The biology of stress

Published: 20-06-2015

Last modified: 29-03-2020

The stress response involves two biological systems – the sympathetic nervous system, which is activated first and hypothalamic–pituitary–adrenal axis (HPA axis), which takes effect after a certain time.

Sympathetic nervous system

The sympathetic nervous system (part of the autonomic nervous system), innervating the internal organs, is responsible for the so-called fight or flight response. In the first moments after stressor occurs it stimulates the adrenal glands to secrete adrenaline and noradrenaline. It increases pulse, breathing and cardiovascular efficiency, dilates the bronchioles and pupils. It also devides fat into fatty acid and glycerol, breakdowns glycogen stored in liver to glucose, causes outflow of blood from limbs to muscles, heart and brain, and stops digestion.

Hypothalamic–pituitary–adrenal axis

The hypothalamic-pituitary-adrenal axis is activated minutes or hours after stressor occurs. Hypothalamus secretes corticoliberyn (corticotropin-releasing hormone – CRH) and stimulates pituitary gland. The front lobe of pituitary gland influenced by CRH, secretes the other hormone – corticotropin (adrenocorticotropic hormone – ACTH). Corticotropin is transported to adrenal cortex which secretes glucocorticoids (cortisol, among others). Cortisol increases the concentration of glucose in the blood and accelerates decomposition of fatty acids to ketone bodies. It also changes the processing of information coming to senses. It reduces the sensitivity of the senses, that is the ability to receive weak stimuli, but increases the ability to distinguish different stimuli. A person influenced by cortisol may not hear a small sound but be able to distinguish two different sounds without difficulty. Glucocorticoids are responsible also for the inhibition of the pituitary and hypothalamus, and thus regulate the activity of the stress response.

Stress and immunological system

Stress influences the immunological system. The cortisol and other corticosteroids secreted by adrenal cortex have negative influence on immunological system. They reduce the reactivity of lymphocytes, block their production in the thymus, and sometimes also activate the process of dying. Since, however, during the stress response the sympathetic nervous system is activated first, and then hypothalamic-pituitary-adrenal axis, short-term stress in most people does not cause increased levels of cortisol (speed and ease of hypothalamic-pituitary-adrenal axis response is individually variable and in some people, even a short-acting stressors increase the concentration of cortisol).

Short-term stress does not reduce immunity, and many studies have shown even its beneficial effect on the immune system. Among others, shows it the study of people performing the first jumps with a parachute. The parachute jump caused transient increase in the concentration of NK cells (natural killer), which are responsible for destruction of virus-infected cells and tumor cells. Research on a different group of persons who perform the first parachute jumps in his life have shown other positive effects of the sympathetic nervous system activity. The most successful jumps have done those persons whose levels of adrenaline and noradrenaline before the jump was the highest. At the same time the worst results achieved those who had the highest concentration of cortisol.

Not only adrenaline…

Adrenaline, noradrenaline and cortisol are the main, but not the only hormones involved in the stress response. It involves also endorphins and enkephalins to reduce pain perception, thyroxine which accelerates release of energy stored in fat, aldosterone which increases blood pressure, as well as melanotropin, thyroid stimulating hormone, vasopressin, renin, growth hormone, glucagon, prolactin, parathyroid hormone, calcitonin, gastrin and many others.