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© 1997-1999 Microsoft Corporation. |
The
Brain and Spinal Cord
Thanks to much research, the brain’s reaction to stress and fear
are well understood. It is difficult
to separate the brain from the reaction of the rest of the central nervous
system because every part of the central nervous system reacts in conjunction
with other areas of the body. The
areas of the brain that stress seems to affect most are: 1. cognitive-affective
integrating centers of the brain 2. adrenal medulla (which heavily affects
the lymphatic system and circulatory
system and will be discussed later) 3. hypothalamic-pituitary-endocrine
axis and finally, 4. midbrain
areas (Zegans 1982).
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© 1997-1999 Microsoft Corporation.
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Stress
Pathway
When a stressful situation
is assessed by the cognitive-affective integration center of the brain,
the sympathetic system of the
autonomic nervous system is activated, causing the
adrenal glands to secrete epinephrine
and norepinephrine (Yeomans and Franklin
1996). This is also known as
the “fight or flight response”. One
of the most reactive areas of the hypothalamic-pituitary-endocrine axis
is the amygdyla. When
the brain detects a stressful situation, the hypothalamus signals the pituitary
gland to secrete adrenocorticotrpic hormone. This hormone stimulates the outer cortex
of the adrenal glands to release glucocorticoids. The glucocorticoids (of which the primary
hormone is cortisol) helps the body turn fats and carbohydrates into
energy. When the nervous system is
chronically stressed, the structures responsible for reacting to stress
become fatigued. Hans Selye,
a pioneer in physiological stress response research found that 1. The cortex of the adrenal glands became enlarged
and hyperactive. 2. The thymus, spleen,
lymph nodes, and other lymphatic
structures decreased in size.
3. And finally, he found that deep bleeding ulcers appeared in
the stomach and upper intestines (1982). Sometimes the stress pathway
does not immediately go to the brain, but instead is taken care of through
the use of reflex arcs in the spinal cord (Yeomans & Franklin
1996). In a reflex arc, receptors excite interneurons, which excite
motor neurons, which excite muscles (Kalat 1998). The reflex arch is activated by an environmental stimulus such
as a loud noise or touching a sharp object.
This signal bypasses the brain, which causes an immediate muscle
contraction. |
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Autonomic Nervous System The autonomic nervous system can be divided into two parts,
the sympathetic system, and the parasympathetic system. Both systems work in opposition to each other. The sympathetic system increases activity
in response to stressful conditions, while the parasympathetic system
works to conserve energy. |
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Parasympathetic System and Stress
This system opposes almost every action of the sympathetic system. This includes reducing the heart rate and strength
of contraction, constriction of coronary vessels, constriction of bronchi,
glandular secretion in the digestive tract, relaxing of sphincter muscles,
secretion of pancreatic juice and insulin, secretion of tear glands and nasal
glands, contraction of urinary bladder, and constriction of pupils. This accounts for why emotional stress can
cause “butterflies” in the stomach, and “froggies” in the throat, and a visit
to the bathroom. Long term stress |
Sympathetic System and Stress
The “fight or flight” response that was discussed in the Brain section of this web site is largely
initiated by this system. The
increased blood flow to muscles, faster and stronger heartbeat, dilation of
coronary vessels, constriction of most blood vessels (which raises blood
pressure), increase in metabolism, inhibition of the Digestive System,
dilation of respiratory bronchi, dilation of pupils, and secretion of sweat
are all initiated in part by the sympathetic system. All of these actions are strengthened and
maintained by the adrenal medulla (Tamar 1982). |
