"Neurochemically, norepinephrine, along with a substance called the corticotropin-releasing factor (CRF), is sent from the amygdala to the hypothalamus, which signals the pituitary gland. The pituitary gland then sends a slow message through your bloodstream to your adrenal glands, telling them to secrete cortisol, a stress hormone that can keep you charged up a little longer than adrenaline does, to deal with the stress. On a short-term basis, cortisol facilitates dopamine, which keeps you alert and activated. However, cortisol can be corrosive to the brain and the body if it stays activated too long. With excessive and prolonged cortisol, the levels of dopamine become depleted, and this makes you feel awful.
"On a short-term basis, however, cortisol is actually very useful. If you encounter stress that requires a prolonged response beyond a quick flight or a fight, your body needs a way to manufacture fuel (glucose). Epinephrine (adrenaline) immediately converts glycogen and fatty acids, but when the stress is longer-lasting, cortisol takes over. It works through the bloodstream, so its effects are slower than adrenaline’s.
"Cortisol works more systemically than adrenaline does. It triggers the liver to make more glucose available in the bloodstream while it also blocks insulin receptors in nonessential organs and tissues so that you get all the glucose (fuel) that you need to deal with the threat. Cortisol’s work is a long-term strategy of insulin resistance, which serves to provide the brain with a sustained level of glucose. However, you don’t always have a lot of glucose floating around, so cortisol works to stockpile energy. It converts protein into glycogen and begins to store fat. If the stress is chronic, the increased body fat is stored in the abdomen. If you have a growing bulge in your midsection, it may be due to cortisol working to store energy. Unfortunately, that’s not the way you want it to be stored. It’s better to burn off such stored energy by exercise.
"One of the many problems associated with chronic stress and high levels of cortisol is that parts of the brain bear the brunt—especially the hippocampus. The hippocampus has many cortisol receptors; under normal circumstances, this helps to trigger the shutting-off of cortisol, much like a thermostat, so that it can turn down the production of cortisol. However, when cortisol production is excessive and prolonged, the hippocampus receptors themselves shut down. The hippocampus then begins to atrophy, and with it your memory capacity."
Source: Rewire Your Brain by John B. Arden