's a gun pointed at you or a group of people looking unhappily at you, these sensory inputs are first relayed to the amygdala. This small, almond-shaped area of the brain located near your ears detects salience, or the emotional relevance of a situation and how to react to it. When you see something, it determines whether you should eat it, attack it, run away from it or have sex with it.
The prefrontal cortex, located above your eyes, is mostly involved in the cognitive and social aspects of fear processing. For example, you might be scared of a snake until you read a sign that the snake is nonpoisonous or the owner tells you it's their friendly pet. Several pathways prepare different body systems for intense physical action. The motor cortex of the brain sends rapid signals to your muscles to prepare them for quick and forceful movements. These include muscles in the chest and stomach that help protect vital organs in those areas. That might contribute to a feeling of tightness in your chest and stomach in stressful conditions.The sympathetic nervous system is the gas pedal that speeds up the systems involved in fight or flight.