Adrenaline, cortisol and exhaustion in the stress response
Adrenaline starts the stress response with a brief burst, while cortisol sustains it and, if stress is prolonged, causes muscle loss, infections and raised blood glucose.
Remember
Adrenaline gives the first burst and fades within minutes, whereas cortisol keeps the response going; when the stress does not end, continued cortisol leads to muscle loss, infections and hyperglycaemia (raised blood glucose).
Memory aid
adrenaline is the sprint and cortisol is the marathon
Adrenaline gives a fast, short burst, whereas cortisol keeps the response going for much longer.
When a threat is sensed, the limbic system (the emotion and survival circuit of the brain) and the cortex together prompt the hypothalamus. The hypothalamus switches on the sympathetic nervous system, the fight or flight branch of the autonomic nervous system. The stress response is described in three phases.
In the alarm phase, the sympathetic nervous system releases noradrenaline and the adrenal medulla (the core of the adrenal gland) releases adrenaline, with cortisol (from the adrenal cortex, the outer layer of the gland) released alongside. Noradrenaline and adrenaline give the initial burst to run away or fight.
By the resistance phase the adrenaline surge is fading, because the half-life of adrenaline (the time for its level to halve) is only 2 to 3 minutes. Cortisol has a much longer half-life, so it keeps the response going if the stress continues. It also raises blood glucose by increasing glucose output from the liver, so fuel stays available for the brain and muscles.
In the exhaustion phase, prolonged stress means continued cortisol release. Over time this causes muscle wasting (loss of muscle), hyperglycaemia (raised blood glucose) and suppression of the immune system, which makes infections frequent. The response can therefore become a problem when a person cannot quickly escape the stress, as with stress in the modern world.
Key facts
- Alarm phase
- noradrenaline and adrenaline are released, with cortisol alongside, giving the initial burst to run away or fight.
- Adrenaline half-life
- it is 2 to 3 minutes, so the high adrenaline levels of the alarm response do not last long.
- Cortisol half-life
- it is much longer than that of adrenaline, so the stress response can be maintained if needed.
- Cortisol and glucose
- cortisol raises blood glucose by increasing glucose output from the liver, so fuel stays available for the brain and muscles.
- Exhaustion phase
- continued cortisol release over time leads to muscle wasting, immune suppression and hyperglycaemia.
- Where they come from
- adrenaline is released from the adrenal medulla and cortisol from the adrenal cortex.
Exam traps
- Cortisol is released in the alarm phase, but muscle loss, infections and raised blood glucose appear when cortisol exposure has continued into the exhaustion phase, not during the initial alarm response.
- In the three-phase model, cortisol rather than adrenaline predominates in the resistance phase: the half-life of adrenaline is only 2 to 3 minutes, so its surge fades, and cortisol takes over and sustains the response.
- In the three-phase model, exhaustion does not mean that cortisol secretion falls because the adrenal glands have become exhausted: in the exhaustion phase, cortisol release continues, and muscle loss, infections and raised blood glucose are effects of too much cortisol.
Read the full notes
Tested in 6 SimpleMed+ questions
Practice it in Neuroanatomy and Psychiatry, with the answer and a full explanation after every question.
Practice these questions