What is stress exactly?
Stress is one of our most primitive emotions. It is tied to our flight-or-fight response, an instinct that evolved to protect our ancestors from predators and other dangers.
When you realize you forgot an important deadline at work, the same basic circuits are being activated in your body as when you ancestor encountered a lion in the wild. A racing heart, sweaty palms, and other classic signs of stress would have prepared your ancestor to fight the lion or flee to safety. In the modern day, these biological reactions are less helpful for dealing with everyday stressors.
The opposite of stress is homeostasis, also called rest-and-digest. When no danger is present, your heartrate slows, your muscles relax, and your gastrointestinal tract resumes digesting food (digestion is an energy-intensive process, so it’s paused during times of perceived danger). Periods of homeostasis are crucial for our minds and bodies to remain healthy.
The world of stress hormones
A few seconds later, these hormones activate a secondary stress circuit called the hypothalamic-pituitary-adrenal axis, or HPA axis for short. As the name implies, the HPA involves three different regions.
The hypothalamus is a brain region located just above the brainstem, which senses stress hormones in the bloodstream and sends a signal to the pituitary gland, a pea-sized brain structure located behind the bridge of your nose. The pituitary is sometimes called the “master gland,” as it can signal to many of the other glands in your body as well as release its own hormones.
When it has been activated, the pituitary gland relays the signal to the adrenal glands located top of the kidneys, which in turn release another hormone called cortisol into the bloodstream. Unlike epinephrine and norepinephrine, which are meant for immediate responses, cortisol helps you respond to longer-lasting stressors. For example, it increases your blood pressure and blood sugar levels, both of which prepare you for physical exertion in the face of danger. It also inhibits less crucial processes including digestion and libido.
Your brain on stress
The hypothalamus and pituitary gland aren’t the only brain regions involved in stress. Once cortisol is released into the bloodstream, it can have widespread impacts on the rest of the brain. The hippocampus, a brain region best known for its role in memory, is particularly sensitive to cortisol. Studies have found that stress causes fewer neurons to be born in the hippocampus, and it can also alter the structure of neuronal connections. As a result, chronic stress can cause the hippocampus to become smaller over time, with some studies suggesting that memory formation can become impaired as well.
Another important brain region is the prefrontal cortex, which is important for advanced brain functions like long-term planning, impulse control, and social behavior. Chronic stress can alter neuronal connections in the prefrontal cortex and lead to gradual atrophy of this region. In contrast, another brain region called the amygdala appears to become larger with repeated stress. The amygdala is important for processing negative emotions, particularly fear and anger. A hyperactivate amygdala may contribute to anxiety or aggressive behavior.
