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- [ S IGN IN ] Anatomy & Physiology (Open + Free) Sy lla bu s Unit 7:: Endocrine System Endocrine Structures and Functions Module 25 / Endocrine Lev els of Organization | Ou t lin e | Help | Mor e This course is not led by an instructor Endocrine Sy stem Hom eostasis and Integration of... Search this course 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. 179 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). learn by doing did I get this