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Anatomy and Physiology for EngineersSlide 13-1

The Endocrine SystemLike nervous system, endocrine system providescommunication and control.Messages are relayed from one cell to another via chemicalmessengers (hormones).Unlike nervous system which provides instant but short-livedcommunication, endocrine system allows for slower-actingbut long-term communication.Many similarities also exist:

Both systems rely on release of chemicals with specific binding ontotarget receptors.Both systems use many of the same chemicals (ex: epinephrine andnorepinephrine).Control set up via negative feedback systems.End goal of both systems is the same: homeostasis.

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The Endocrine System

Includes all endocrine cellsand tissues in the body.Endocrine cells are secretorygland cells that releaseproduct internally (asopposed to exocrine glandsthat release onto epithelialsurfaces).Chemicals released may actlocally (prostaglandins), oron other cells elsewhere(hormones).

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Hormone Structure

Hormones can be divided into three basic groups based on chemical structure.Amino acid derivatives

Small molecules that are structurally similar to amino acids.Include epinephrine, norepinephrine, thyroid hormones, melatonin.

Peptide hormonesChains of amino acids.Include short chains (antidiuretic hormone(ADH), oxytocin).And long protein-like chains (growth hormone, prolactin).Largest class of all hormones and includes all hormones secreted byhypothalamus, pituitary gland, heart, kidneys, thymus, digestive tract, andpancreas.

Lipid derivativesTwo classes: steroid hormones (derived from cholesterol), and hormones derivedfrom arachidonic acid.Steroid hormones are structurally similar to cholesterol, and are released byreproductive organs and adrenal glands.Arachidonic acid-derived hormones include the prostaglandins and are importantin various extracellular activities including blood clotting.

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We have learned about the importance of proteins indefining the general shape and metabolism of the cell.Structural proteins provide shape.Enzymes direct metabolic processes.Hormones affect the identity, activity or quantity ofspecific structural and enzymatic proteins.Hormones have specific cellular targets (target cells).Sensitivity of target cell to particular hormonedepends on the presence or absence of specificreceptors (on membrane or in cytoplasm) to thatparticular hormone.

Mechanism of Hormone Action

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Mechanism of Hormone Action

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First and Second Messengers

Certain hormones cannot diffuse through the cellmembrane due to their insolubility in lipids and theirlarge size.These include epinephrine, norepinephrine, peptidehormones.These hormones are known as first messengersbecause they target specific cell receptors onmembrane.When this binding occurs, a second messenger in thecytoplasm is activated.Second messenger will modulate cellular activities(activation or inhibition).

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First and Second Messengers

Cyclic AMP (c-AMP) isa common and importantsecond messenger.1st messenger binds tocell membrane.This activates enzymeadenylate cyclase.Enzyme catalyzesconversion of ATP tocAMP.

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Intracellular ReceptorsSteroid hormones and thyroidhormones can cross cellularmembrane.Receptors are present withincytoplasm.Hormone-receptor complexbinds to specific DNAsegments, allowing activationor inhibition of specific genes.For example: testosteronestimulates production ofenzymes and proteins withinskeletal muscle causingincrease in muscle mass andstrength.

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Control of Endocrine ActivityControl of hormonal activity may be direct orindirect.Direct pathway involves endocrine cells releasingsecretions into blood stream as a result of changes inextracellular fluid composition.

Decrease in calcium levels causes release of parathyroidhormone, causing release of calcium by target cells.

Indirect pathway set up via negative feedbackthrough the hypothalamus.

Controls adrenal cells via autonomic pathways.Releases various hormones.Secretes specialized regulatory hormones which regulateactivity of pituitary gland.

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The Pituitary Gland

Also called thehypophysis.Located in sellaturcica withinsphenoid bone.All pituitary hormones bind to membrane receptorson target cells and use cAMP as a secondmessenger.Infundibulum connects pituitary gland tohypothalamus.Contains distinct anterior and posterior regions.

