Stress (cont.)
How does the response to stress work?
While
the complete story is not fully known, scientists understand much about how the
response to stress works. The two main systems involved are the
hypothalamic-pituitary-adrenal (HPA) axis and the SNS. (These systems are
described later.) Triggered (activated) primarily by an area in the brain stem
(lowest part of brain) called the locus coeruleus, the SNS secretes epinephrine
and norepinephrine. The five most important concepts to remember about these two
systems are that:
1. They are governed by a feedback loop to regulate their response. (In a
feedback loop, increased amounts of a substance—for example, a hormone—inhibit the
release of more of that substance, while decreased amounts of the substance
stimulate the release of more of that substance.)
2. They interact with
each other.
3. They influence other brain systems and functions.
4.
Genetic (inherited) variability affects the responses of both systems. (That
is, depending on their genes, different people can respond differently to
similar stresses.)
5. Prolonged or overwhelming responses of these systems
can be harmful to an individual.
What is the role of the hypothalamus-pituitary-adrenal (HPA)
axis (grouping) in stress?
The HPA axis is a grouping of responses to stress by the brain and the
pituitary and adrenal glands. First, the hypothalamus (a central part of the
brain) releases a compound called corticotrophin releasing factor (CRF), which
was discovered in 1981. The CRF then travels to the pituitary gland, where it
triggers the release of a hormone, adrenocorticotrophic hormone (ACTH). ACTH is
released into the bloodstream and causes the cortex of the adrenal gland to
release the stress hormones, particularly cortisol, which is a corticosteroid
hormone. Cortisol increases the availability of the body's fuel supply
(carbohydrate, fat, and glucose), which is needed to respond to stress. However,
if cortisol levels remain elevated for too long, then muscle breaks down, there
is a decreased inflammatory response, and suppression of the immune (defense)
system occurs.
Because they suppress the immune system, corticosteroids in measured doses
are used to treat many illnesses that are characterized by inflammation or an
overactive immune system, such as asthma and inflammatory bowel disease. For the
same reason, they are used to help reduce the chances that our body will
immunologically reject a transplanted organ. Corticosteroids also can cause
fluid retention and high blood pressure. Therefore, it is critical that the
response to corticosteroids be carefully controlled (modulated). This control
usually is accomplished by a feedback mechanism in which increased cortisol
levels feeding back to the hypothalamus and pituitary turn off production of
ACTH. In addition, extremely high levels of cortisol can cause depression and
psychosis, which disappear when the levels return to normal.
Next: What is the role of the locus coeruleus in stress? »
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