Hypothalamus & Pituitary Gland PDF

FLG 222: Endocrinology Dr Gumede [email protected] Endocrine System vs Nervous System Nervous system Endocrine system Electrical communication Chemical communication Transmission by Neurons Transported via Blood stream Voluntary/involuntary...

FLG 222: Endocrinology Dr Gumede [email protected] Endocrine System vs Nervous System Nervous system Endocrine system Electrical communication Chemical communication Transmission by Neurons Transported via Blood stream Voluntary/involuntary Involuntary localised Often distant Endocrine System - organisation Endocrine glands - Hormones secreting glands Basic introduction to endocrine physiology Definition of HORMONE: Chemical released into BLOOD by specialized cells (ENDOCRINE GLAND) in response to specific stimulus (neural or blood borne). Travel through bloodstream Amount of HORMONE released is generally SMALL but depends on the strength of the stimulus. Receptors are specific for a hormone Location of receptors determines where hormone will produce its effects (ie. which tissues are affected) eg. ACTH receptors in adrenal cortex; Cortisol receptors in most tissues INPUT Gland Target tissue RESPONSE Hormones: effect of secretion Autocrine action: hormone has an effect on same cell it produces it Paracrine action: locally acting chemicals that affect cells adjacent to it General endocrine hormones: transported via the bloodstream to other tissues/organs  Hormones: Three major chemical classes Amines Peptides Steroids  Hormone Classification - Amines Small group of hormones derived from tyrosine; Examples – Epinephrine, norepinephrine, thyroid hormone Catecholamines (example – Adrenaline): Are water-soluble Do not require carrier proteins in plasma Receptor are located on membrane surface (of cell) Hormone Classification - Amines Thyroid hormones (Example – T3) Solubility in water – poor (circulate in protein) Require carrier proteins in the blood Receptors are located intracellularly (inside cell) Hormone Classification - Peptide & Protein Hormones Examples - Growth hormone, insulin, glucagon, prolactin etc Largest class of hormones Generally water soluble Do not require carrier molecules in the blood Receptors located on membrane surface Guyton and Hall Textbook of Medical Physiology, 12th edition Copyright © 2011 by Saunders imprint of Elsevier, Inc. All rights reserved.  Hormone Classification - Steroid hormones Example - cortisol, aldosterone, oestrogen, progesterone etc. Synthesised from cholesterol Produced in adrenal cortex and gonad (testes and ovaries) Are not stored in vesicles Rapidly diffuse out of cell once synthesised (due to high lipid solubility) Require carrier proteins in blood (due to low water solubility) Steroid hormones enter target cells primarily by passive diffusion Steroid hormones:  Summary Hormones are chemical messengers that are produced in glands , released into the blood and act on distant target tissues Hormones belong to 3 chemical classes depending on how they are synthesized Amino acid derived hormones are small and include hydrophilic catecholamines & dopamine and hydrophobic thyroid hormones. Peptide/protein hormones are hydrophilic , stored in vesicles ,released on stimulus , soluble in plasma Steroid hormones are small, hydrophobic, not stored in vesicles, poorly soluble in plasma, transported by binding globulins and slowly metabolized.  THE HYPOTHALAMUS Hypothalamus Concerned with homeostasis Located in Brain below thalamus Sphenoid bone at base of brain Funnel-shaped stalk infundibulum connects hypothalamus to pituitary Pituitary Anterior and posterior Liberins & Statins – regulate pituitary hormone secretion Anatomical and functional relationship between the hypothalamus and the pituitary. The hypothalamus is anatomically and functionally linked with the anterior and posterior pituitary. They are closely related because of the portal system of blood supply. The superior, medial, and inferior hypophyseal arteries provide arterial blood supply to the median eminence and the pituitary. Magnocellular neurons of the supraoptic and paraventricular nuclei have long axons that terminate in the posterior pituitary. The axons of parvicellular neurons terminate in the median eminence where they release their neuropeptides. The long portal veins drain the median eminence, transporting the peptides