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Anterior Pituitary Gland

Contains endocrine cells withincapillary network.Controlled by regulatory hormonesproduced by hypothalamus.The vascular network here is calledthe hypophyseal portal system.Entry of hormones produced bypituitary gland into vascular systemoccurs at the hypophyseal port.

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Hypothalamic Control of the AnteriorPituitary

Regulatory hormones released by hypothalamus aretransported directly to anterior pituitary via theyhypophyseal system.

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Anterior Pituitary HormonesThyroid stimulating hormone (TSH)

Stimulates hormone production by the thyroid gland.Increase in thyroid hormones inhibit TSH production via negative feedback throughhypothalamus.

Adrenocorticotropic hormone (ACTH)Stimulates adrenal glands to release steroid hormones.Important in the metabolism of glucose.

Follicle-stimulating hormone (FSH)Promotes reproductory functions (egg development and sperm production)

Luteinizing hormone (LH)Also supports reproductory functions.Induces ovulation in women, promotes secretion of various hormones including progesterone(women) and testosterone (men).

Prolactin (PRL)Stimulates development of mammary glands and production of breast milk.

Growth hormone (GH)Stimulates cell growth and replication by increasing rates of protein synthesis.

Melanocyte-stimulating hormone (MSH)Stimulates melanocytes (normal human function is still unknown).

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The Posterior PituitaryContains axons from thehypothalamus.Two hormones are secreted inresponse to hypothalamicstimulation.Antidiuretic hormone is releasedwhen electrolytes in blood rise, orwhen blood pressure or volumefall.

ADH causes kidneys to retain morewater.ADH also causes vasoconstriction.

Oxytocin is also released to causeuterine muscles in women tocontract during later stages ofpregnancy.

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Pituitary Hormones and their Targets

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The Thyroid Gland

Located below the anterior portion of the larynx.Red in color due to high degree of vascularity.Thyroid follicles are spheroid-shaped structures thatare present throughout thyroid.Cavity within each follicle is filled with colloidalfluid (fluid suspended with proteins).Thyroid hormones are stored within these follicles.Upon stimulation, epithelial cells on surface movehormones from inside to outside.

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The Thyroid Gland

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Thyroid Hormones

Released hormones are instantly bound by carrier proteinspresent within blood.Hormones are derivatives of amino acid tyorosine (plus 3 – 4iodine atoms).Thyroxine (T4) is the most common thyroid hormone.Thyroid hormones act by diffusing into cytoplasm of the cellto reach receptor sties on mitochondria and in the nucleus.Causes increase in ATP production, and activates genesresponsible for enzymes that increase cellular metabolism.Normal production of thyroid hormones establish backgroundrates of cellular metabolism.

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C Cells and Calcitonin

Additional group of endocrine cells present inthyroid gland.These are called C-cells (produce hormonecalcitonin).Calcitonin assists in the regulation of calcium ionlevels in body fluids.C-cells release calcitonin when calcium levels rise.Calcitonin targets bones and kidneys.

Decreases osteoclast activity and stimulates calciumexcretion by kidneys.

Reduction in calcium levels turns off calcitoninswitch.

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The Regulation of Calcium Ions

Know thepathways bywhich calciumis regulated inthe body.Includesthyroid andparathyroidglands.

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The Parathyroid GlandTwo pairs of parathyroid glands are embedded on posterior surfaces of thethyroid gland.Two different cell populations found in parathyroid gland.Chief cells produce parathyroid hormone.

These monitor calcium levels also and release parathyroid hormone (PTH)when calcium levels fall.PTH stimualtes osteoclasts, reduces urinary elimination of calcium, promotesreabsorption of calcium by intestine.

Function of other group of cells is still unknown.

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The Thymus

Specialized secretory gland located in thoraciccavity, just behind the sternum.Increase in size as child grows; decreasesafter puberty.Responsible for producing various hormones(thymosins) that are important in maintainingimmunologic defenses.