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 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 could 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 3 different types of stimuli. The
first is the humoral stimuli, second neural stimuli, and third hormonal stimuli. Let's take a
closer look. With the 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 osteoclasts 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 the neural stimulus, nerve fibers are going to stimulate hormone release. For example,
sympathetic nervous system fibers stimulate the adrenal medulla to secrete catecholamines.
Remember that the sympathetic nervous system is our fight or flight system. And also,
catecholamines are going to be hormones such as dopamine, norepinephrine, and epinephrine
which you may know as adrenaline. Lastly, we have the hormonal stimulus. With the hormonal
stimulus, hormones are going to stimulate other endocrine glands to release their hormones.
A good example of this is the hormones of the hypothalamus. Hypothalamic hormones stimulate
the release of most 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 fight or flight response.