Endocrine System Endocrine System Major homeostatic control system

Endocrine System

Endocrine System Major homeostatic control system with nervous system. Effects mediated by hormones Responses slower and longer lasting than nervous system Influence much broader Regulates virtually all types of cells Consists of endocrine glands several organs containing endocrine tissue

Chemical Regulating Systems: Overview Metabolite is a substance produced during metabolism or taking part in metabolism can be intracellular and extracellular Hormones: cell to cell communication molecules Made in gland(s) or cells Transported by blood and extracellular fluid Distant or local target tissue receptors Activates physiological response Pheromones: organism to organism communication

Types of hormones Functional Endocrine Hormones – Travel through the blood to act at a site distant from the secreting cell or gland Paracrine Hormones – Act on cells near the secreting cell Autocrine Hormones – Act on the secreting cell Neurocrine Hormones – Secreted by neural cells neurotransmitters neurohormones

Types of hormones Chemical Two classes of hormones: water soluble Amines Modified amino acids Proteins Eicosanoids Derived from arachadonic acid lipid soluble Steroids Derived from cholesterol Thyroid hormones Iodine added to tyrosine Nitric oxide

Hormone action Water soluble hormones act via second messengers Cyclic AMP hormone binds to membrane receptor coupled to adenylate cyclase (AC) via G protein AC catalyses synthesis of cyclic AMP cyclic AMP changes metabolism Eventually deactivated by phosphodiesterase

Hormone action Lipid soluble hormones Bind to and activate receptors in cytosol or nucleus Activated receptor-hormone complex alters gene expression Directs synthesis of new proteins Alters cellular metabolism

Hormone action Hormones only affect cells with receptors for that hormone Responsiveness of target cell to hormone depends on: Hormone concentration Abundance of receptors Receptors constantly turned over Up-regulation - increases sensitivity Down-regulation - reduces sensitivity Influences exerted by other hormones Permissive effect Presence of another hormone increases response to first hormone by: up-regulating receptors for first hormone stimulating production of enzyme required for expression of first hormones effect

Control of hormone secretion Secretion of most hormones occurs in short pulsatile bursts Secretion regulated by: Nervous system Chemical changes in blood Other hormones (tropins) Secretion regulated by negative feedback for most Some operate via positive feedback (eg oxytocin) Blood levels reflect balance between secretion and degradation/excretion. Half-lives vary

Endocrine glands Endocrine glands ductless well vascularised secrete hormones into extracellular fluid

Classification Phylogenetic branchial group (thyroid gland, parathyroid glands and thymus gland), medullary process group (pituitary [neurohypophysis] and pineal glands), chromaffin group (adrenal [suprarenal] glands, chromaffin paraganglia) Ontogenetic (histogenetic) ectodermal glands (adenohypophysis, thyroid gland, parathyroid glands and thymus gland), coelomic [mesodermal] glands (adrenal cortex, genitals), entodermal glands (insular apparatus of the pancreatic gland), neural glands (neurohypophysis, adrenal medulla, epiphysis, paraganglia) Functional - functional interrelation of the organs of the endocrine system

Classification

HYPOTHALAMUS Integrates functions that maintain chemical and temperature homeostasis Functions with the limbic system Controls the release of hormones from the anterior and posterior pituitary

HYPOTHALAMUS Synthesizes & releases hypophysiotropic hormones: Thyrotropin-releasing hormone (TRH) Corticotropin-releasing hormone (CRH) Gonadotropin-releasing hormone (GnRH) Growth hormone-releasing hormone (GHRH) Growth hormone-inhibiting hormone (GHIH) Prolactin-releasing factor (PRF) Prolactin-inhibitn hormone (PIH)

HYPOTHALAMUS Synthesizes hypophysiotropic hormones in cell bodies of neurons located in the hypothalamus Transports hormones down the axon and stored in the nerve endings Secretion of hormones is in pulses

HYPOTHALAMUS: Secretion of Hypophysiotropic Hormones Is influenced by emotions Can be influenced by the metabolic state of the individual Delivered to the anterior pituitary via the hypothalamic-hypophyseal portal system Usually initiates a three-hormone sequence

Negative Feedback Controls: Long & Short Loop Reflexes

Endocrine Control: Three Levels of Integration Figure 7-13: Hormones of the hypothalamic-anterior pituitary pathway

Anterior Pituitary gland Anterior pituitary hormones: secretion controlled by negative feedback releasing and inhibiting factors from hypothalamus Synthesised by neurosecretory cells in hypothalamus Packaged in vesicles at axon terminals Released in response to nerve impulses Reach anterior pituitary via hypophyseal portal system

Anterior Pituitary gland Anterior pituitary cells secrete 7 hormones: Growth hormone Secreted in bursts every few hours (especially during sleep) Stimulates general body growth and regulates metabolism by stimulating production of IGFs Increase protein synthesis Decrease protein catabolism Increase uptake of amino acids Thyroid stimulating hormone Controls secretions and activities of thyroid gland Gonadotropins (FSH and LH) regulate function of gonads

Anterior Pituitary gland Anterior pituitary cells secrete 7 hormones: Prolactin Initiates and maintains milk secretion by mammary glands Other hormones exert permissive effects (eg estrogens, progesterone) Adrenocorticotrophic hormone (ACTH) stimulates adrenal cortex to release glucocorticoids Melanocyte stimulating hormone precise role unknown but causes darkening of skin

Posterior Pituitary gland Posterior pituitary Stores and releases: Oxytocin - uterine contractions, milk ejection, sexual arousal, nurturing ADH - stimulates reabsorption of water by kidneys, reduces sweating, promotes constriction of arterioles Formed in neurosecretory cells in hypothalamus Fast axonal transport to secretory vesicles in axon terminals in posterior pituitary Released in response to nerve impulses

