The chemistry of fear explained

A screenshot of a video by the American Chemical Society that seeks to explain the body's chief responses to fear, and the chemistry behind the response itself. (American Chemical Society)

It’s a classic horror movie scene: Fleeing from a crazed killer (or tentacled alien or hairy beast or whatever), the frightened victim ducks into a closet ... and freezes, paralyzed.

But why freeze? What governs that response? What is fear?

The American Chemical Society knows.

A new video put out by the science organization offers a concise explanation of the reactions in the body that cause those reactions -- and it all comes down to chemistry.

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"Fear is the expectation or the anticipation of possible harm ... We know that the body is highly sensitive to the possibility of threat, so there are multiple pathways that bring that fear information into the brain," explained Abigail Marsh, associate professor of psychology at Georgetown University.

Imagine the following scenario: you’re watching TV late at night, when you hear a crash from the porch. An intruder? A spook? Tentacled alien?

“The nerves in your ears that transduce that sound are the first part of the nervous system,” Marsh said. That signal is relayed to the thalamus, a telephone switching station in your brain, and then directly to the amygdala, which releases neurotransmitters throughout the body -- notably glutamate, essentially the chemical behind fear.

“The actions of glutamate in the amygdala in response to the fearful thing you’ve heard set off this cascade of other responses,” Marsh explained.

A reciprocal response comes from an area of the brain called the “periaqueductal gray,” a region deep within the ancient brain that controls two classic responses to fear: jumping and freezing. Sound familiar? The hypothalamus controls the fight or flight responses -- increased heart rate and so on.

A signal sent to the adrenal glands in your torso causes them to send out cortisol and adrenaline. The fear response also a release of glucose into the bloodstream -- a power up to get you running for your life.

Depending on the level of risk, the body regulates the response from these various systems to control whether we fight, freeze -- or flee like scared little kids we all are.