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endocrine system.docx

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Biological Sciences
Kenneth Welch

The Endocrine System Intercellular Communication Endocrine versus Nervous system • Nervous system performs short term crisis management • Endocrine system regulates long term ongoing metabolic • Endocrine communication is carried out by endocrine cells releasing hormones • Alter metabolic activities of tissues and organs • Target cells • Paracrine communication involves chemical messengers between cells within one tissue An Overview of the Endocrine System Endocrine system • Includes all cells and endocrine tissues that produce hormones or paracrine factors Hormone structure • Amino acid derivatives • Structurally similar to amino acids • Peptide hormones • Chains of amino acids • Lipid derivatives • Steroid hormones and eicosanoids Hormones can be • Freely circulating • Rapidly removed from bloodstream • Bound to transport proteins Mechanisms of hormone action • Receptors for catecholamines, peptide hormones, eicosanoids are in the cell membranes of target cells • Thyroid and steroid hormones cross the membrane and bind to receptors in the cytoplasm or nucleus Control of endocrine activity • Endocrine reflexes are the counterparts of neural reflexes • Hypothalamus regulates the activity of the nervous and endocrine systems • Secreting regulatory hormones that control the anterior pituitary gland • Releasing hormones at the posterior pituitary gland • Exerts direct neural control over the endocrine cells of the adrenal medullae Endocrine System The Pituitary Gland Hypophysis • Releases nine important peptide hormones • All nine bind to membrane receptors and use cyclic AMP as a second messenger Gland The anterior lobe (adenohypophysis) • Subdivided into the pars distalis, pars intermedia and pars tuberalis • At the median eminence, neurons release regulatory factors through fenestrated capillaries • Releasing hormones • Inhibiting hormones The Endocrine System Hypophyseal portal system • All blood entering the portal system will reach the intended target cells before returning to the general circulation Hormones of the adenohypophysis • Thyroid stimulating hormone (TSH) • Triggers the release of thyroid hormones • Thyrotropin releasing hormone promotes the release of TSH • Adrenocorticotropic hormone (ACTH) • Stimulates the release of glucocorticoids by the adrenal gland • Corticotrophin releasing hormone causes the secretion of ACTH Hormones of the adenohypophysis • Follicle stimulating hormone (FSH) • Stimulates follicle development and estrogen secretion in females and sperm production in males • Leutinizing hormone (LH) • Causes ovulation and progestin production in females and androgen production in males • Gonadotropin releasing hormone (GNRH) promotes the secretion of FSH and LH Hormones of the adenohypophysis • Prolactin (PH) • Stimulates the development of mammary glands and milk production • Growth hormone (GH or somatotropin) • Stimulates cell growth and replication through release of somatomedins or IGF • Growth-hormone releasing hormone (GH-RH) • Growth-hormone inhibiting hormone (GH-IH) Melanocyte stimulating hormone (MSH) • May be secreted by the pars intermedia during fetal development, early childhood, pregnancy or certain diseases • Stimulates melanocytes to produce melanin The posterior lobe of the pituitary gland (neurohypophysis) • Contains axons of hypothalamic nerves • neurons of the supraoptic nucleus manufacture antidiuretic hormone (ADH) • Decreases the amount of water lost at the kidneys • Elevates blood pressure The posterior lobe of the pituitary gland (neurohypophysis) • Neurons of the paraventricular nucleus manufacture oxytocin • Stimulates contractile cells in mammary glands • Stimulates smooth muscle cells in uterus The Thyroid Gland The thyroid • Lies near the thyroid cartilage of the larynx • Two lobes connected by an isthmus Thyroid follicles and thyroid hormones • Thyroid gland contains numerous follicles • Release several hormones such as thyroxine (T4) and triiodothyronine (T3) • Thyroid hormones end up attached to thyroid binding globulins (TBG) • Some are attached to transthyretin or albumin Thyroid hor
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