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ENDOCRINE SYSTEM General Overview Major Control System Ductless Glands; Produce Hormones Diffuse into Blood Slow Initial Effects, but Effects Persist much longer Hormones act on specific Targets Regulated by Negative Feedback Classification of Glands Endocrine – ductless glands that secrete hormones into the blood stream Exocrine – ducted glands that secrete their products onto a surface Hormones - General Function Modify activity of target cells Targets have specific receptors for the particular hormone Distributed by blood Location of Receptors Cell Surface - cell membrane Intracellular - Cytoplasm - Nucleus There are Two Types of Hormones Steroid Hormones (cholesterol based) - Attach to intracelluar receptor - Directly activate genes Non-steroid Hormones (amino acid based) - Attach to receptor on plasma membrane - A second messenger forms inside the cell to change cell’s activity Mechanisms of Hormone Action Steroids (Synthesized from Cholesterol) - Hormone enters cell - Hormone forms complex with receptor - Complex binds to DNA in nucleus - Transcription of mRNA - New protein is synthesized in cytoplasm (Translation) - E.g. Testosterone Mechanisms of Hormone Action continued Non-Steroids (Proteins/Peptides/Amine) - Hormone does not enter cell & acts as 1st messenger - Hormone binds to receptor on cell membrane - Enzyme is activated - 2nd messenger is produced inside cell - 2nd messenger affects cell function - E.g. Pituitary hormones How do Cells Respond to Hormones? Precise response depends on target cell type or rates of normal cellular processes: - Activates/deactivates enzymes - Stimulates protein synthesis - Opens/closes ion channels - Stimulates mitosis What Stimulates Endocrine Glands? Hormonal - Tropic hormones alter activity of other endocrine glands - Most common Humoral (Chemical) - Changing levels of ions or nutrients trigger release of hormones Neural - Nerve Impulses stimulate hormone release What if Hormone Levels are too High or too Low? Cells can change the number of receptors they have: - Up Regulation * Low hormone levels * Number of receptors increases * Target’s responsiveness increases - Down Regulation * Excess hormone levels * Number of receptors decreases * Target’s responsiveness decreases Endocrine Organs Hypothalamus Pituitary - Anterior - Posterior Thyroid Gland Parathyroid Glands Adrenal Glands - Cortex - Medulla Pancreas Pineal Gland Thymus Gonads Hypothalamus Controls anterior pituitary - Releasing hormones stimulate secretion - Antagonistic Inhibiting hormones - Portal system carries hormones to Anterior Pituitary Secretes & stores hormones in posterior pituitary (neural tissue carries & stores) Neurally stimulates adrenal medulla (Master controller of ANS) Pituitary Gland Attached to Hypothalamus via infundibulum Protected by sella turcica of sphenoid Divided into 2 lobes: - Anterior (glandular) - Posterior (neural) Anterior Pituitary “Master Gland” (makes & secretes various tropic hormones) Tropic Hormones - Act on other Endocrine Glands - Serve as Hormonal Stimuli - Stimulate release of another hormone Anterior Pituitary Hormones Human Growth Hormone (GH) - Target: All/Most Body Cells (esp. Bone & Muscle) - Action: Promotes Growth & Repair Prolactin (PRL) - Target: Mammary tissue - Action: Promotes milk production Anterior Pituitary continued Thyroid Stimulating Hormone/Thyrotropic Hormone (TSH) - Target: Thyroid Gland - Action: Stimulates production of Thyroid Hormone Anterior Pituitary continued Adrenocorticotropic Hormone (ACTH) - Target: Adrenal Cortex - Action: Controls production/secretion adrenal cortex hormones Gonadotropic Hormones - Follicle Stimulating Hormone (FSH) * Target: Gonads (Ovaries/Testes) * Action: Production of gametes & sex hormones Anterior Pituitary continued - Luteinizing Hormone (LH/ICSH) *Target: Gonads *Functions: Produce sex hormones Prepare uterus for implantation Develop corpus luteum in ovary Posterior Pituitary Does NOT synthesize Hormones Neural rather than glandular Stores & releases two hormones produced by hypothalamus - Oxytocin *Target: Uterus & Mammary Glands *Action: Stimulates contractions to eject fetus & milk Posterior Pituitary continued - Antidiuretic Hormone (ADH) also called vasopressin *Target: Kidneys *Action: Conserves water (decreases urine volume) Raises blood pressure Thyroid Gland Thyroid Hormone - T3 (Triiodothyronine), T4 (Thyroxine) - Target: All body cells - Action: *Increase metabolic rate *Regulate body temperature *Influence development Thyroid Gland Calcitonin - Target: Bone/Osteoclasts - Action: *Inhibit osteoclasts *Increase Ca2+ deposition in bone *Decrease blood Ca2+ Parathyroid Glands Parathyroid Hormone - Target: Bone/Osteoclasts - Action: *Stimulate osteoclasts *Stimulate bone resorbtion *Increase absorption from urine & small intestine *Increase blood Ca2+ Adrenal Glands Adrenal Cortex (outer/glandular) - Cortisol * Target: Many tissues * Action: make glucose, decrease inflammation - Aldosterone * Target: Kidneys * Action: Increase blood Na+ & water; decrease blood K+ Adrenal Glands continued Adrenal Medulla (inner/nervous) - Epinephrine/Adrenalin (Fear/Flight) - Norepinephrine/Noradrenalin (Anger/Fight) - Target: Heart, Liver, Muscles, Blood Vessels, etc. - Action: Intensify sympathetic responses (threat response) 41 Pancreas Glucagon *Target: Liver *Action: Glycogen breakdown; raises blood glucose Insulin *Target: Most/All Cells *Action: Promotes glucose transport into cells; decreases blood glucose) Pineal & Thymus Glands Pineal Gland (Endocrine & CNS) - Melatonin * Target: Hypothalamus * Action: Sets biological rhythms & promotes sleep Thymus (atrophies at puberty) - Thymosins * Target: T cells of immune system * Action: T cell maturation (immunocompetence) Gonads Ovaries - Estrogen * Target: Uterus, Ovaries, other * Action: prepare for fetus, secondary sexual characteristics - Progesterone * Target: Uterus * Action: maintains pregnancy, prevents uterine contractions Gonads continued Testes - Testosterone *Target: Various body parts *Action: Secondary sexual characteristics