Previously, in Part 1: We explored how cortisol, long blamed as the villain of stress, actually acts as a messenger within a much more complex system, one where context, control, and the brain’s own circuitry determine whether stress strengthens or damages us. Read this post first
If cortisol isn’t the villain, then who is? Enter the amygdala: the small, almond-shaped structure buried deep in your brain that acts like a drama queen with a panic button. Its job is to detect threats and keep you alive, but under pressure it tends to overreact. When the amygdala senses danger, real or imagined, it floods your system with alarm signals, and before you know it, your body is bracing for impact. That’s how stress hijacks your calm.
The amygdala belongs to the limbic system, the emotional core of the brain. It works hand in hand with the hippocampus and the prefrontal cortex, but when the stress response is activated for too long, that delicate balance collapses. The amygdala grows more reactive, its connections strengthen, and emotional responses become exaggerated. At the same time, the hippocampus, which normally helps you maintain perspective and regulate memory, begins to shrink and lose efficiency. The result is a mind that feels constantly under threat, even when nothing is actually wrong.
This is the second part in English of the original article in Spanish, which you can find here.
This change has been observed in animal studies as well. When rats are exposed to chronic stress, their amygdalas literally expand, more neural connections, more reactivity, more fear. The hippocampus, meanwhile, loses volume. This is the brain’s version of burnout: the part that screams “danger!” gets louder, while the one that says “calm down, we’re fine” fades into the background.
Once the amygdala takes control, the rest of the brain is forced into emergency mode. Suddenly, everything feels urgent, dangerous, or personal. Your prefrontal cortex, the rational decision-maker, watches helplessly as the amygdala calls the shots. That’s why people make impulsive decisions under stress: the brain isn’t trying to reason, it’s trying to survive. This system works beautifully if you’re being chased by a lion. It’s a disaster when the “lion” is your inbox or your boss.
When stress becomes chronic, this overactivation leads to what neuroscientist Bruce McEwen called allostatic load, the cumulative wear and tear on the body and brain from staying in survival mode for too long. The stress system, meant to help you adapt, turns against you. Allostatic load affects everything: metabolism, blood pressure, immune function, even bone density. In the brain, it changes how structures like the hippocampus and amygdala communicate, tipping the balance further toward anxiety, impulsivity, and fatigue.
In simple terms, allostatic load is the biological price you pay for staying alert all the time. When cortisol amplifies a signal of danger that never fades, your system burns out. But when cortisol accompanies voluntary effort (like exercise, learning, or any controlled challenge) the story changes. The same hormone that once fueled chaos now supports growth. In this context, cortisol promotes neuroplasticity, strengthens adaptive circuits, and boosts resilience. The difference lies not in the hormone itself, but in the meaning of the experience. Stress with control and purpose strengthens you. Stress without it breaks you.
ALLOSTATIC LOAD
Allostatic load refers to the physiological burden that accumulates when the body’s allostatic systems (those responsible for maintaining stability through change) are repeatedly or chronically activated. Under normal conditions, stress responses mediated by the hypothalamic–pituitary–adrenal (HPA) axis and the autonomic nervous system help the organism adapt to challenges and restore equilibrium. However, when these mechanisms are engaged too frequently or fail to shut down properly, the adaptive process itself becomes damaging. The sustained secretion of glucocorticoids such as cortisol, along with altered immune and metabolic activity, leads to structural and functional changes in multiple organs, including the brain. In this sense, allostatic load represents the long-term physiological price of adaptation: the measurable imprint of chronic stress on the body’s regulatory systems.
In humans, the pattern mirrors what we’ve seen in other mammals. The fundamental biology of stress hasn’t changed much through evolution. What varies is the complexity of our stressors. Our threats are rarely physical; they’re psychological, social, or existential. But the body reacts the same way it did thousands of years ago. The amygdala screams, the cortisol rises, and the system prepares to fight or flee, even when the “danger” is just an unanswered message or a performance review.
So the next time someone tells you that cortisol is the root of all your problems, remember this: cortisol is just the messenger. The amygdala interprets the situation, the hippocampus tries to make sense of it, and together they decide whether you experience stress as a challenge or as an apocalypse. Cortisol doesn’t create chaos; it amplifies whatever signal the brain sends. The real question isn’t how much cortisol you have, it’s what story your brain is telling when it shows up.
And when you finish explaining that, feel free to walk away with your head high and your metaphorical cape fluttering behind you.