How It All works:
The Stress Response, as mentioned above, is a survival mechanism which becomes maladaptive when it begins to be triggered repeatedly on a chronic basis. People vary in how effectively they cope with the routine stresses of life and not everyone experiences activation of the Stress Response on a frequent basis. Those who do, however, face a significant potential challenge to their heath and well-being because the Stress Response affects your nervous system, your endocrine system, your immune system and through these systems, your whole body. These effects operate through two major pathways: the Hypothalamic-pituitary-adrenocortical axis (HPA axis) and the Sympathetic Nervous System.
When the brain perceives a threat, it alerts the hypothalamus, a small collection of cells deep in the brain that is responsible for coordinating a great deal of the body’s physiological activity. By secreting hormones that act on the pituitary gland, the hypothalamus is the key link between the brain and the endocrine system. Once alerted, the hypothalamus then does two things in sequence:
First, it sends signals directly to the adrenal glands through the nerves of the Sympathetic Nervous System. These signals stimulate the adrenal glands to release increased amounts of epinephrine and norepinephrine. The increase in these chemicals has several effects, including increases in heart rate, respiratory rate, muscle tone, and blood sugar levels; blood vessels in the skin and abdominal organs constrict and those in the brain dilate and the brain becomes more alert and efficient.
These physiological changes serve to mobilize your energy reserves, increase your mental and physical capabilities and strength, decrease your likelihood of bleeding from cuts on the body and wounds to the gut, and so on.
If the threat is something that’s removed fairly quickly and is over and done with then this is as far as the process goes and your levels of epinephrine and norepinephrine return to normal. But if the stress or threat isn’t fairly quickly removed then the hypothalamus begins to secrete corticotrophin releasing hormone (CRH) which travels through the blood stream to the pituitary gland which is, in turn, stimulated by it to release adrenocorticotropin hormone (ACTH). This ACTH then travels to the adrenal glands, where it causes those glands to release increased amounts of cortisol and other corticosteroids, which mobilize fat reserves, keep blood sugar levels high, inhibit allergic response, decrease inflammation, and decrease pain perception.
Just as is the case when epinephrine and norepinephrine are increased, these actions in the body are mobilizing you to fight or run and are also preparing to limit your risk of impairment or death if you’re injured. For example, inflammation is a natural immune response that normally begins as soon as your body gets hurt but, if you are under threat, inflammation in the muscles and joints can slow you down and thereby get you killed- by releasing corticosteroids, which have the effect of damping down normal immune responses, the adrenal glands are helping to insure that you won’t be hindered by an inflamed musculoskeletal system.
Feedback loops in the HPA axis normally insure that the function of the adrenal glands returns to normal and that cortisol and other stress hormone levels decrease once a threat is escaped. However, when a person begins to become “stressed out” on a chronic basis, the feedback loops begin to fail so that cortisol and related hormones are regularly staying too high. Then these physiological changes such as suppression of the immune system, increases in blood sugar levels, and elevated cholesterol can become chronic and you see increased rates of infection and the development of diseases like diabetes, arteriosclerosis, hypertension, and osteoporosis (chronically elevated cortisol causes bone demineralization). Eventual physical changes may occur in the tissues and organs of the body, such as atrophy of the thymus gland.
Although the classic model of the Stress response is based on what happens in the body when a threat is encountered, it’s very important to understand that many things short of a real threat to life or safety may provoke the same chain of events through the sympathetic nervous system and the HPA, with resulting effects on endocrine function, immunity and health. As little as a week of inadequate sleep (75% of your “normal” sleep) will raise cortisol levels and therefore blood sugar levels and if this becomes chronic, diabetes can result. Common anxieties such as worry over money, fear of failure, stress in the home and so on create feelings of “being on edge” and lead to over-stimulation of the HPA.