Study topic · about 8 minutes

Emotion and stress: the amygdala and fight or flight

Emotions come from a network of brain areas, not from one center. The amygdala spots threats and learns fear, the prefrontal cortex calms it down, and the insula turns body signals into feelings. Stress works at two speeds: a fast fight-or-flight response floods the body with adrenaline, and a slower hormone chain, the HPA axis, releases cortisol.

See the amygdala in 3D

The limbic system: an emotion network

In 1878, the French doctor Paul Broca named a ring of cortex around the brain's core the limbic lobe, from the Latin for border. In the 1950s, Paul MacLean made the limbic system famous as the brain's emotion system. Its best-known parts are these:

Scientists still debate which parts belong to the limbic system, and the hippocampus works mostly on memory. Emotion is spread across the brain, including the prefrontal cortex and the insula.

Good feelings have a circuit too. Dopamine neurons in the ventral tegmental area signal the nucleus accumbens when something turns out better than expected, which drives wanting. Drugs can hijack this reward circuit, as how drugs affect the brain explains.

The amygdala: detecting threats and learning fear

The amygdala is an almond-shaped cluster of neurons deep in each temporal lobe. It scans what you see and hear for danger, like an angry face or a snake-shaped stick. When it finds some, its central nucleus alerts the hypothalamus and brainstem: your heart races, you freeze, and stress hormones flow.

It also learns what to fear, through fear conditioning, a kind of classical conditioning. In the classic experiment, a rat hears a tone just before a mild shock. Soon the tone alone makes it freeze. The link is thought to form in the input side of the amygdala, the basolateral amygdala.

The neuroscientist Joseph LeDoux found two routes in. The low road runs straight from the thalamus to the amygdala. It is fast but rough, enough to make you jump at a curved shape on the path. The high road runs through the sensory cortex. It is slower but detailed, and tells you whether the shape is a snake or a stick. The low road was mapped mostly in rats, and its role in people is still debated.

The amygdala also makes emotional moments stick, by helping the hippocampus store them more strongly, with help from stress hormones. Such vivid memories feel certain, but they are not always accurate. Read more in how memory works.

Without both amygdalae, fear fades. S.M., a woman whose amygdalae were destroyed by a rare condition called Urbach-Wiethe disease, held snakes without fear and struggled to recognize fear in faces. In monkeys, removing both temporal lobes, amygdalae included, causes Klüver-Bucy syndrome: the animals lose their fear and put objects in their mouths. In people, it can follow herpes encephalitis, a brain infection.

Calming the alarm: the prefrontal cortex and its partners

The prefrontal cortex works as a brake on the amygdala. Its lower middle part, the ventromedial prefrontal cortex, which includes the medial orbitofrontal cortex, helps a fear fade once something proves safe. This fading is called extinction, and it is the idea behind exposure therapy for phobias.

You can also change a feeling by reappraisal: rethinking what a situation means. "This test is a chance to show what I know" feels different from "this test will ruin me". When people reappraise, brain scans show prefrontal areas working harder and the amygdala calming down.

The orbitofrontal cortex, just above the eyes, weighs rewards and punishments and helps you follow social rules. In 1848, an iron rod blasted through the left frontal lobe of Phineas Gage, a railroad foreman, damaging this underside area. He survived, but friends said he was "no longer Gage": he had become rude and impulsive. Historians think he later recovered much of his ability to work.

The insula, hidden deep inside the side of the brain, reads the body's inner state: heartbeat, breathing, a churning stomach. It turns those signals into feelings and is central to disgust, whether at a rotten smell or at a disgusted face. The front of the cingulate cortex, the anterior cingulate cortex, notices conflicts and mistakes and makes pain feel unpleasant. Its frontmost part also helps calm fear.

Theories of emotion: which comes first?

Do you tremble because you are afraid, or feel afraid because you tremble? AP Psychology and the MCAT test four classic answers. Here they are for one moment: you see a snake on a hiking trail.

Each theory has support. In a 1962 experiment by Stanley Schachter and Jerome Singer, people were injected with adrenaline without being told its effects. They tended to report feeling happy or angry to match an actor in the room. Robert Zajonc argued that some emotions come before any thinking, like the jolt from a sudden bang, which fits LeDoux's low road. Most researchers now think body signals, fast circuits and thinking all shape emotion together.

A related idea, the facial feedback hypothesis, says your facial expression feeds back into how you feel, so smiling can lift your mood slightly.

The stress response: fast and slow

A threat, a big exam or a near miss in traffic all set off the same stress response, run by the hypothalamus. It works at two speeds. The fast one, which the physiologist Walter Cannon named the fight-or-flight response, runs through the nerves of the autonomic nervous system. The slow one runs through hormones.

In the HPA axis, the hypothalamus releases CRH (corticotropin-releasing hormone), which makes the pituitary release ACTH (adrenocorticotropic hormone) into the blood. ACTH makes the adrenal glands, on top of the kidneys, release cortisol. Cortisol then acts back on the hypothalamus and pituitary to stop CRH and ACTH. This loop is called negative feedback, like a thermostat that turns off the heat once a room is warm. The hippocampus helps apply this brake too.

When stress lasts: the general adaptation syndrome

In the 1930s, Hans Selye exposed rats to many different stressors, such as cold, injury and exhausting exercise. All of them caused the same signs: enlarged adrenal glands, a shrunken thymus and lymph nodes, and stomach ulcers. Selye called the body's general response the general adaptation syndrome (GAS). It has three stages:

1. Alarm reaction: the fight-or-flight response kicks in, and the body gathers its resources to meet the threat.
2. Resistance: if the stressor continues, the body adapts. It stays on alert, with stress hormones still raised, but less intensely.
3. Exhaustion: if stress goes on too long, the body can no longer adapt. Its resources run down, and illness becomes more likely.

Long-term stress keeps cortisol high, and that has costs. It weakens the immune response, partly by reducing lymphocytes, the white blood cells that fight infection. It is linked to high blood pressure and heart disease. Studies also suggest that long-lasting high cortisol can harm the hippocampus, which is rich in cortisol receptors. People with too much cortisol often have a smaller hippocampus and weaker memory.

Not all stress is harmful. Short, manageable challenges, sometimes called eustress, can sharpen focus and performance. Problems come when stress is intense, constant or out of your control.

In these study guides

• MCATEmotion and stress
• Intro psychologyHunger, emotion and stress (sections 10.2, 10.4 and 14.1 to 14.3)
• Medical schoolLimbic system and hypothalamus
• A-level PsychologyThe endocrine system and fight or flight

Take it further in 3D

• Emotion and rewardFear, drive and the value of things. A short guided lesson in the 3D app.

Sources

1. Psychology 2e, 10.4 Emotion, OpenStax
2. Psychology 2e, 14.1 What Is Stress?, OpenStax
3. Physiology, Cortisol, StatPearls, NCBI Bookshelf (NIH)

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