One UAB expert explains the chemistry that takes place in the body when the “fight-or-flight” response is activated.A racing heart, quickened breath and tense muscles are common reactions in the body to fear, often before one even realizes what they are afraid of. This built-in alarm system mobilizes every body part to survive, whether the threat comes from the world around or from within.

“Fear can freeze us in place, push us to fight or flee, or even drive us to seek comfort in others,” said Ashley Reno, Psy.D., associate professor and chief psychologist in the University of Alabama at Birmingham’s Department of Psychiatry and Behavioral Neurobiology. “This rapid response, often called the ‘fight-or-flight’ reaction, is essential to human survival. However, when fear systems become overactive or misfire, they can contribute to conditions like PTSD, phobias and chronic anxiety.”

Reno provides insight into what happens inside the brain when fear takes hold.

Inside the brain’s alarm system

Fear begins deep in the brain. The amygdala, buried within the temporal lobe, acts as an alarm system, detecting threats and signaling regions like the hypothalamus and brainstem to activate the body, producing a racing heartbeat, faster breathing or a startle reflex.

“At the same time, higher-level regions such as the prefrontal cortex and hippocampus provide context,” Reno said. “These two systems interact constantly, with the cortex influencing and sometimes calming the amygdala’s activity. They weigh memory and reasoning against the amygdala’s alarm, helping us distinguish between a true threat and a false one.”

When this balance holds, fear protects individuals. When it breaks down, imagined threats can feel as real as physical danger.

The chemistry of fear

Fear is not just a feeling; it is chemistry in motion. The brain relies on a balance of several key neurotransmitters to decide how strongly fear is felt, how the body reacts and how the experience is stored in memory. Glutamate, the brain’s primary excitatory messenger, sparks fear circuits into action, while GABA, the primary inhibitory messenger, applies the brakes.

“Neurotransmitters such as norepinephrine sharpen focus and heighten arousal, serotonin regulates mood and anxiety, and dopamine helps the brain learn from the experience by linking threats to responses,” Reno said. “Striking the right balance, especially between glutamate’s accelerator and GABA’s brake, is critical. When that balance tips, the brain can shift from healthy vigilance to chronic anxiety and fear.”

Chronic fear, however, does not only linger in the mind; it can physically reshape it.

“The amygdala may become overactive, causing exaggerated fear, even when there is no real threat,” Reno said. “The hippocampus, which plays a key role in the formation and storage of memories and provides context, may have reduced volume, or atrophy, in the face of chronic stress. As a result, the individual may experience difficulty distinguishing between traumatic memories and actual threats or danger.”

Together, these changes lock the brain into a heightened state of fear, fueling cycles of anxiety, flashbacks and hypervigilance.

Turning neuroscience into treatment

What science reveals about fear also opens the door to healing. Understanding the neuroscience of fear gives psychiatrists and psychologists a roadmap for treatment. Pinpointing whether it is an overactive amygdala, a weakened hippocampus or a disrupted chemical balance, clinicians can tailor therapies to restore regulation.

“We can tailor specific treatments to patients by predicting who will respond best to certain treatments,” Reno said. “These treatments may include cognitive behavioral therapy, cognitive processing therapy, exposure therapy or medications, used alone or in combination, to address the patient’s symptoms and help calm overactive circuits, strengthen regulation and address traumatic memories.”

This knowledge drives psychiatric and psychological research and, importantly, allows clinicians to explain symptoms to patients in a way that validates and normalizes their experience. Ultimately, by uncovering how fear operates in the brain, through circuits, chemistry and lived experience, science is transforming how trauma and anxiety are treated.