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Endocrine: Bioscience (Source, stimulus for secretion & main actions…
Endocrine: Bioscience
- Endocrine gland & various ways hormones alter target cell activity
ENDOCRINE GLAND: effects manifested more slowly than NS. Endocrine system consists of ductless glands: produces and secretes hormones directly into blood. Hormones only interact with the target cells that have appropriate receptors. When hormone binds to receptor, it alters the target cell activity , and therefore acts as a chemical messenger
ALTERATION: 1. alters permeability of plasma membrane 2. stimulate protein synthesis 3. activate/deactivates enzymes within cell 4. increases/decreases secretory activity of cell 5. stimulates cell division (mitosis) and therefore regulate reproduction, growth and development
- Amino acid based hormones and steroid hormones : defined by chemical structure
Amino acid based-most hormones, size varies, usually lipid-insoluble (water-soluble). - cannot diffuse through plas. mem of target cell, therefore receptor on surface of plas mem. - receptors usually coupled with G-proteins (activates an enzyme) to an intracellular second messenger -> mediares target cell repsonse to hormone
Steroid - synthesised from cholesterol: lipid soluble and include sex hormones made by ovaries and testes (estrogen, testosterone, progesterone) & hormones produced by adrenal cortex (cortisol & aldosterone) -can diffuse across plas mem. of target cell to bind to receptors in cytoplasm. - when hormone binds creates receptor-hormone complex -> which enters nucleus to activate genes and direct synthesis of new proteins. - new proteins alter cell activity and cause typical hormone response.
- Three stimuli that control hormone secretion
- Humoral stimulus - hormones secreted in response to changing levels of certain ions/nutrients in blood (eg: glucose, calcium) 2. Neural stimulus - hormone secretion controlled by NS. 3. Hormonal stimulus - hormones secreted in response to hormones produced by other endocrine glands
-Hypothalamus: produces no. of diff hormones, including those that control the endocrine function of pituitary gland - Pituitary gland: produce and secrete hormones that regulate endocrine activities of various organs - Pineal gland: produce and secrete melatonin which regulates circadian rhythm - Thyroid gland: produce and secrete thyroid hormone and calcitonin - Parathyroid gland: produce and secrete parathyroid hormone- Thymus: produce and secrete hormones that develop and maintain immune defences - Adrenal gland: produce and secrete adrenaline and nor-adrenaline Pancreas: produce and secrete insulin and glucagon Gonads: (ovaries, testis) produce sex hormones responsible for maturation of reproductive organs and development of secondary sex characteristics
- Vascular & neural relationships between pituitary and hypothalamus
Pituitary gland connected to inferior surface of hypothalamus by infundibulum. consists of two lobes: anterior (vascular) and posterior (neural)
Posterior: -composed of neural tiss. and doesn't produce own hormones, but secretes those produced by hypothalamus. - connected to hypothalamus via bundle of neuron axons called hypothalamic-hypophyseal tract. these axons originate in neurons in hypothalamus and terminate in posterior. - cell bodies of neurons synthesise oxytocin and anti-diuretic hormone (ADH), and the axons transport hormones to posterior, where they are stored in vesicles in axon terminal. -when hypothalamic neuron stimulated stored hormones released into capillary bed with posterior for distribution across boy.
Anterior -comprised of endocrine cells ->produce and secrete a no. of hormones. - secretion of hormones regulated by regulating hormones (inhibiting/releasing hormones) produced by hypothalamus. - releasing hormones: stimulate the anterior to secrete particular hormone - inhibiting hormones: prevent anterior from secreting hormone. - Vascular connection to hypothalamus (hypophyseal portal system). - this connection links blood supply of hypothalamus with that of anterior and transports regulating hormones produced by hypothalamic neurons to anterior.
- Source, stimulus for secretion & main actions of hormones
Posterior Pituitary 1. ADH: -ADH produced by hypothalamus released from posterior. STIM: increase in blood sodium concentration / decrease in blood volume and Blood pressure ACT: targets kidneys. increases amt. water reabsorbed from urine into blood -> which decrease blood sodium concentration to homeostatic levels, decreases urine output & conserves body water to regulate blood volume and BP. - causes vasoconstriction of arterioles at high concentration-> aka Vasopressin 2. Oxytocin : produced by hypothalamus released by posterior STIM: stretching of cervix during labour, and the suckling action of the infant after birth ACT: stimulates uterine contractions during birth and ejection of milk from mammary glands during breastfeeding.
