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CONTROL OF THE ENDOCRINE SYSTEM. NEGATIVE FEEBACK MECHANISM Regulates the endocrine system through a negative-feedback mechanism to maintain homeostasis. Occurs when an increase in a substance inhibits the process leading to the increase.
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CONTROL OF THE ENDOCRINE SYSTEM • NEGATIVE FEEBACK MECHANISM • Regulates the endocrine system through a negative-feedback mechanism to maintain homeostasis. • Occurs when an increase in a substance inhibits the process leading to the increase. • Works like a thermostat in a house keeping the temp. constant. • TSH stimulates the thyroid and is regulated by the hypothalamus • Causes it to release thyroxine and increase metabolism • When level of thyroxine drops the cell’s metabolic activity drops • Hypothalamus is sensitive to body temp. as well as thyroxine. How would this affect metabolic rates? • HORMONE ACTION (Polypeptide and Steroid Hormones) • Polypeptide: Large proteins composed of amino acids: thyroxine • Steroid: Lipids produced from cholesterol: progesterone
HORMONE ACTION • Polypeptide: Don’t enter target cells; instead bind to receptors on the cell membrane. • This activates enzymes on inner surface of membrane which convert ATP into cAMP (cyclic AMP) 2nd mesenger (calcium ions are as well) • cAMP diffuses through membrane and activates other enzymes • Different target cells respond in different ways to the change in cAMP levels, so cells may speed up or slow down. • Example: Adrenaline binds to liver cell membrane which stimulates the break down of glucogen to glucose into bloodstream. • Steroid hormones: Can easily diffuse through lipid layer entering the cytoplasm of target cells. • Attach to receptor cells of proteins forming a hormone-receptor complex. Drifts through the cell until it reaches the cell nucleus. • Can affect gene expression by attaching tightly to gene sequences that previously were not expressed. (dramatic changes)
PROSTAGLANDINS: LOCAL HORMONES • Prostaglandins: Small amount of hormonelike substance produced in other cells and tissues. • Originally found in the prostrate of males. • Fatty acids that function only within the same cells in which they are produced. (local hormones) • Effects: • Ability to cause contractions in smooth muscles: uterus, bronchioles • Sensation of pain: Aspirin stops the synthesis of pain-causing prostaglandins.