What causes hormones to be released from
their endocrine glands?
Most of the time, blood levels of
hormones are controlled by a negative
feedback system.
This happens when an increased hormone
is going to affect the target organ. And
this the effect on the target organ will
then inhibit further hormone release.
The levels that are going to vary
between release and inhibition are very
narrow and a very small range.
Hormone release can be triggered by
endocrine gland stimuli
as well as nervous system modulation.
Endocrine glands can be stimulated to
synthesize and release hormones in
response to three different types of
stimuli.
The first is the humoral stimuli,
second neural stimuli, and third
hormonal stimuli. Let's take a closer
look.
With a humoral stimulus, changing blood
levels of certain ions and nutrients are
going to directly stimulate the
secretion of hormones.
A good example of this is the calcium in
our blood.
Declining blood calcium levels
stimulates the parathyroid gland to
secrete the hormone parathyroid hormone.
In turn, parathyroid hormone causes the
calcium concentration to rise by
activating osteoclast which break down
the bone minerals and release calcium
into the blood. Once there is a
sufficient amount of calcium in the
blood, the stimulus is removed and the
parathyroid hormone is also removed.
A second type of stimulus is a neural
stimulus. With a neural stimulus, nerve
fibers are going to stimulate hormone
release.
For example, sympathetic nervous system
fibers stimulate the adrenal medula to
secrete catakolamines.
Remember that the sympathetic nervous
system is our fightorflight system. And
also catakolamines are going to be
hormones such as dopamine,
norepinephrine, and epinephrine, which
you may know as adrenaline.
Lastly, we have the hormonal stimulus.
With a hormonal stimulus, hormones are
going to stimulate other endocrine
glands to release their hormones.
A good example of this are the hormones
of the hypothalamus.
Hypothalamic hormones stimulate the
release of most or of our anterior
pituitary hormones.
Anterior pituitary hormones stimulate
targets to secrete more hormones.
Activity of the hypothalamic pituitary
target endocrine organ feedback loop are
going to cause hormones from the final
target organs to inhibit release of the
anterior pituitary organs. The anterior
pituitary releases hormones that are
going to trigger the release of things
like testosterone.
Consequently, high testosterone levels
inhibit the release of those hormones.
The nervous system can make adjustments
to hormone levels when needed. This is
referred to as nervous system
modulation.
It can modify stimulation or inhibition
of certain endocrine glands.
The nervous system can also override
normal endocrine controls.
An example of this is when you're under
severe stress, the hypothalamus and your
sympathetic nervous system can override
the insulin from the pancreas to allow
blood glucose levels to increase and
prepare your body for the fightor-flight
response.
Heat. Heat.
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