Comprehensive Overview of the Endocrine System: Functions, Hormones, and Mechanisms

1. Endocrine System

2. Overview • Body system that consists of a group of glands that produces regulatory chemicals called hormones, and along with the nervous system, controls and coordinates all other systems of the body • Exocrine glands are not part of endocrine sys. –secrete prod.’s that are passed outside body (i.e. salivary, digestive, sweat,…) • Endocrine-ductless, exocrine-ducts

3. Nervous system vs. Endocrine system • Nervous sys. coordinates rapid activity &precise response • by means of electrical stim. (action potentials) and chemical stim. (neurotransmitters) • Neurotransmitters travel across a narrow space (synaptic cleft), binds to receptors on target cell and act locally to control nerve cell functions. • Endocrine system effects occur slower & over longer period of time • Involve chemical stimuli only (hormones) • Hormones are chemical messenger released by glands or specialized cells into circulating blood and influence the function of cells at another location in the body. (target cells)

4. Target cells • Hormones act by binding to receptor sites on target cell, producing a response by the target cell

5. Target Cell Response • Attached to the cell, the hormone affects cell activities: • Regulating the manufacture of proteins • Changing permeability of the membrane • Affecting metabolic reactions

6. Hormone Function • Play key role in regulating almost all body functions, including: • Metabolism • Growth & development • Water and electrolytes • Reproduction • Behavior

7. Generalized Specific Other hormones only affect specific target tissues Examples: ACTH Ovarian hormones • Some hormones affect many diff types of cells of the body • Examples: • GH • Thyroxin • Insulin

8. Antagonistic Pairs • Some hormones come in antagonistic pairs that have opposite affects on target organs • Ex. Insulin and glucagon

9. Classes of Hormones • Hormones are grouped into three classes based on structure • Steroids • Peptides • Amines

10. Steroids • Lipids • Chem. Structure sim. to cholesterol • In most instances synthesized from cholesterol itself • Consist of 3 cyclohexyl rings and one cyclopentyl ring • Very little hormone storage in steroid producing endocrine cells • Large stores of cholesterol esters in cytoplasm vacuoles can be rapidly mobilized for steroid synthesis after a stimulus • Secreted by the adrenal cortex: • Cortisol • Aldosterone • Secreted by ovaries: • Estrogen • Progesterone • Secreted by testes: • Testosterone • Secreted from Placenta: • Estrogen • Progesterone

12. Peptides, Polypeptides, & Proteins • Short chains of Amino Acids • Most hormones are polypep.’s and proteins • Range and size from small peptides with 3 AA’s (thyrotropin releasing hormone TRS) to proteins with almost 200 AA’s (GH and prolactin) • Polpypepetides with 100 AA’s or more are called proteins • Fewer than 100 = peptides • They are water soluble • Secreted by pituitary, parathyroid, heart, hypothalamus stomach, liver, kidney and pancreas • Precursor molecules processed by ER and golgi are stored in secretory granules

13. Amines • Derived from amino acid Tyrosine • Secreted from thyroid and adrenal medulla • Water soluble • Stored as granules in cytoplasm until needed • Ex. Epinephrine (adrenaline)

14. Mechanism of Action • Endocrine sys. acts by releasing hormones that trigger actions in specific target cells • Receptors selective to one type hormone • Receptors are located on target cell membrane whereas others are in cytoplasm or nucleus • First step of hormone action is to bind to specific receptor on target cell (first messenger) • First messenger triggers 2nd messengers to affect cell’s activity • Binding hormone changes shape of receptor causing the response to the hormone • 2 mechanisms of hormone action on all target cells: non-steroid hormones and steroid hormones

15. Non-Steroid Hormones • Water soluble • Don’t enter cell but bind to plasma membrane receptors (first messenger) • Generates chem signal (second messenger) inside target cell • Second messenger activates other intracellular chemicals to produce the target cell response • Resulting complex activates an enzyme that catalyzes the synthesis of cyclic AMP (cAMP) from ATP • cAMP activates other enzymes to produce target cell response

18. Steroid Hormones • Pass through plasma membrane  act in a 2 step process • Steroid hormones bind to nuclear membrane receptors or receptors in cytoplasm producing activated hormone-receptor complex • Complex enters nucleus  binds with chromatin (DNA) and activates specific genes  increasing production of proteins • Act more slowly than peptide hormones

20. Onset of hormone secretion after stimulus and duration of action of different hormones • Some hormones (i.e. norepinephrine and epi.) are secreted within seconds after glands stimulated and may develop full action within another few seconds or minutes • While other hormones (i.e. thyroxine or GH) may require months for full effect • Each hormone has own characteristic onset and duration of action – each tailored to perform its specific function

21. Hormone receptors - location • The locations for the different types of hormone receptors are generally: 1. In or on the surface of the cell membrane (protein, peptide and catecholamine hormones) 2. In the cell cytoplasm (steroid hormones) 3. In the cell nucleus (thyroid hormones) • Hormonal receptors are large proteins • Each cell that is to be stimulated usually has 2,000 – 100,000 receptors

22. Transport of Hormones in the Blood • Water soluble hormones are dissolved in the plasma & transported from their site of synthesis to target tissue • They diffuse out of capillaries into the interstitial fluid & ultimately to target cells • Steroid and thyroid hormones circ. In blood bound to plasma proteins (<10% free in soln) • Protein bound hormones can’t easily diffuse across capillaries to get to target cells • They are inactive until disassociate from plasma proteins

23. Feedback Control of Hormone Secretion • Although hormone concen. Fluctuate in response to various stimuli, they are closely controlled • In most instances negative feedback mechanisms insure proper level of hormone activity at the target tissue • After stim causes release of hormone  conditions or products resulting from hormone action suppress further release • The control variable is not the secretory rate of the hormone, but the degree of activity of the target tissue • Thus when target tissue activity rises to approp. Level than feedback signals to endocrine gland to slow further secretion of hormone • Promotes balance and homeostasis

24. (this is the last slide ) • In a few instances positive feedback occurs when action of hormone causes additional secretion of the hormone • i.e. LH luteinizing hormone • Also periodic variations in hormone release that are influenced by seasonal changes, stages of development and aging, diurnal cycle, and sleep • i.e. GH • Periodic variations due to changes in activity of neural pathways involved in controlling hormone release