# Fear (DD-748) > When the amygdala perceives danger, it signals the hypothalamus. The hypothalamus activates the sympathetic nervous system, and the adrenal glands release epinephrine (adrenaline), raising heart rate, blood pressure and blood sugar. If the threat persists, the HPA axis takes over: CRH from the hypothalamus triggers ACTH from the pituitary, which prompts the adrenal cortex to release cortisol. The amygdala is central to fear of external threats, but a 2013 study showed that people without functioning amygdalae can still feel fear and panic triggered from inside the body. - Department: Science - First published: 2026-10-07 - Last updated: 2026-10-07 - Permalink: https://detailenjoyer.com/case/fear/ - Original Reel title: -'^s - Author: Marcus Easterling (DetailEnjoyer), The Detail Department ## Quick facts - **DD-748-F01**: Cortisol comes from the adrenal cortex, the last step of the HPA chain. _(Write-up · Exhibit C (The slow wave))_ - **DD-748-F02**: People with bilateral amygdala damage still felt fear when they inhaled carbon dioxide. _(Write-up · Exhibit D (Not only the amygdala))_ ## Write-up What your brain and body do when danger appears. A fast adrenaline surge, then a slower cortisol wave. 1. **The alarm**: Information from the eyes and ears reaches the amygdala. When it perceives danger, it sends a distress signal to the hypothalamus. This cascade can begin before the brain's visual centers have fully processed what is happening. 2. **The fast wave**: The hypothalamus activates the sympathetic nervous system through nerves to the adrenal glands, which release epinephrine (adrenaline). Heart rate, blood pressure and breathing go up, and glucose and fats are released for energy. 3. **The slow wave**: As the initial epinephrine surge subsides, the HPA axis takes over. The hypothalamus releases CRH, the anterior pituitary releases ACTH, and the adrenal cortex releases cortisol. Cortisol then feeds back to shut the response down. 4. **Not only the amygdala**: Patient SM and two others with bilateral amygdala damage felt fear, and had panic attacks, after inhaling 35% carbon dioxide. The authors concluded the amygdala is not required for fear triggered from inside the body. 5. **The brake**: When the threat passes, cortisol falls and the parasympathetic nervous system dampens the stress response. ## Related case files - [Anger (DD-745)](https://detailenjoyer.com/case/anger/): The same sympathetic surge in another emotion. Anger, the series' threat response: an amygdala-to-hypothalamus threat system with weaker prefrontal control. - [Surprise (DD-747)](https://detailenjoyer.com/case/surprise/): Same emotion series. Surprise, the series' broken-expectation response: a burst of noradrenaline from the locus coeruleus. - [Disgust (DD-746)](https://detailenjoyer.com/case/disgust/): Same emotion series. Disgust, the series' contamination response: tracked by activity in the anterior insula. - [Happiness (DD-743)](https://detailenjoyer.com/case/happiness/): Same emotion series. Fear, the series' danger response: the amygdala signals the hypothalamus, and adrenaline then cortisol follow. - [Sadness (DD-744)](https://detailenjoyer.com/case/sadness/): Same emotion series. Fear, the series' danger response: the amygdala signals the hypothalamus, and adrenaline then cortisol follow. - [SSRIs (DD-749)](https://detailenjoyer.com/case/ssris/): More of the body's chemical signaling: adrenaline and cortisol in the fear response. ## Reel vs. record - **Mechanism**: the HPA 'axis' doesn't release cortisol in one step. The hypothalamus releases CRH, the pituitary releases ACTH, and the adrenal cortex then releases cortisol. - **Timing**: the HPA response isn't simultaneous. Harvard Health describes it as a second wave that follows as the initial epinephrine surge subsides. - **Simplification**: the amygdala is central to fear of external threats but not essential to all fear. Amygdala-damaged patients still felt fear and panic after inhaling carbon dioxide (Feinstein et al., 2013). - **Missing context**: the amygdala and hypothalamus can start the cascade before the brain's visual centers have fully processed the scene. - **Missing context**: the parasympathetic nervous system acts as the 'brake', calming the body once the threat passes. ## Video: Fear: What Your Brain and Body Do When Danger Appears - Duration: 23.9 s · Published: 2026-10-07 - File: https://detailenjoyer.com/assets/videos/DeMZJ40x3Eg.mp4 ### Transcript DID YOU KNOW? Fear begins as the brain rapidly detects potential danger with the amygdala coordinating signals from sensory systems and triggering defensive response. The hypothalamus activates the sympathetic nervous system, causing the adrenal glands to release adrenaline and noradrenaline. At the same time, the hypothalamic pituitary adrenal axis releases cortisol, helping maintain energy availability during prolonged stress. NOW YOU KNOW! ## Questions & answers **Q: Which brain region sends the first distress signal to the hypothalamus?** A: The amygdala signals the hypothalamus when it perceives danger. _(Source: Write-up · Exhibit A (The alarm) · Harvard Health: Understanding the stress response)_ **Q: Which hormone causes the fast surge in heart rate?** A: The adrenal glands pump epinephrine into the blood. _(Source: Write-up · Exhibit B (The fast wave) · Harvard Health: Understanding the stress response)_ **Q: Which part of the adrenal gland releases cortisol?** A: ACTH prompts the adrenal cortex to release cortisol. _(Source: Write-up · Exhibit C (The slow wave) · Harvard Health: Understanding the stress response; Cleveland Clinic: Hypothalamic-Pituitary-Adrenal (HPA) Axis)_ **Q: What triggered fear in the patients with amygdala damage?** A: They inhaled 35% carbon dioxide. _(Source: Write-up · Exhibit D (Not only the amygdala) · Feinstein et al. (2013), Fear and panic in humans with bilateral amygdala damage, Nature Neuroscience (PMC))_ **Q: Which system calms the body after the threat passes?** A: The parasympathetic system is the 'brake'. _(Source: Write-up · Exhibit E (The brake) · Harvard Health: Understanding the stress response)_ ## Sources - [Harvard Health: Understanding the stress response](https://www.health.harvard.edu/healthy-aging-and-longevity/understanding-the-stress-response) - [Cleveland Clinic: Hypothalamic-Pituitary-Adrenal (HPA) Axis](https://my.clevelandclinic.org/health/body/hypothalamic-pituitary-adrenal-hpa-axis) - [Feinstein et al. (2013), Fear and panic in humans with bilateral amygdala damage, Nature Neuroscience (PMC)](https://ncbi.nlm.nih.gov/pmc/articles/PMC3739474/) ## Cite this case file Easterling, Marcus. "Fear." The Detail Department (DetailEnjoyer), case file DD-748, last updated Oct 7, 2026. https://detailenjoyer.com/case/fear/