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HUBS 191 MODULE 5 NOTES MODULE 5 : ENDOCRINE SYSTEM Endocrine System - consists of endocrine (gland) cells that secrete hormones (chemical messengers) that are carried in the bloodstream to target cells upon which they act Endocrine glands = ductless glands ! secrete products (hormones) directly into blood made of glandular epithelium whose cells manufacture & secrete hormones some made of neurosecretory tissues which are modified neurons that secrete chem messengers that diffuse into bloodstream rather than across synapse (here chem messenger is hormone not NT) Exocrine glands = secrete products (eg enzymes for digestion) into ducts Functions of Endocrine system - Regulates internal environment by secreting hormones that travel through bloodstream to target cells ! homeostasis 4 main functions = regulation of internal environment (homeostasis), growth, energy metabolism, reproduction & development Remember! - Homeostasis = maintenance of a relatively constant internal environment despite changes in the internal or external environment - set point range = normal range of values for that variable Major Endocrine Glands - hypothalamus = hypothalamus links nervous system to endocrine system & controls secretion of many endocrine glands ! principle regulator / master gland pituitary gland thyroid gland parathyroid gland adrenal gland pancreas (pineal, thymus, testis, ovaries) NOTE : - hypothalamus = brain tissue that secretes hormones (neurons that secrete hormones into blood) - many of the hypothalamic hormones control function of the pituitary gland which itself secretes hormones that affect other parts of the body Hormones - chemical messengers secreted in 1 location and transported (usually via bloodstream) to a second location (target cells) where they exert their effects hormones usually have secondary functions in body which modulate activity of other regulatory mechanism unused hormones are usually quickly excreted by kidneys or broken down by metabolic processes ! so that they wont stimulate target cells when not required - SYNERGISM = some hormones work together to enhance each other's influence on target cell PERMISSIVENESS = combined hormone actions may exhibit permissiveness eg when a small amount of one hormone allows a second hormone to have its full effect on a target cell ANTAGONISM = antagonism between hormones ! fine tune activity of target cells with great accuracy, signaling cell when to increase or decrease a certain cellular process Specificity of Hormone Action - hormones only affect cells with specific receptors that hormones can bind to - Signal transduction = binding of hormones to receptors which activates mechanism of action / pathway (depending on type of hormone & receptor) - Signal transduction may :- initiate synthesis of new proteins - trigger activation or inactivation of certain enzymes - open / close specific ion channels in the PM Regulation of Target Cell Sensitivity - sensitivity depends on number of receptors expressed - receptors constantly degraded & synthesised to ensure efficiency - up-regulation = synthesis > degradation ! cells become more sensitive - down-regulation = degradation > synthesis ! cells become less sensitive Control of Hormone Secretion - negative feedback mechanism (inhibitory, stabilise physiological variables – keep them within set range) - endocrine reflexes = responses that result from operation of feedback loops within endocrine system - endocrine cells are sensitive to the physiological changes produced by their target cells - secretion by many endocrine glands is regulated by hormone produced by another gland eg secretion by anterior pituitary can be regulated by releasing hormones/inhibiting hormones secreted by hypothalamus - target gland may be able to adjust its own output but additional controls exerted by long feedback loops involving APG & hypo allow more precise regulation of hormone secretion and hence internal env - input from nervous system also influence secretion of hormones eg secretion by posterior pituitary is regulated by direct nervous input from hypo, sympathetic nerve impulses reaching medulla of adrenal glands trigger secretion of E & NE Properties Water-soluble hormones Chem - peptides (75% of all classification hormones) ! some aren’t very soluble in water - catecholamines ! involved in stress response Storage - made & stored until required (released by exocytosis) Lipid-soluble hormones - steroid hormones - thyroid hormones (modified peptide hormone that gives it lipid-soluble properties) - steroids made only when required (cannot store them cuz hormones will dissolve in the lipid membranes wrapping them) - thyroid hormones made in thyroid cells & 3. Activated G protein reacts with GTP which activates membrane-bound enzyme adenyl cyclase 4. Adenyl cyclase removes 2 phosphates from ATP converting it into cAMP (second messenger) 5. cAMP activates/inactivates protein kinases 6. Protein kinases activate specific intracellular enzymes which influence specific cellular reactions, producing target cell's response to hormone Mobile receptor Model - for lipid-soluble hormones - amount of steroid hormone present determines magnitude of target cell's response - more hormones ! more hormone-receptor complexes ! more mRNAs ! more proteins - steroid hormones may also have additional secondary effects such as influencing signal transduction pathways at PM 1. Lipid-soluble hormones attach to soluble plasma protein & travel in blood 2. Lipid-soluble hormones detach from carrier protein & passes through PM 3. Hormone molecules enter cell where they bind with a mobile receptor either in cytoplasm / nucleus to form a hormone-receptor complex 4. Complex acts as transcription factor & activates a certain gene sequence to begin transcription of mRNA 5. Newly-formed mRNA moves out of nucleus into cytosol where they associate with ribosomes, undergoes translation & begins synthesis of protein molecules 6. New protein (usually enzyme or channel protein) produces specific effects in target cells Comparison of Nervous & Endocrine Systems Nervous system - Neuronal (AP in axons, NT released at synapse) - Fast ! minimise response delay, tissue damage - Good for brief responses Endocrine System - Hormonal (hormones released into bloodstream) - Slow - Good for widespread, sustained responses BUT BOTH CAN ONLY INTERACT WITH CELLS WITH SPECIFIC RECEPTORS A) Pituitary Glands