Pineal gland Pineal gland secretes: melatonin (inhibited by light) sets diurnal rhythm Inhibits reproductive function in animals that breed seasonally Causes atrophy of gonads May contribute to Seasonal Affective Disorder (SAD)

Thyroid gland Only endocrine gland that stores secretory product in large quantity Approx 100 day supply Located inferior to larynx Two highly vascular lobes Made up mostly of thyroid follicles Wall of each follicle contains: Parafollicular cells Produce calcitonin Reduces blood calcium by inhibiting osteoclasts Follicular cells become secretory under influence of TSH (from anterior pituitary) Produce: Thyroxine (tetraiodothyronine; T4) Triiodothyronine (T3)

Thyroid gland Synthesis and secretion of T3 and T4: (1) Iodide trapping Follicular cells actively transport iodide ions from blood (2) Synthesis of thyroglobulin (TGB) Follicular cells synthesise TGB and release into follicle lumen (3) Oxidation of iodide Some tyrosine amino acids in TGB will become iodinated negatively charged iodide will not bind to tyrosine so must be oxidised (ie electron removed) Catalysed by peroxidase enzyme (4) Iodination of tyrosine 1-2 iodine ions bind to tyrosine to form T1or T2 (5) Coupling of T1 and T2 2 x T2 molecules join to form T4 1 x T1 + 1 x T2 join to form T3

Thyroid gland Synthesis and secretion of T3 and T4: (6) Pinocytosis and digestion of colloid Droplets of colloid from lumen enter follicular cells by pinocytosis Merge with lysosomes T3 and T4 cleaved from TGB in lysosome (7) Secretion of thyroid hormones T3 and T4 lipid soluble Diffuse through plasma membrane T3 usually secreted in greater quantity than T3 T3 more potent than T4 Most T4 converted to T3 in body cells through cleavage of one iodine (9) Transport in the blood >99% combine with transport proteins in blood Mainly thyroxine-binding globulin (TBG)

Parathyroid glands Chief cells secrete parathyroid hormone in response to decreasing blood calcium Increases osteoclast number and activity Promotes Ca2+ release from bone Increases Ca2+ reabsorption in kidney Enhances calcitriol (vitamin D) production by kidney Increases Ca2+ absorption in GI tract

Thymus Located in mediastinum Hormones produced promote maturation of T cells

Pancreas Pancreas is both endocrine and exocrine gland 99% of pancreatic cells arranged in clusters called acini produce digestive enzymes Rest clustered into pancreatic islets (Islets of Langerhans) Islets contain 4 types of hormone-secreting cells Alpha cells (~15%) – secrete glucagon Raises blood glucose Beta cells (~80%) – secrete insulin Lowers blood glucose Delta cells (~5%) – secrete somatostatin Inhibits glucagon and insulin secretion Slows nutrient absorption from GI tract F cells (<1%) - secrete pancreatic polypeptide Inhibits somatostatin secretion Inhibits gall bladder contraction Inhibits secretion of pancreatic digestive enzymes

Adrenal Glands Located superior to kidneys Outer cortex (80-90%) and inner medulla (10-20%) Cortex secretes steroid hormones essential for life: mineralocorticoids (mainly Aldosterone) Targets kidneys Na+ and H2O resorption K+ and H+ secretion glucocorticoids (mainly cortisol) resist stress (mobilise fuel) anti-inflammatories immunosuppressive Androgens (mainly dehydroepiandrosterone) Converted to estrogens in females Promote libido Only source of androgens after menopause

Adrenal Glands Adrenal medulla secretes catecholamines Epinephrine Norepinephrine Increase fuel mobilisation and cardiac output

Ovaries Gonads Produce gametes Sperm Oocytes Ovaries Produce estrogens (estradiol and estrone - maturation of female reproductive system and development of secondary sex characteristics Progesterone - works with estrogen to establish menstrual cycle Inhibin – inhibits secretion of FSH Relaxin – increases flexibility of pubic symphisis and cervical dilation during pregnancy

Ovaries and testes Gonads Produce gametes Sperm Oocytes Testes Testosterone - maturation of male reproductive organs, development of secondary sex characteristics, production of sperm Inhibin – inhibits secretion of FSH

Stress response Any stimulus that produces a stress response is a stressor May be pleasant (ie long ride at royal show) or unpleasant Homeostatic mechanisms attempt to counteract stress Variety of different stressors elicit similar sequence of responses (ie the stress response) Stress response occurs in 3 stages: (1) Fight-or-flight response Sympathetic division of ANS mobilises fuel stores etc ready for physical activity Non-essential functions inhibited (2) Resistance reaction Initiated by hypothalamic releasing hormones shich make ATP available Corticotropin releasing hormone Growth-hormone releasing hormone Thyrotropin-releasing hormone Helps body continue fighting stressor after fight-or-flight response dissipates (3) Exhaustion Resistance reaction cannot be maintained due to depletion of resources

Endocrine system and aging Production of hGH decreases with age Contributes to muscle atrophy Thyroid gland reduces production of thyroid hormones Reduces metabolic rate PTH increases and calcitonin decreases with age Contribute to age-related reduction in bone mass Pancreas releases insulin more slowly and receptor sensitivity to glucose declines Blood glucose increases faster and returns to normal more slowly with age Ovaries decrease in size with age Reduced estrogen output contributes to osteoporosis, high BP and atherosclerosis Testes decrease testosterone production with age Effects on sperm not evident until very old age

Summary

Summary

Summary