2. 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

3. 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) 2 nd 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)

4. 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.