Anterior Pituitary 1. Prolactin: PRL produced and secreted by anterior. STIM: controlled by hypothalamuc regulating hormone, prolactin-inhibinting hormone. low levels of PIH lead to increase in release of PRL. - ACT: stimulates milk production Follicle Stimulating Hormone & Luteininsing Hormone : - both produced and secreted by anterior STIM: controlled by gonadotropin -releasing horomone ACT: target ovaries and testes GH :- growth hormone produced & secreted by anterior STIM: controlled by 2 horm: - Growth hormone releasing hormone and growth hormone inhibiting hormone. ACT: GH mediates growth of body tissues(skeletal muscle and bone). stimulate liver, muscle, bone to produce insulin like growth factor hormone which stimulates action required for growth (cell division etc.) Adrenocorticotropic hormone :- ACTH : anterior pro and sec. STIM: corticotropin-releasing hormone ACT: stimulate adrenal cortex to secrete the hormone cortisol Thyroid Stimulating Hormone:- TSH pro & sec anterior STIM: thyrotropin-releasing hormone ACT: stimulates thyroid gland to secrete hormone TH
Insulin: - produced and secreted by beta cells in pancreas STIM: blood glucose levels increase above normal range ACT: decrease BGL by stimualting target cells to take glucose from blood. stimulates cells to utilise the glucose for ATP (glycolysis) and stimulates liver and skeletal muscles to convert glucose to glycogen for storage (glycogenesis)Glucagon: - produced & secreted by alpha cells in pancreas STIM: BGL decrease below normal range ACT: increase BGL. targets liver to stimulate - the breakdown of stored glycogen to glucose (glycogenolysis) - production of glucose from amino acids from glyerol (glucogenesis) - release glucose into blood
cortisol: pro and secr. by adrenal cortex STIM: controlled by ACTH. ACT: helps maintain BGL by stimulating liver to produce glucose when BGL low. Cortisol = stress hormone as it helps body resist stress by providing energy required for fight/flight .( HPA AXIS: series of interaction between hypothalamus, pituitary gland and adrenal gland.)Aldosterone: - pro and sec. adrenal gland. STIM: response t increase in blood potassium levels or hormone angiotensin II which is produced by liver due to decrease in bloof volume and BP. -ACT: regulate blood concentration of K+ and Na+ -> therefore it is a mineralocorticoid. - helps maintain BP and blood volume -primarily targets kidneys to increase secretion of K+ from blood into urine, and re-absorption of Na+ from urine into blood
Adrenalin & Noradrenalin : - pro & sec. from adrenal medulla. 80% adrenaline from adrenal medul. STIM: controlled by sympathetic division of ANS - ACT: bind to adrenergic receptors on cells in various organs and enhance fight/flight. Main actions those of flight or flight(eg: pupil dilate)
Erythropoietin:- EPO sec. & pro by kidneys STIM: low oxygen levels in blood(hypoxia). ACT: EPO signals bone marrow to increase rate of blood cell production
Thyroid hormone:- pro & sec by specialised follicular cells in thyroid gland. STIM: TSH - ACT: affects virtually every cell in body and acts to - increase basal metabolic rate (amt. of energy required by body to metabolise) - increase body temp (through increase in metabolism, therefore increase in heat prod. - regulate tissue growth and developmentCalcitonin :-pro & sec. by parafollicular cells in thyroid gland STIM: when blood Ca2+ livels increase ACT: stimulate uptake of Ca2+ from blood into bone deceasing Ca2+ levels Parathyroid hormone: - PTH pro & sec by parathyroid glands. located in posterior aspect of thyroid gland STIM: when blood Ca2+ levels decrease ACT: stimulate osteoclasts to breakdown bone and release Ca2+ into blood - increase re-absorption of Ca2+ at kidneys - activate Vit D to promote absorption of Ca2+ from GIT.
- Hormonal secretion regulated by positive feedback using oxytocin as example & neg feedback using thyroid hormone as example
Oxytocin: release is regulated through pos. feedback as increased stretcing of cervix during birth stimulates the posterior pituitary to release more oxytocin as does the suckling of the infant Thyroid hormone: regulated via neg. feedback. Increase in TH levels inhibit secretion of TRH from hypothalamus and TSH from anterior to decrease production of TH.