The Stress Response – The Fight-or-Flight Response 

The Sympatho-adrenal response: Understanding a mechanism linked to depression



A primitive man encounters a lion; it threatens its survival. He feels an enormous stress mounting. He either has to fight the animal or he has to escape (to flee – to “fly”). Either behavior is linked to the stress response.


Similarly, when we have to give a talk before an audience we are confronted with the same stress.


The link below points to an interesting animation showing a primitive man encountering a lion; it goes on to suggest the relation to a man giving a talk in front of a big audience:




What is required?

In order to confront the lion, the man needs to run or engage in combat, both of which require intense muscle work.


To support intense muscle activity you need energy. Therefore, you need fuels like glucose and fat; and oxygen to burn these substrates and liberate energy.


First of all you need to make fuels available from the internal stores. Then you have to direct them to muscle.


How will you direct substrates and oxygen to muscle? By blood circulation. You need to activate the blood pump, the heart, to provide blood to muscle and dilate the corresponding vessels to make it easier for blood to run where needed.


Similarly, you need to shut down all unrelated activities to conserve energy and divert the blood supply. You need to shut down digestion and constrict the corresponding vessels so that blood is shunted to muscle.



How would you design such a biological scenario?

First of all you need a receptor unit for the danger/alert signal.

This receptor is the amygdala.


Then you will need a coordinating unit or chamber for the response.

This unit would be the hypothalamus.


Then you will need connection routes from the hypothalamus to the whole body.

You can use electricity wires like nerves or blood pipes like vessels in combination with molecules like hormones travelling in the bloodstream.


You use both. Nerves that function in autonomous function (that will take this crucial decision on their own – no time to think) and hormones flowing in blood.


The hypothalamus will activate the autonomic nervous system and specifically the sympathetic nervous system (the “activating” branch). It will also engage in blood transmission by secreting a hormone.


What mechanism will you choose to liberate fuels like glucose and fat from internal stores?


A hormone that will circulate in the blood to reach all body places that have fuel stores.


Where will you place the organ that will liberate this hormone?

At about the center of the body, right and left.

This would be the adrenal glands - on top of the kidneys (ad renes).


Would you make this an emergency mechanism? Or would you try to upregulate the everyday mechanism for this, if one exists?




Is there an everyday mechanism for release of fuels from internal stores?

There is a mechanism at the beginning of the day, after awakening that gets our body going.

At around 7.00 a.m. there is a specific hormone that rises in our blood for this purpose.

It is secreted from the cortices of the adrenal glands and therefore it has been named cortisol.


Although cortisol plays a role in the mobilization of internal stores, it is not sure if this is why it gets our body going after awakening.


Let us say that we will be upregulating an everyday mechanism that gets our body going.


What would be the emergency mechanism?

Another hormone that we will call adrenaline (produced by adrenal glands).


So we would connect nerves to the adrenal gland to make it liberate adrenaline.

We would also have the hypothalamus secreting a hormone that would eventually liberate cortisol.


Nature has used an intermediate step in this process: it has connected a gland to the hypothalamus, called the pituitary gland (hypophysis). The hypothalamus secretes hormone A, and this in its turn activates the pituitary gland to secrete hormone B that will act on the adrenal gland, and specifically on the cortices of the adrenal gland for the secretion of cortisol.


How would you call hormone B which makes the adrenal cortices or adrenocortices act in a certain way (“tropos” in greek)? The answer is adrenocorticotropic hormone (ACTH) or corticotropin. And how would you call hormone A which induces the release of corticotropin?

The answer is corticotropin releasing hormone (CRH).




Candace Pert in her book says that suicide victims have a 10 fold increase in CRH in their cerebrospinal fluid. This is a very important piece of information which links chronic stress responses like depression to this mechanism. This is why we can understand better the phenomenon of depression if we know the fight-or-flight response.



It is noted that the hormones mentioned are associated with a lot of other functions. E.g. adrenaline makes the heart beat faster.


I suggest the following resource to obtain an understanding of the response.


I have also copied some information from Wikipedia on the autonomic nervous system on this page.