The Central Endocrine Glands Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Endocrine System Consists of ductless endocrine glands scattered throughout body Glands secrete hormones which travel through blood to target cells Target cells have receptors for binding with specific hormone Regulates or directs particular function Two hormone categories based on solubility Hydrophilic Peptide hormones Catecholamines Lipophilic Steroid hormones Thyroid hormone Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Endocrine System Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Endocrine System Overall functions Regulate organic metabolism and H2O and electrolyte balance Induce adaptive changes to help body cope with stressful situations Promote smooth, sequential growth and development Control reproduction Regulate red blood cell production Along with autonomic nervous system, control and integrate both circulation and the digestion and absorption of food Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Tropic Hormones Regulates hormone secretion by another endocrine gland Stimulates and maintains their endocrine target tissues Example Thyroid-stimulating hormone (TSH) secreted from anterior pituitary stimulates thyroid hormone secretion by thyroid gland Also maintains structural integrity of thyroid gland Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Hormones Plasma concentration of each hormone is normally controlled by regulated changes in rate of hormone secretion Direct regulatory inputs that influence secretory output of endocrine cells Neural input Input from another hormone Effective plasma concentration also influenced by Rate of removal from blood by metabolic inactivation and excretion Rate of activation or its extent of binding to plasma proteins Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Endocrine Dysfunction Can arise from a variety of factors Most commonly result from abnormal plasma concentrations of a hormone caused by inappropriate rates of secretion Hyposecretion Too little hormone is secreted Hypersecretion Too much hormone is secreted Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Hyposecretion Primary hyposecretion Too little hormone is secreted due to abnormality within gland Causes Genetic Dietary Chemical or toxic Immunologic Other disease processes such as cancer Iatrogenic Idiopathic Secondary hyposecretion Gland is normal but too little hormone is secreted due to deficiency of its tropic hormone Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Hypersecretion Causes Tumors that ignore normal regulatory input and continuously secrete excess hormone Immunologic factors Primary hypersecretion Too much hormone is secreted due to abnormality within gland Secondary hypersecretion Excessive stimulation from outside the gland causes oversecretion Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Hormone Influence at Target Cell Hormone can influence activity of another hormone at given target cell in one of three ways Permissiveness One hormone must be present in adequate amounts for full exertion of another hormone’s effect Synergism Occurs when actions of several hormones are complimentary Combined effect is greater than the sum of their separate effects Antagonism Occurs when one hormone causes loss of another hormone’s receptors Reduces effectiveness of second hormone Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Pituitary Gland Hypophysis Small gland located in bony cavity just below hypothalamus Thin stalk connects pituitary gland to hypothalamus Consists of two anatomically and functionally distinct lobes Posterior pituitary (neurohypophysis) Composed of nervous tissue Anterior pituitary (adenohypophysis) Consists of glandular epithelial tissue Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Anatomy of the Pituitary Gland Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Pituitary Gland Release of hormones from both anterior and posterior pituitary is controlled by hypothalamus Posterior pituitary Along with hypothalamus forms neuroendocrine system Does not actually produce any hormones Stores and releases two small peptide hormones Vasopressin Conserves water during urine formation Oxytocin Stimulates uterine contraction during childbirth and milk ejection during breast-feeding Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Relationship of the Hypothalamus and Posterior Pituitary The paraventricular and supraoptic nuclei both contain neurons that produce vasopressin and oxytocin. The hormone, either vasopressin or oxytocin depending on the neuron, is synthesized in the neuronal cell body in the hypothalamus. The hormone travels down the axon to be stored in the neuronal terminals within the posterior pituitary. On excitation of the neuron, the stored hormone is released from these terminals into the systemic blood for distribution throughout the body. Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Pituitary Gland Anterior pituitary Secretes six different peptide hormones that it produces itself Tropic hormones Thyroid-stimulating hormone (TSH) Stimulates secretion of thyroid hormone Adrenocorticotropic hormone (ACTH) Stimulates secretion of cortisol by adrenal cortex Follicle-stimulating hormone (FSH) In females, stimulates growth and development of ovarian follicles; promotes secretion of estrogen by ovaries In males, required for sperm production Luteinizing hormone (LH) In females, responsible for ovulation and luteinization; regulates ovarian secretion of female sex hormones In males, stimulates testosterone secretion Growth hormone (GH) Primary hormone responsible for regulating overall body growth; important in intermediary metabolism Not a tropic hormone Prolactin (PRL) Enhances breast development and milk production in females Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Pituitary Gland Hypothalamic releasing and inhibiting hormones help regulate anterior pituitary hormone secretion Two most important factors that regulate anterior pituitary hormone secretion Hypothalamic hormones Feedback by target-gland hormones Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Hypothalamic Releasing and Inhibiting Hormones Secretion of each anterior pituitary hormone is stimulated or inhibited by one or more hypothalamic hypophysiotropic hormones Hormone Effect on Anterior Pituitary Thyrotropin-Releasing hormone (TRH) Stimulates release of TSH Corticotropin-Releasing hormone (CRH) Stimulates release of ACTH Gonadotropin-releasing hormone (GnRH) Stimulates release of FSH and LH Growth-hormone releasing hormone (GHRH) Stimulates release of growth hormone Growth-hormone inhibiting hormone (GHIH) Inhibits release of growth hormone and TSH Prolactin-releasing hormone (PRH) Stimulates release of prolactin Prolactin-inhibiting hormone (PIH) Inhibits release of prolactin Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Vascular Link Between the Hypothalamus and Anterior Pituitary Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Endocrine Control of Growth Growth depends on growth hormone but is influenced by other factors as well Genetic determination of an individual’s maximum growth capacity An adequate diet Freedom from chronic disease and stressful environmental conditions Normal levels of growth-influencing hormones Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Growth Rate Not continuous Factors responsible for promoting growth are not the same throughout growth period Fetal growth Promoted largely by hormones from placenta GH plays no role in fetal development Postnatal growth spurt Displayed during first two years of life Pubertal growth spurt Occurs during adolescence Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Normal Growth Curve Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Growth Hormone Primarily promotes growth indirectly by stimulating liver’s production of somatomedins Primary somatomedin is insulin-like growth factor (IGF-1) Acts directly on bone and soft tissues to bring about most growth-promoting actions Stimulates protein synthesis, cell division, and lengthening and thickening of bones Exerts metabolic effects not related to growth Increases fatty acid levels in blood by enhancing breakdown of triglyceride fat stored in adipose tissue Increases blood glucose levels by decreasing glucose uptake by muscles Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Growth Hormone Abnormalities Growth hormone deficiency Due to pituitary defect or hypothalamic dysfunction Hyposecretion of GH in child is one cause of dwarfism Deficiency in adults produces relatively few symptoms Growth hormone excess Most often caused by tumor of GH-producing cells of anterior pituitary Symptoms depend on age of individual when abnormal secretion begins Gigantism Caused by overproduction of GH in childhood before epiphyseal plates close Acromegaly Occurs when GH hypersecretion occurs after adolescence Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Growth Other hormones besides growth hormone are essential for normal growth Thyroid hormone Growth severely stunted in hypothyroid children Hypersecretion does not cause excessive growth Insulin Deficiency often blocks growth Hyperinsulinism often spurs excessive growth Androgens Play role in pubertal growth spurt, stimulate protein synthesis in many organs Effects depend on presence of GH Estrogens Effects of estrogen on growth prior to bone maturation are not well understood poorly Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Pineal Gland Tiny, pinecone-shaped structure Located in center of brain Secretes melatonin Hormone of darkness Secretion falls to low levels during light of day Functions Helps keep body’s circadian rhythms in synchrony with light-dark cycle Promotes sleep Influences reproductive activity, including onset of puberty Acts as antioxidant to remove free radicals Enhances immunity Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Circadian Rhythms Suprachiasmatic nucleus (SCN) Body’s master biological clock Self-induced cyclic variations in clock protein concentrations within SCN bring about cyclic changes in neural discharge from SCN Cycle takes about a day Drives body’s circadian (daily) rhythms SCN must be set daily by external cues so body’s biological rhythms are synchronized with activity levels driven by surrounding environment Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Circadian Rhythms Daily changes in light intensity Major environmental cue used to adjust SCN master clock Photoreceptors in retina pick up light signals and transmit them directly to SCN SCN relays message regarding light status to pineal gland Chapter 18 The Central Endocrine Glands Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning