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Anatomy & Physiology (Open + Free)
Sy lla bu s
Unit 7:: Endocrine System
Endocrine Structures and
Functions
Module 25 /
Endocrine Lev els of
Organization
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Endocrine Sy stem
Hom eostasis and
Integration of...
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Stimuli Regulating Hormone Production
List and com pare the
m echanism s of horm onal
stim ulation.
There are three mechanisms by which endocrine glands are stimulated to synthesize and release hormones:
humoral regulation, hormonal regulation, and neural regulation.
Humoral Stimuli
The term humoral is derived from the term humor, which refers to bodily fluids such as blood and other
extracellular fluids. Humoral stimuli regulate the release of hormones in response to specific changes in
extracellular fluids, such as the concentration of a particular ion or solute in the blood or even the overall
solute levels in the blood.
EXAMPLE
A rise in blood glucose level triggers the pancreatic release of insulin. Insulin causes blood glucose levels
to drop, which signals the pancreas to decrease insulin production through a negative feedback loop.
Similarly, low blood calcium stimulates the release of parathyroid hormone from the parathyroid gland
which stimulates the release of calcium from bone, decreases calcium excretion in urine and promotes
calcium absorption in the digestive system.
Tropic Hormonal Stimuli
With tropic hormonal stimuli, a hormone is produced and released by an endocrine gland in response to
another hormone (known as "tropic hormones"). These hormones controlling release of another hormone are
called tropic (meaning "turn toward," pronounced “tro’-pick”; not same as geographical “trop-ic”).
EXAMPLE
The hypothalamus produces hormones that stimulate the anterior pituitary. The anterior pituitary in turn
releases hormones that regulate hormone production by other endocrine glands. For example, the
anterior pituitary releases thyroid-stimulating hormone, which stimulates the thyroid gland to produce the
hormones T3 and T4. As blood concentrations of T3 and T4 rise, they inhibit further hormone production by
both the pituitary and the hypothalamus in a negative feedback loop.
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Sometimes students get confused between tropic and trophic hormones. Tropic
hormones stimulate release of other hormones from endocrine cells, such as those discussed
here. Trophic (meaning “nourishment or nurse”) horomones stimulate non-endocrine cell
growth and development, such as growth hormone, estrogen and testosterone.
NOT E...
Neural Stimuli
The nervous system can also directly stimulate endocrine glands to release hormones through a mechanism
known as neural stimuli.
EXAMPLE
Neuronal signaling from the sympathetic nervous system directly stimulates the adrenal medulla to
release the hormones epinephrine and norepinephrine in response to stress.
Length Scale of Hormonal Signaling
Cells communicate with one another via chemical messengers. The communication may happen between cells
close by or far away from the cells that produces the messenger (signal). For example, released hormones
travel throughout the body and affect any cells with receptors for the specific hormones. Autocrine
signaling (auto- means self) affects the cells that released the signaling molecule. Autocrine signaling
(auto- means self) affects local cells other than the secreting cells. While traditionally a hormone is thought to
have its effect at a distances from where it is secreted, the definition of hormones now include paracrine and
autocrine mechanisms as well. The all-inclusive term, Endocrine signaling, includes all types of
communication where chemical molecules produced from a cell affect the metabolism of another cell
(paracrine or endocrine) or that of its own (autocrine).
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