18.3 Endocrine System
18.3 Endocrine System BMS 508
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This 5 page Class Notes was uploaded by Katherine Loiselle on Monday February 22, 2016. The Class Notes belongs to BMS 508 at University of New Hampshire taught by Mary Katherine Lockwood, PhD in Spring 2016. Since its upload, it has received 16 views. For similar materials see Human Anatomy and Physiology II in Biological Sciences at University of New Hampshire.
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Date Created: 02/22/16
18-3 The bilobed pituitary gland is an endocrine organ that releases nine peptide hormones Anatomy of Pituitary Gland (Hypophysis) Small, oval gland Lies nestled within the sella turcica o Depression in sphenoid bone Inferior to hypothalamus Connected by infundibulum o Slender, funnel-shaped structure o Base lies between optic chiasm and mammillary bodies Held in position by sellar diaphragm o Dural sheet that encircles infundibulum o Isolates pituitary gland from cranial cavity Has anterior and posterior lobes o Differ in function and developmental anatomy Releases nine peptide hormones o Seven from anterior lobe o Two from posterior lobe o Bind to membrane receptors o Use cAMP as second messenger Anterior Lobe of Pituitary Gland (Adenohypophysis) Contains variety of endocrine cells Three regions o Pars distalis largest, most anterior region o Pars tuberalis Extension, wraps around infundibulum o Pars Intermedia Borders posterior lobe Slender, narrow band Capillary network o radiates through the three regions o provides endocrine cells access to bloodstream The Hypophyseal Portal System Chemical communication o One-way Named for destination Ensures hypothalamic hormones reach target cells in anterior lobe before being diluted by general circulation Hypothalamus controls production of hormones in anterior lobe of pituitary gland o Secretes specific regulatory hormones Median eminence o swelling near attachment of infundibulum o hypothalamic neurons release regulatory hormones into interstitial fluids Fenestrated capillaries o allow relatively large molecules to enter or leave bloodstream Capillary networks in median eminence are supplied by the superior hypophyseal artery o unite to form a series of larger vessels that spiral around infundibulum to reach anterior lobe Vessels form a second capillary network branches among endocrine cells Portal vessels o blood vessels that link two capillary networks Steps 1. Superior hypophyseal artery delivers blood to the capillary networks in the median eminence 2. Portal vessels deliver blood containing regulatory factors to the capillary network within the anterior lobe of the pituitary gland 3. Inferior hypophyseal artery delivers blood to the posterior lobe of the pituitary gland 4. Hypophyseal veins carry blood containing the pituitary hormones to the cardiovascular system for delivery to the rest of the body Hypothalamic Control of Anterior Lobe Two classes of hypothalamic regulatory hormones o releasing hormones o inhibiting hormones Releasing hormones (RH) o stimulates synthesis and secretion of one or more hormones at the anterior lobe Inhibiting hormones (IH) o Prevents synthesis and secretion of hormones from anterior lobe Control endocrine cells in anterior lobe Travel to anterior lobe by hypophyseal portal system Negative feedback Hormones of Anterior Lobe Tropic hormones o turn on endocrine glands or support the functions of other organs Thyroid-Stimulating Hormone (TSH) o thyrotropin o targets the thyroid gland o triggers release of thyroid hormones o released in response to thyrotropin-releasing hormone (TRH) from hypothalamus o As circulating concentrations of thyroid hormones rise, rates of TRH and TSH production declines Adrenocorticotropic Hormone (ACTH) o corticotropin o stimulates release of steroid hormones by the adrenal cortex o target cells that product glucocorticoids affect glucose metabolism o released in response to corticotropin-releasing hormone from hypothalamus o as glucocorticoid levels increase, rates of CRH release and ACTH release decline Gonadotropins o regulate activities of the gonads o released in response to gonadotropin-releasing hormone (GnRH) o abnormally low production results in hypogonadism o Two gonadotropins follicle-stimulating hormone luteinizing hormone o Inhibited by estrogens, progesterone, and androgens Follicle-Stimulating Hormone (FSH) o Follitropin o promotes follicle development in females and, in combination with luteinizing hormone, stimulates the secretion of estrogens by ovarian cells o Stimulates nurse cells in males promotes physical maturation of developing sperm o Inhibited by inhibin peptide hormone released by cells in testes and ovaries Luteinizing Hormone (LH) o Lutropin o Induces ovulation o Promotes ovarian secretion of estrogens and progesterone prepares body for pregnancy o Called interstitial cell-stimulating hormone (ICSH) stimulates production of sex hormone of interstitial cells of testes androgens Prolactin (PRL) o Works with other hormones to stimulate mammary gland development o stimulates milk production by mammary glands o May help regulate androgen production in males by making interstitial cells more sensitive to LH o Inhibited by neurotransmitter dopamine prolactin-inhibiting hormone (PIH) o Hypothalamus secretes prolactin-releasing factors (PRF) o Circulating PRL stimulates PIH release and inhibits secretion of PRF Growth Hormone (GH) o Samatotropin o Stimulates cell growth and replication by accelerating rate of protein synthesis o Stimulation of growth by GH involves two mechanisms: direct and indirect Primary mechanism is indirect Liver cells respond to GH by synthesizing and releasing somatomedins o Insulin-like growth factors (IGFs) o Stimulate tissue growth by binding to receptors on a variety of plasma membranes In skeletal muscle fibers, cartilage cells, and other target cells, somatomedins increase the uptake of amino acids and their incorporation into new proteins Direct mechanism Stimulates stem cell division and differentiation of daughter cells Stimulates breakdown of adipocytes (fat cells) o Releases fatty acids into blood o Glucose-sparing effect Stimulates breakdowns of glycogen reserves by liver cells o Releases glucose into blood stream o Diabetogenic effect Regulated by growth hormone-releasing hormone (GH-RH or somatocrinin) and growth hormone-inhibiting hormone (GH-IH or somatostatin) from the hypothalamus Somatomedins inhibit GH-RH and stimulate GH-IH Melanocyte-Stimulating Hormone (MSH) o Melanotropin o Two forms o Stimulates melanocytes of the skin increases production of melanin o Inhibited by dopamine o human pars intermedia secretes MSH during fetal development, very young children, pregnant women, diseases The Posterior Lobe of the Pituitary Gland Neurohypophysis Contains axons of hypothalamic neurons o Neurons of the supraoptic and paraventricular nuclei manufacture antidiuretic hormone (ADH) and oxytocin (OXT) o Move along axons in infundibulum to axon terminals End in basement membranes of capillaries in posterior lobe Travel by axoplasmic transport Antidiuretic Hormone (ADH) o Vasopressin (VP) o Released in response to stimuli Blood pressure o Osmoreceptors Respond to change in osmotic concentration of body fluids Stimulate neurosecretory neurons that release ADH o Acts on kidneys to retain water and decrease urination o Causes vasoconstriction in high concentrations Narrowing of peripheral blood vessels that helps elevate blood pressure o Inhibited by alcohol Oxytocin o Stimulates smooth muscle contraction in walls of uterus, promoting labor and delivery o Promotes ejection of milk by stimulating the contraction of myoepithial cells o Both part of neuroendocrine reflex Milk let-down reflex o Functions in sexual activity