The HPA axis is a multi-step biochemical pathway
where information is transmitted from one area of the body to the next
via chemical messengers Each step in this pathway, as in many
biochemical pathways, not only passes information along to stimulate the
next region but also receives feedback from messengers produced later
in the pathway to either enhance or suppress earlier steps in the
pathway – this is one way a biochemical pathway can regulate itself, via
a feedback mechanism.
When the hypothalamus receives signals from one of its many inputs
(e.g., cerebral cortex, limbic
system, visceral organs) about
conditions that deviate from an ideal homeostatic state (e.g., alarming
sensory stimulus, emotionally charged event, energy deficiency), this
can be interpreted as the initiation step of the stress-response cascade. The hypothalamus is stimulated
by its inputs and then proceeds to secrete corticotropin-releasing
hormones. This hormone is transported to its target, the pituitary
gland, via the hypophyseal portal system (short
blood vessels system), to which it binds and causes the pituitary gland
to, in turn, secrete its own messenger, adrenocorticotropic hormone,
systemically into the body’s blood stream. When adrenocorticotropic
hormone reaches and binds to its target, the adrenal gland, the adrenal
gland in turn releases the final key messenger in the cascade, cortisol.
Cortisol, once released, has widespread effects in the body. During an
alarming situation in which a threat is detected and signaled to the
hypothalamus from primary sensory and limbic structures, cortisol is one
way the brain instructs the body to attempt to regain homeostasis – by
redistributing energy (glucose) to areas of the body that need it most,
that is, toward critical organs (the heart, the brain) and away from digestive
and reproductive organs,
during a potentially harmful situation in an attempt to overcome the
challenge at hand.
After enough cortisol has been secreted to best restore homeostasis
and the body’s stressor is no longer present or the threat is no longer
perceived, the heightened levels of cortisol in the body’s blood stream
eventually circulate to the pituitary gland and hypothalamus to which
cortisol can bind and inhibit, essentially turning off the HPA-axis’
stress-response cascade via feedback inhibition. This prevents
additional cortisol from being released. This is biologically identified
as a normal, healthy stress mechanism in response to a situation or
stressor – a biological coping mechanism for a threat to homeostasis.
It is when the body’s HPA-axis cannot overcome a challenge and/or is
chronically exposed to a threat that this system becomes overtaxed and
can be harmful to the body and brain. A second major effect of cortisol
is to suppress the body’s immune system during a stressful situation,
again, for the purpose of redistributing metabolic resources primarily
to fight-or-flight organs. While not a major risk to the body if only
for a short period of time, if under chronic stress, the body becomes
exceptionally vulnerable to immune system attacks. This is a
biologically negative consequence of an exposure to a severe stressor
and can be interpreted as stress in and of itself – a detrimental
inability of biological mechanisms to effectively adapt to changes in
homeostasis.
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