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Endocrine System.doc

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Department
Physiology
Course
Physiology 2130
Professor
Sarah Mc Lean
Semester
Summer

Description
Endocrine System The endocrine system consists of a series of glands that secrete different hormones into the blood. These hormones then travel throughout the body to their target site to initiate their effect. Function - maintenance of internal environment - adaptation to stress - control of growth - metabolism - control of reproduction Actions of endocrine system are slower to take effect than nervous system but they last longer and are generally more widespread throughout the body. Glands - group of specialized cells that synthesize, store and release hormones - hormones circulate throughout the body to specific target cells that have receptors for the hormone Hormones 1) Tyrosine (thyroxine, triiodothyronine)  thyroid gland 2) Protein (calcitonn, parathyroid, pituitary and pancreatic) hypothalamus 3) Steroid (cortisol, aldosterone, estrogen, progesterone and testosterone)  cholesterol Protein Hormones  Hydrophilic = circulate freely in blood = cannot diffuse through the cell membrane  Receptor = must be located on the cell membrane of target cell Steroid/Thyroid Hormones  Hydrophobic = require a protein carrier to help circulate = diffuse easily  Receptor = located inside target cell - Hormones are secreted into the blood in pulses by a very specific stimulus and in amounts that vary with the strength of stimulus. Receptors - Unique structure in or on a cell that interacts with hormone Hydrophobic Hormones o can diffuse through the cell membrane, the receptor will be located in the cytoplasm or in the nucleus o hormone must be released by its carrier protein before it can enter cell Hydrophilic Hormones o unable to diffuse through the cell membrane o hormone attaches to the receptor and initiates a sequence of chemical reactions that will eventually alter the activity of the cell o 3 ways: through second messenger, G protein, tyrosine kinase Second messenger • when hormone binds to its receptor, it causes a G-protein nd on the inside of the membrane to produce a 2 messenger • cAMP is most common Tyrosine Kinase • hormone receptor complex activates tyrosine kinase on the inside surface of the membrane • tyrosine kinase then alters existing proteins that will then alter activity of cell Ion Channels - when hormone attaches to its receptor, a G protein is activated that lies within the cell membrane - G-protein can then open adjacent ion channels Control of secretion - controlled by negative feedback Hypothalamus Structure/Function - located at the base of the brain just above pituitary gland and below thalamus - receives information from all over the brain - involved with homeostatic mechanisms: temperature, water balance, energy production, behavioural drives (thirst, hunger, sexual) Hormones and releasing factors - secretion of many types of hormones:  Prolactin Releasing Hormone (PRH)  Prolacting Inhibiting Hormone (PIH)  Thyrotropin Releasing Hormone (TRH)  Corticotropin Releasing Hormone (CRH)  Growth Hormone Releasing Hormone (GHRH)  Growth Hormone Inhibiting Hormone (GHIH)  Gonadotropin Releasing Hormone (GnRH) I.e. Prolacting Releasing hormone is secreted from the hypothalamus to cause the ‘release’ of the hormone prolactin from the anterior pituitary. Pituitary Gland Structure - divided into two distinct regions (anterior and posterior) Anterior: o develops from tissue that forms the roof of the mouth o made up of endocrine tissue  endocrine cells secrete pituitary hormones directly into blood o Regulated by circulatory system: hypothalamic-hypophyseal portal system o Communicates with anterior pituitary by secreting releasing/inhibiting hormones Posterior: o Develops from neural tissue at the base of the brain o Contains axons and nerve terminals of neurons whose cell bodies lie in the hypothalamus (hypothalamic-hypophyseal tract) o Neurons produce neurohomrones (antidiuretic and oxytocin) o Secreted into posterior in response to AP Function - hormones from anterior pituitary control metabolic functions - two other anterior hormones control growth of the ovaries and testes and regulate their reproductive functions - hormones from posterior pituitary regulate water reabsorption in kidney and milk release from the breasts + contraction of uterus Regulation by Negative Feedback - Hypothalamus secretes a releasing hormone (H1) which causes the release of an anterior pituitary hormone (H2) into the blood - Hormone (H2) can feed back to the hypothalamus to decrease the release of hormone (H1) in a ‘short loop’ - The anterior pituitary hormone (H2) will circulate to an endocrine gland to cause the release of (H3) - (H3) can feed back to the hypothalamus and pituitary to decrease the release of H1 and H2 in a ‘long loop’ Thyroid Gland Structure - lies directly below the larynx and consists of two lobes that almost completely surround the trachea - made up of follicles (functional units of gland) - follicles consist of a central region of colloid surrounded by epithelial cells Function - produce triiodothyronine (T3) and thyroxine (T4)  regulating basal metabolic rate - hormones are made from tyrosine (hydrophobic)  require protein carrier to circulate in the blood Production of T3 and T4 - produced inside the follicles of the thyroid gland by combining iodine and tyrosine with the help of a glycoprotein called thyroglobulin - epithelial cells take up molecules of tyrosine from the circulation, then they combine with thyroglobulin. - Epithelial cells also actively take up iodine (from diet) - As the tyrosine-thyroglobulin complex is secreted into the colloid, one or two molecules of iodine attach to each tyrosine - In colloid, two tyrosine molecules will join together while attached to the thyroglobulin - Depending on number of iodine molecules attached to tyrosine  T3 or T4 Secretion of T3 and T4 - Thyroid stimulating hormone (TSH) is released from anterior pituitary gland - TSH binds to a receptor on membrane of epithelial cell which will stimulate rxn - Reactions:  Trapping and taking up of circulating iodine (in order to form hormones)  Stimulating endocytosis of the T3/T4 thyroglobulin complex into the cells  Enzymatic removal of thyroglobulin from T3/T4 in epithelial cells and stimulating the secretion of T3/T4 into the blood - TSH also stimulates the thyroid to grow  Hyperplasia Regulation of Secretion (T3/T4) - hormones will feed back to the hypothalamus and pituitary to ultimately inhibit the release of thyrotropin releasing hormone + TSH - With less TSH circulating to the thyroid  less T3/T4 will be released into blood Circulation of T3/T4 - 90% of hormones released from the thyroid gland consist of T4 - 10% is T3 - Most of T4 is converted to T3 (T3 is more biologically active) Effects of T3 and T4 hormones - Receptors for hormones are found within the nucleus of almost all cells in the body and can alter the transcription of genes to form many different proteins - Responsible for regulating body’s basal metabolic rate (BMR), proper development of nervous system in foetus - Help maintain person’s alertness, responsiveness, and emotional state o Increase in thyroid hormone will:  Increase body temperature  Increase cardiac output  Increas
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