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Fear

Short answer: 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.

VideoWrite-up3 sources

In collections:The Brain

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What should you know about Fear?

Quick facts. Each one stands alone and has a stable ID you can cite.

  1. Cortisol comes from the adrenal cortex, the last step of the HPA chain.

    DD-748-F01 · Write-up · Exhibit C (The slow wave)

  2. People with bilateral amygdala damage still felt fear when they inhaled carbon dioxide.

    DD-748-F02 · Write-up · Exhibit D (Not only the amygdala)

Video transcript: Fear: What Your Brain and Body Do When Danger Appears

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!

Burned-in captions, read frame by frame (see transcript.md) · 23.9s Reel ·

Video coming soonFear · Instagram Reel
Fear: What Your Brain and Body Do When Danger Appears Instagram Reel · · 23.9s

The write-up

What your brain and body do when danger appears. A fast adrenaline surge, then a slower cortisol wave.

  1. Exhibit A

    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. Exhibit B

    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. Exhibit C

    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. Exhibit D

    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. Exhibit E

    The brake

    When the threat passes, cortisol falls and the parasympathetic nervous system dampens the stress response.

Questions people ask about Fear

Which brain region sends the first distress signal to the hypothalamus?

The amygdala signals the hypothalamus when it perceives danger.

Source: Write-up · Exhibit A (The alarm) · Harvard Health: Understanding the stress response

Which hormone causes the fast surge in heart rate?

The adrenal glands pump epinephrine into the blood.

Source: Write-up · Exhibit B (The fast wave) · Harvard Health: Understanding the stress response

Which part of the adrenal gland releases cortisol?

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

What triggered fear in the patients with amygdala damage?

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)

Which system calms the body after the threat passes?

The parasympathetic system is the 'brake'.

Source: Write-up · Exhibit E (The brake) · Harvard Health: Understanding the stress response

Test Your Knowledge

All quizzes →

5 questions, all from this case file. Answers are revealed instantly.

  1. Which brain region sends the first distress signal to the hypothalamus?

    1. Cerebellum
    2. Amygdala
    3. Hippocampus
    4. Occipital lobe
    Show the answer

    Answer: B. Amygdala The amygdala signals the hypothalamus when it perceives danger. Source: Write-up · Exhibit A (The alarm) · Harvard Health: Understanding the stress response

  2. Which hormone causes the fast surge in heart rate?

    1. Insulin
    2. Epinephrine (adrenaline)
    3. Melatonin
    4. Oxytocin
    Show the answer

    Answer: B. Epinephrine (adrenaline) The adrenal glands pump epinephrine into the blood. Source: Write-up · Exhibit B (The fast wave) · Harvard Health: Understanding the stress response

  3. Which part of the adrenal gland releases cortisol?

    1. Adrenal medulla
    2. Adrenal cortex
    3. Pituitary
    4. Thyroid
    Show the answer

    Answer: B. Adrenal cortex 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

  4. What triggered fear in the patients with amygdala damage?

    1. Spiders
    2. Carbon dioxide inhalation
    3. Loud noises
    4. Heights
    Show the answer

    Answer: B. Carbon dioxide inhalation 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)

  5. Which system calms the body after the threat passes?

    1. Sympathetic
    2. Parasympathetic
    3. Lymphatic
    4. Skeletal
    Show the answer

    Answer: B. Parasympathetic The parasympathetic system is the 'brake'. Source: Write-up · Exhibit E (The brake) · Harvard Health: Understanding the stress response

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/

Permalink: https://detailenjoyer.com/case/fear/ · Last updated

Markdown version

Cite a single fact by its ID, for example DD-748-F01 → https://detailenjoyer.com/case/fear/#DD-748-F01

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