Biology and stress

Hans Selye (1979) defined stress as the non-specific response of the body to any demand made upon it.  Threats on the body activate a general response to stress called the General Adaptation Syndrome (GAS)[1], marked by fever and other signs of illness (the body reacts to prolonged stress by activating the adrenal cortex and the immune system, with the resulting increase in cytokines[2] producing the same reactions that an infection would.


Cytokines are part of the human body’s immune system response. They are typically released to coordinate a complex series of changes to help the body fight injury[3].

Emotional states, stress, cytokines

Emotional states which once seemed too ephemeral for scientific study are now part of mainstream biology. Or in other words:

  • stress = threatening events that elicit behaviour and physiological responses.

Long-term inescapable issues activate the GAS which is harmful to our health over time.

Stress activates two systems in our bodies:

– the sympathetic nervous system (fight or flight response that prepares the body for brief emergency responses

– the HPA axis – the hypothalamus, pituitary gland and adrenal cortex[4] (see diagram).

In response to a stressful experience, the nervous system activates the immune system.

Immune system increases production of natural killer cells, leukocytes and cytokines. The cytokines can trigger symptom of illness as a reaction to the stress itself.

Prolonged stress of longer than a month significantly increases the likelihood of illness.

Prolonged stress can also be harmful to the hippocampus and can affect memory.

Cortical enhances metabolic activity in the body.

When metabolic activity is high in the hippocampus, the neurons are more sensitive to damage by toxins or over-stimulation.

Stress also impairs the adaptability and the production of new hippocampal neurons.

[1] The General Adaptation Syndrome: 1) Alarm stage – characterized by increased sympathetic nervous system activity. 2) Resistance stage – sympathetic response declines, the adrenal cortex releases cortisol and other hormones that enable the body to maintain prolonged alertness. 3) Exhaustion stage – occurs after prolonged stress and is characterized by inactivity, vulnerability, and decreased energy to sustain heightened responses.

[2] Small proteins combat infection and communicate with the brain to inform of illness, producing symptoms of illness. Fever, sleepiness, lack of energy etc. Sleep and inactivity are the bodies way of conserving energy to fight illness.

[3] Felicity Allen (2010). Biological Bases Of Health

[4] The HPA axis becomes the dominant response to prolonged stressors. Activation of the hypothalamus induces the pituitary gland to secrete adrenocorticotropic hormone (ACTH). ACTH stimulates the adrenal cortex to secrete cortisol. Cortisol (a stress hormone) helps to mobilize energies to fight a difficult situation. Prolonged increased cortisol levels impair the immune system.



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