Stress: Your brain and body



What is stress and can it be good for you? Does it kill off your brain cells and can it cause depression? How does your brain perceive a terrifying situation and prepare your body for survival?


Your body’s stress response kicks in when you perceive you are under threat.

Mammals have evolved this superb mechanism to ensure we have the best possible chance of survival when faced with a life-threatening situation.


Imagine you are in the jungle and you hear movement behind you. You stop still, heart pounding and turn your head to orientate your eyes and ears to the sound. You see the undergrowth trampled as you hear an animal pounding towards you – then you hear the lions roar.


At times like this you’d want every muscle in your body to work to the peak of its ability – and your brain to be super-alert.

Evolution has obliged, and given you the stress response.


What happens in your body?

Putting energy into digesting your lunch, optimising your immune system or ensuring you are fertile all become rather unimportant in life-threatening situations. All these non-essential body functions cease and you divert all energy to your muscles and brain.


Your heart beats faster, your blood pressure increases and you breathe faster pumping maximum oxygen and energy-rich blood to your muscles. Your liver releases more sugar into your blood ready for action.


In evolutionary terms, this is a remarkable system that has helped our species survive.


Perceiving threats?

Encounters with lions, muggers in dark alleys or the loss of a loved one are fairly universal in eliciting the stress response.


Most other situations are subjective. Life-events, exams or types of work can be hugely stressful to one person yet easier to cope with by others.


As with many forms of perception, scientists don’t know the actual neural mechanisms that allow you to combine your prior experience with information coming in through your senses, and produce your brains judgement that a situation is dangerous.


There are three main parts of you that control your stress response – your hypothalamus and your pituitary (both in your brain), and your adrenal glands by your kidneys.




What happens in your brain?

Once your brain has decided there’s a danger, it sends immediate nerve signals down your spinal cord to your adrenal glands telling them to release the hormone adrenaline. Once released, adrenaline increases the amount of sugar in your blood, increases your heart rate and raises your blood pressure (and has many other actions).


Your brain’s remarkable hypothalamus also sends signals to your pituitary gland at the bottom of your brain, telling it to release factors that within a few minutes have travelled through your blood stream and stimulated your adrenal cortex to produce a stress hormone – cortisol.


Cortisol is very important in your stress response – keeping your blood sugar and blood pressure up to help you escape from danger.




Long-term effects of stress

Your body’s stress response is perfect in the short-term, but damaging if it goes on for weeks or years. Raised levels of cortisol for prolonged periods can damp down your immune system and decrease the number of brain cells so impairing your memory. It can also affect your blood pressure and the fats in your blood making it more likely you will have a heart attack or stroke.


Cortisol levels in people under prolonged stress


So, does stress kill brain cells?

The answer seems to be yes.

Stress causes the release of a hormone called cortisol.


Giving rats daily injections of corticosterone (rat cortisol) for several weeks kills certain brain cells. Stressing the rats each day for the same amount of time has an identical effect.


Cortisol has been shown to damage and kill cells in the hippocampus (the brain area responsible for your episodic memory) and there is robust evidence that chronic stress causes premature brain aging.


Without cortisol you would die – but too much of it is not a good thing. It seems it makes your brain more vulnerable to damage such as strokes, ageing and stressful events.


Stress excites brain cells to death

The cortisol released in stress travels into the brain and binds to the receptors inside many neurons (in the cytoplasm). Through a cascade of reactions, this causes neurons to admit more calcium through channels in their membrane.


In the short-term cortisol presumably helps the brain to cope with the life-threatening situation. However, if neurons become over-loaded with calcium they fire too frequently and die – they are literally excited to death.


Stress and depression

It’s quite clear that chronic stress is related to depression.

A common feature of depression is an excess release of cortisol into the blood. Some neuroscientists and psychiatrists are now suggesting that the major changes in serotonin and other neurotransmitters seen in depression are not the.


Growing new brain cells

Contrary to traditional ideas, the adult brain does make new neurons, but only in very restricted areas. For example, the hippocampus of an adult rat makes between 5000-10 000 new neurons each day.


Joe Herbert’s lab in Cambridge has showed that cortisol dramatically decreases the rate new brain cells are made. So perhaps some of the adverse effects of stress are related to fewer brain cells being created in the hippocampus.


Along the same lines, anti-depressant drugs that increase serotonin (eg SSRI’s including Prozac) boost the rate new neurons are made. Perhaps depression or recovery from it may be related to the formation of new neurons.




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