Boo! How Your Brain Reacts to Fear

WRITTEN BY: Neurosurgery
Monday, October 27th, 2014

Here’s the scenario. Your house loses power. You hear a creaky noise coming from upstairs. You swear someone is whispering your name. A shadow creeps up from behind. You become so scared that you can’t move. Your heart races. You feel a rush of adrenaline. You’re ready for whatever might be coming for you. But then the lights come back on and you realize your dog is upstairs and your friend has been calling your name from the bathroom. All goes back to normal and you laugh it off. But what just happened is the result of your brain taking the necessary actions to help you survive, should that monster have been real.

So what actually happens?

Your brain’s window to the world, also known as the thalamus, receives sensory input from your eyes, ears and other sources. When your brain perceives fear or a threat, signals are sent from the thalamus to two regions of the brain – the amygdala and hypothalamus.

When the signal reaches the amygdala — a region near the base of the brain that plays a key role in processing emotions and is linked to fear responses and pleasure, it releases a brain chemical called glutamate, which makes you freeze or involuntary jump. This is a known evolutionary response that is designed to keep you hidden from any potential danger, like that ghost lurking in the attic.

Now, here comes the part that makes you feel like Superman. When the signal reaches the hypothalamus, a portion of the brain that maintains the body’s internal balance and helps regulate and produce hormones, it elevates your blood pressure and heart rate and pumps adrenaline throughout your body. This is a key component to your body’s “fight or flight” response – an evolutionary adaptation that allows you to react to fear or danger quickly.

The adrenaline increases blood flow to the muscles, giving you a surge of increased physical strength. The adrenaline also increase oxygen to the lungs, allowing you to run faster than you normally would. Your ability to feel pain also decreases. Basically, your body is prepared for whatever might try to attack or scare it – whether it’s a werewolf, or a mouse.

Luckily, for most of us, we don’t need to kick into the fight or flight mode very often. After the initial fear response turns out to be just a bug you didn’t know was there, the parasympathetic nervous system reverses the flow of adrenaline and lowers your heart rate back to normal.

So the next time you think you spot a ghost or goblin, or a house centipede, remember it’s your brain causing you to run faster, jump higher and lift cars. Well maybe not cars, but very heavy things.



UPMC is ranked among the top hospitals in the nation for neurosurgical care by U.S. News & World Report. In addition, UPMC is one of the few medical centers in the U.S. – and throughout the world – with expertise in the full array of surgical options for brain and spine conditions. Our world-renowned experts are committed to finding the best treatment approach for each patient, maximizing the benefits of surgery while minimizing the risks. Read More