from the primary capillary plexus to the secondary plexus that provides blood supply to the anterior pituitary. The release of hypothalamic neuropeptides is regulated by afferent signals from other brain regions, from visceral afferents by circulating levels of substrates and hormones. The sleep/awake state of the individual, the light variations, noise, fear, anxiety, and visual images all are examples of signals that are integrated by the hypothalamus and that are involved in the regulation of hypothalamic neuropeptide release and control of pituitary function. The hormones released from the anterior and posterior pituitary regulate vital body functions maintaining homeostasis. Citation: The Hypothalamus and Posterior Pituitary Gland, Molina PE. Endocrine Physiology, 5e; 2018. Available at: https://accessmedicine.mhmedical.com/content.aspx?bookid=2343&sectionid=183488081 Accessed: July 06, 2019 Copyright © 2019 McGraw-Hill Education. All rights reserved  Hypothalamus – Secretes Liberins Thyrotropin-releasing hormone (TRH) – stimulates secretion of TSH Corticotropin-releasing hormone (CRH) – stimulates secretion of ACTH Gonadotropin-releasing hormone (GnRH) – stimulates secretion of FSH & LH Growth hormone-releasing hormone (GHRH) – stimulates secretion of GH Hypothalamus – Secretes Statins Statins – Inhibiting Factors or Inhibiting Hormones Inhibit hormone secretion Growth hormone inhibiting hormone or GHIH (Somatostatin) Prolactin inhibiting hormone Pituitary Gland - Hypophysis Has two major lobes = anterior pituitary & posterior pituitary Anterior Pituitary (adenohypophysis) vascular connection (blood vessel) Posterior Pituitary (neurohypophysis) David W. Cooke, Sara A. DiVall and Sally Radovick Extension of neural components of hypothalamus Williams Textbook of Endocrinology, 25, 937- 1022.e41 Clinical key Contain axons that end next capillaries neural connection POSTERIOR PITUITARY Posterior Pituitary is a downgrowth of hypothalamic neural tissue neural connection with the hypothalamus (hypothalamic-hypophyseal tract) Produced - cell bodies of neurosecretory cells Packaged in vesicles/Transported through axons Stored in axon terminals that lie in posterior pituitary Stimulation of neurosecretory cells = triggers release of oxytocin / ADH from axon terminals Hormone released into capillaries within posterior pituitary  Anti-diuretic hormone (ADH) Secreted by posterior pituitary (synthesized in hypothalamus) Stimulus – ↑ECF osmolarity/ dehydration Target – Renal tubules Effects: main effect = water reabsorption  Oxytocin Small peptide synthesized in hypothalamus Secreted from posterior pituitary Oxytocin stimulates milk ejection Oxytocin stimulates uterine contraction during labour and return to normal size after birth Stimulated by suckling and birth ANTERIOR PITUITARY Anterior pituitary (adenohypophysis) is an outpocketing of the oral mucosa no direct neural contact with the hypothalamus (portal vein) Six hormones: GH, TSH, ACTH, FSH, LH, and PRL Regulate activity of other endocrine glands The tropic hormones that are released are: Thyroid-stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH) Follicle-stimulating hormone (FSH) Luteinizing hormone (LH) Anterior pituitary (adenohypophysis) Hypothalamic trophic hormones Stimulates the pituitary gland Control: The hypothalamus sends a chemical stimulus to the anterior pituitary Releasing hormones from the hypothalamus stimulate release of tropic Releasing hormones (PRH, TRH, CRH, GHRH, GnRH) stimulate the synthesis and release of anterior pituitary hormones hormones from the anterior pituitary that act on various target organs. Inhibiting hormones (PIH, GHIH) shut off the synthesis and release of anterior pituitary hormones Anterior pituitary cells 5 major cell types in anterior pituitary & hormones they secrete: Somatotropes – GH Thyrotropes – TSH Corticotropes – ACTH Gonadotropes – LH & FSH Lactotropes – prolactin Regulation of hormone release Negative/positive Feedback Overview Hypothalamic-pituitary target organ axes Hypothalamic ↑ TRH ↑CRH ↑ GnRH ↑ GHRH ↓dopamine hormone ↓somato- ↑ADH ↓somato- PIH statin statin (GHIH) THREE HORMONE SEQUENCE Anterior pituitary TSH ACTH FSH GH Prolactin hormone LH Target organ Thyroid Adrenal Ovary / Liver / breasts cortex testes tissue Target organ T3/T4 Cortisol Estradiol IGF-1 hormone progesterone Testosterone