What happens in the first seconds of a stressful event? (The SAM System)

The moment your brain perceives a threat—whether it’s a physical danger like a swerving car or a psychological one like your boss calling you into an unexpected meeting—it initiates a lightning-fast response. Your amygdala (the threat detector) signals the hypothalamus, which activates the Sympathetic-Adrenal-Medullary (SAM) system. This is the first alarm. It triggers your adrenal glands to release a flood of adrenaline (epinephrine).

• Your heart rate and blood pressure increase, pumping more oxygen to your muscles and brain.
• Your breathing quickens to maximize oxygen intake.
• Your pupils dilate and your senses sharpen, making you hyper-aware of your surroundings.

What happens in the following minutes? (The HPA Axis)

Following the initial adrenaline surge, your brain activates a second, slower, and more sustained response to keep you fueled for the challenge. This is the Hypothalamic-Pituitary-Adrenal (HPA) axis. The hypothalamus releases a hormone (CRH) that tells the pituitary gland to release another hormone (ACTH). This hormone travels to the adrenal glands and tells them to release cortisol, the body’s primary stress hormone. Cortisol’s main job is to mobilize energy by flooding your bloodstream with glucose, ensuring you have enough fuel to sustain a fight-or-flight effort.

Why is this short-term stress response actually good for you?

This state is known as “eustress,” or positive stress. In short bursts, the combination of adrenaline and cortisol is incredibly beneficial. It creates a state of heightened focus, improved memory retrieval, and increased physical performance. This is the biological state that helps you deliver a compelling presentation, rise to an athletic challenge, or ace a final exam. The acute stress response is designed to help you perform at your peak when it matters most.

How does the brain turn the alarm off?

Once the stressful event is over, your brain has a built-in “off switch.” The hippocampus and prefrontal cortex act as sensors for cortisol. When they detect that cortisol levels are high (and the threat has passed), they send inhibitory signals back to the hypothalamus, telling it to shut down the HPA axis alarm. This is a crucial negative feedback loop that allows your body to return to its baseline state of “rest and digest,” conserving energy and beginning cellular repair.

How does chronic high cortisol damage the brain?

While helpful in the short term, prolonged exposure to high levels of cortisol is neurotoxic. It has two particularly damaging effects on the brain:

1. It weakens the Prefrontal Cortex (PFC): Chronic cortisol impairs the function of your brain’s CEO, leading to poor impulse control, difficulty concentrating, and impaired decision-making.

2. It shrinks the Hippocampus: Cortisol can damage and kill neurons in the hippocampus, the area crucial for learning, memory, and—critically—shutting off the stress response. This creates a vicious cycle: stress damages the very part of the brain that’s supposed to turn the stress off, making it harder to recover.

Why does chronic stress lead to exhaustion and burnout?

Constantly running the HPA axis and mobilizing energy via cortisol is metabolically expensive. It’s like keeping a car’s engine red-lining for weeks on end. Your body’s resources become depleted, your adrenal system can become dysfunctional, and your cells suffer from the inflammatory effects of chronic stress. This biological depletion manifests as the profound physical and emotional exhaustion we call burnout.

How does chronic stress lead to anxiety or depression?

The link is direct and powerful. Chronic stress depletes the key neurotransmitters responsible for mood, like serotonin and dopamine. It promotes neuroinflammation, a process strongly linked to the onset of depression. Furthermore, a constantly firing amygdala (the “what if” fear center) combined with a weakened PFC (the “calm down, it’s okay” center) creates the perfect neurological storm for an anxiety disorder to develop.

• Depletes mood-regulating neurotransmitters like serotonin and dopamine.
• Damages the hippocampus, a key region for mood regulation.
• Promotes brain inflammation, which is strongly linked to depression.

Why do I get sick more often when I’m stressed?

Cortisol’s job includes temporarily suppressing the immune system (to save energy for the immediate threat). In short bursts, this is fine. But when cortisol levels are chronically high, it leads to a dysregulated and weakened immune response over time. This leaves you more vulnerable to infections, from the common cold to more serious illnesses, and can worsen autoimmune conditions.

Building Stress Resilience: The Path Forward

While you can’t eliminate stress from your life, you can absolutely change your brain’s and body’s response to it. “Stress resilience” is the biological capacity to experience a stressful event and efficiently return to your baseline. It means strengthening the “off switch” for your stress response. Neuroscientifically, this involves practices that bolster the health and function of your prefrontal cortex and hippocampus, giving them more control over your amygdala and HPA axis. Tools like physical exercise, mindfulness, quality sleep, and social connection are not just relaxing; they are powerful, evidence-based interventions that physically rebuild a brain worn down by chronic stress.