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Fear & the Brain

A Full-Body Experience

We have to react quickly to potential danger in order to stay safe. Once the brain jump starts the fear response, it doesn’t take long for physiological changes to affect the entire body.

First, the sensory organs – our eyes, ears, tongue, nose and skin – pick up cues from our surroundings and feed them to the brain.

The brain’s threat center, a structure called the amygdala, is constantly on the lookout for danger. If it identifies a possible threat, it sounds the alarm, immediately kicking the fight or flight response into gear. Before we know it, our heart’s beating like crazy, we’re taking quick, shallow breaths and sweating in case we have to defend ourselves or make a quick getaway.

These changes are controlled by a part of the peripheral nervous system called the autonomic nervous system, which regulates automatic changes to the body's vital functions.

Scroll over Mr. Goose Bumps to see how fear affects the various parts of his body.

Did you experience the same changes the last time you were scared?


A pair of structures deep within the brain, called the amygdalae, orchestrates the fear response. Before we even know we’re afraid, these almong-shaped structures can send signals to jump-start the physiological changes that help us respond to potential danger.

Once the fear response is under way, other parts of the brain detect what’s happening in the body and make us feel scared.

Our brains also retain memories of fearful experiences, pairing them with details related to those events. Even memories we can’t consciously stir up can help us recognize potential threats and keep us safe. On the other hand, memories can also trigger the fear response when nothing's wrong.

Heart & Lungs

Our large muscles may have to do an unusual amount of work during the fight or flight response. This means they'll need more oxygen than normal. To help meet the increased demand for oxygen, the heart pumps blood to the muscles faster and more forcefully.

Our breathing also picks up. Respiration carries oxygen to the lungs where it gets incorporated into the blood. The circulatory system then delivers it to the muscles.

Adrenal Glands

The brain’s threat center, the amygdala, activates the autonomic nervous system when it detects danger, causing the adrenal glands, located just above the kidneys, to release hormones, such as adrenaline, into the bloodstream. The flood of hormones triggers a host of defensive behaviors.


Sometimes the small hairs on our arms and legs stand up when we get scared. This reflex, called piloerection, is responsible for goose bumps, sometimes called gooseflesh or goose pimples. This response may seem pointless or silly to us today, but it probably helped our hairier ancestors survive by making them look bigger to potential predators. Many animals, especially cats, still use this scare tactic today.

Fear also makes us sweat. Think of this as the body’s cooling system. Sweat is one of the ways our bodies anticipate the hard work of running away or fighting. The excess moisture, responsible for our sweaty palms and clammy hands when we're scared, evaporates and helps cool us down.


The fear response shunts extra blood to our large muscles to prepare us for a fight or to make a speedy getaway. The additional muscle tension can make us literally quake in our boots.

head heart & lungs adrenal glands arm knees