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Endocrine System: OverviewEndocrine system-a control system which influences metabolic activities of cells by means of hormonesEndocrine glands- pituitary, thyroid, parathyroid, adrenal, pineal, and thymusThe pancreas and gonads produce both hormones and exocrine productsThe hypothalamus has both neural functions and releases hormonesOther tissues and organs that produce hormones-adipose cells, pockets of cells in the walls of small intestine, stomach, kidneys, and heartAutocrines and ParacrinesAutocrines: chemicals that exert effects on the same cells that secrete themParacrines: locally acting chemicals that affect cells other than those that secrete themThese are not considered hormones since hormones are long-distance chemical signalsHormonesHormones: chemical substances secreted by cells into the extracellular fluidsRegulate the metabolic function of other cellsHave lag times ranging from seconds to hoursTens to have prolonged effectsAre classified as amino acid based hormones, or steroidsEicosanoids: biologically active lipids with local hormone-like activityTypes of hormonesAmino acid basedAmines, thyroxin, peptide, and protein hormonesSteroids- gonadal and adrenocortical hormonesEicosanoids- leukotrines and prostaglandinsHormone ActionHormones alter target cell activity by one of two mechanisms1. Second messengersRegulatory G proteinsAmino acid-based hormones2. Direct Gene ActivationSteroid hormonesThe precise response depends on the type of the target cellMechanism of Hormone ActionHormones produce one or more of the following cellular changes in target cellsAlter plasma membrane permeabilityStimulate protein synthesisactivate or deactivate enzyme systemsInduce secretory activityStimulate mitosisAmino Acid Based Hormone Action: Second Messenger (cAMP)Hormone (first messenger) binds to its receptor, which then binds to a G proteinThe G protein is then activated as it binds GTP, displacing GDPActivated G protein activates the effector enzyme adenylate cyclaseAdenylate cyclase generates camp (second messenger) from ATPcAMP activates protein kinases, which then cause cellular effectsAmino Acid-Based Hormone Action: PIP-CalciumHormone binds to the receptor and activates G proteinG protein binds and activates phospholipasePhospholipase splits the phospholipid PIP­2 into discylglycerol (DAG) and IP3 (both act as second messengers)DAG activates protein kinases; IP3 triggers release of Ca2+ storesCa2+ (third messenger) alters cellular responsesSteroid HormonesThis interaction prompts DNA transcription to produce mRNAThe mRNA of translated into proteins, which bring about a cellular effectTarget cell SpecificityHormones circulate to all tissues but only activate cells referred to as target cellsTarget cells must have specific receptors to which hormones bindThese receptors may be intracellular or located on the plasma membraneExamples of hormone activityACTH receptors are only found on certain cell of the adrenal cortexThyroxin receptors are found on nearly all cells of the bodyTarget Cell ActivationTarget cell activation depends on three factorsBlood levels of the hormoneRelative number of receptors on the target cellThe affinity of those receptors for the hormoneUp-regulation-target cells form more receptors in response to the hormoneDown-regulation-target cells lose receptors in response to the hormoneHormone concentrations in the bloodHormones circulate in the blood in two forms - free or boundSteroids and thyroid hormone are attached to plasma proteinsAll others are unencumberedConcentrations of circulating hormone reflect:Rate of releaseSpeed of inactivation and removal from the bodyHormones are removed from the blood byDegrading enzymesThe kidneysLiver enzyme systemsInteraction of Hormones at Target CellsThree types of hormone interactionPermissiveness: one hormone cannot exert its effects without another hormone being presentSynergism: more than one hormone produces the same effects on a target cellAntagonism: one or more hormones opposes the action of another hormoneControl of hormone releaseBlood levels of hormones:Are controlled by negative feedback systemsVary only within a narrow desirable rangeHormones are synthesized and released in response to:Humoral stimuliNeural stimuliHormonal stimuliHumoral stimuliHumoral stimuli: secretion of hormones in direct response to changing blood levels of ions and nutrientsExample: concentration of calcium ions in the bloodDeclining blood Ca2+ concentration stimulates the parathyroid glands to secrete PTH (parathyroid hormone)PTH causes Ca2+ concentrations to raise and the stimulus is removedNeural stimuliNeural stimuli: nerve fibers stimulate hormone releasePreganglionic sympathetic nervous system (SNS) fibers stimulate the adrenal medulla to secrete catecholamine’sHormonal StimuliHormonal stimuli: release of hormones in response to hormones produced by other endocrine organsThe hypothalamic hormones stimulate the anterior pituitaryIn turn, pituitary hormones stimulate targets to secrete till more hormonesNervous System ModulationThe nervous system modifies the stimulation of endocrine glands and their negative feedback mechanismsThe nervous system can override normal endocrine controlsFor Example, control of blood glucose levelsNormally the endocrine system maintains blood glucoseUnder stress, the body needs more glucoseThe hypothalamus and the sympathetic nervous system are activated to supply ample glucoseMajor Endocrine Organs: pituitary (hypophysis)Pituitary gland: two-lobed organ that secretes nine major hormonesNeurohypophysis: posterior lobe (neural tissue) and the infundibulumReceives, stores, and releases hormones from the hypothalamusAdenohypophysis: anterior lobe, made up of glandular tissueSynthesizes and secretes a number of hormonesPituitary-Hypothalamic Relationships: Posterior LobeThe posterior lobe is a down growth of hypothalamic neural tissueHas a neural connection with the hypothalamus (hypothalamic-hypophyseal tract)Nuclei of the hypothalamus synthesize oxytoxin and antidiuretic hormone (ADH)These hormones are transported to the posterior pituitaryPituitary-Hypothalamic Relationships: Anterior LobeThe anterior lobe of the pituitary is an outpocketing of the oral mucosaThere is no direct neural contact with the hypothalamusAdenophypophyseal HormonesThe six hormones of the adenohypophysis:Abbreviated as GH, TSH, ACTH, FSH, LH, and PRIRegulate the activity of the other endocrine glandsIn addition,


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FSU PET 3322 - Endocrine System

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