system. It is this pathway that stimulates the autonomic nervous sys-
tem to release epinephrine, which has a quick action and, among
other actions, increases the heart rate, affects breathing, and enhances
senses. The activity reflects a form of implicit memory in that it does
not require a conscious awareness.
The hormones released by stressful stimuli work at different
speeds. Epinephrine (adrenaline) works quickly, whereas the action
of cortisol (a glucocorticoid) is slower. The amygdala plays a major
role in the response to stress through the autonomic nervous system
(epinephrine) and the cortico-releasing hormone pathways.
The hypothalamus stimulates the pituitary gland to produce
adrenocorticotropic hormone (ACTH), which stimulates the adrenal
gland to produce cortisol. The slower-acting HPA stress pathway leads
to the release of cortisol, which affects the function of cells in differ-
ent parts of the body, including the brain, and has a longer-lasting ef-
fect than epinephrine.
Cortisol can affect gene activation in different organs, including
the brain. Through these pathways, cortisol affects metabolic path-
ways and vulnerability to health problems such as type II diabetes
and coronary artery disease. Cortisol has major effects on cognition
and memory through its action on receptors in the brain, particularly
the hippocampus. Increased cortisol levels in the blood interact with
receptors in the hypothalamus and hippocampus to shut down the
stimulus (cortico-releasing hormone) from the hypothalamus.
In this dynamic system, emotional stimuli to the amygdala can
override the normal regulation of the pathway, leading to continuous
stimulus for cortisol production from the adrenal gland. This system
can be thought of as similar to a thermostat in that its operation nor-
mally maintains an appropriate balance.
McEwen (2002) and others have shown that normal cortisol levels
increase when you get up in the morning and return to low levels at
the end of the day if it has not been too stressful and the LHPA path-
way functions normally. McEwen describes this dynamic regulation
as the maintenance of stability through change, and he refers to this
process as allostasis. If the pathway does not return to a normal bal-
ance, this can be considered as increasing the allostatic load.
52 J. Fraser Mustard