Endocrine System & Hypothalamus PDF

Physiology Endocrine System Chapter 17 | Hypothalamic Regulation of Hormonal Functions Endocrine System | Hypothalamic Regulation Objectives of this chapter : Introduction 3 Relation To The Pituitary Gland 9 Thirst 28 Control Of Posterior Pituitary Secretion 22 Vasopressin & Oxytocin 23 Effects of V...

Physiology Endocrine System Chapter 17 | Hypothalamic Regulation of Hormonal Functions Endocrine System | Hypothalamic Regulation Objectives of this chapter : Introduction 3 Relation To The Pituitary Gland 9 Thirst 28 Control Of Posterior Pituitary Secretion 22 Vasopressin & Oxytocin 23 Effects of Vasopressin 32 Effects of Oxytocin 35 Control of anterior pituitary secretion 37 Chapter Summary 46 Endocrine System | Hypothalamic Regulation Introduction : Many of the complex autonomic mechanisms that maintain the chemical constancy and temperature of the internal environment are integrated in the hypothalamus. The hypothalamus also functions with the limbic system as a unit that regulates emotional and instinctual behavior. Endocrine System | Hypothalamic Regulation Relation To The Pituitary Gland : There are neural connections between the hypothalamus and the posterior lobe of the pituitary gland and vascular connections between the hypothalamus and the anterior lobe. Embryologically, the posterior pituitary arises as an evagination of the floor of the third ventricle. It is made up in large part of the endings of axons that arise from cell bodies in the supraoptic and paraventricular nuclei and pass to the posterior pituitary via the hypothalamohypophysial tract. Endocrine System | Hypothalamic Regulation Human hypothalamus, with a superimposed diagrammatic representation of the portal hypophysial vessels. Endocrine System | Hypothalamic Regulation Secretion of hypothalamic hormones. Endocrine System | Hypothalamic Regulation Most of the supraoptic fibers end in the posterior lobe itself, whereas some of the paraventricular fibers end in the median eminence. The anterior and intermediate lobes of the pituitary arise in the embryo from the Rathke pouch, an evagination from the roof of the pharynx. Sympathetic nerve fibers reach the anterior lobe from its capsule, and parasympathetic fibers reach it from the petrosal nerves, but few, if any, nerve fibers pass to it from the hypothalamus. Endocrine System | Hypothalamic Regulation Endocrine System | Hypothalamic Regulation However, the portal hypophysial vessels form a direct vascular link between the hypothalamus and the anterior pituitary. Arterial twigs from the carotid arteries and circle of Willis form a network of fenestrated capillaries called the primary plexus on the ventral surface of the hypothalamus. Endocrine System | Hypothalamic Regulation Endocrine System | Hypothalamic Regulation Capillary loops also penetrate the median eminence. The capillaries drain into the sinusoidal portal hypophysial vessels that carry blood down the pituitary stalk to the capillaries of the anterior pituitary. This system begins and ends in capillaries without going through the heart and is therefore a true portal system. Endocrine System | Hypothalamic Regulation In birds and some mammals, including humans, there is no other anterior hypophysial arterial supply other than capsular vessels and anastomotic connections from the capillaries of the posterior pituitary. The median eminence is generally defined as the portion of the ventral hypothalamus from which the portal vessels arise. This region is outside the blood–brain barrier. Endocrine System | Hypothalamic Regulation Thirst Endocrine System | Hypothalamic Regulation Thirst : Decreases in ECF volume also stimulate thirst by a pathway independent of that mediating thirst in response to increased plasma osmolality. Thus, hemorrhage causes increased drinking even if there is no change in the osmolality of the plasma. The effect of ECF volume depletion on thirst is mediated in part via the renin–angiotensin system. Endocrine System | Hypothalamic Regulation Diagrammatic representation of the way in which changes in plasma osmolality and changes in ECF volume affect thirst by separate pathways. Endocrine System | Hypothalamic Regulation Renin secretion is increased by hypovolemia and results in an increase in circulating angiotensin II. The angiotensin II acts on the subfornical organ, a specialized receptor area in the diencephalon, to stimulate the neural areas concerned with thirst. Endocrine System | Hypothalamic Regulation Some evidences suggest that it acts on the organum vasculosum of the lamina terminalis (OVLT) as well. These areas are highly permeable and are two of the circumventricular organs located outside the blood–brain barrier. However, drugs that block the action of angiotensin II do not completely block the thirst response to hypovolemia, and it appears that the baroreceptors in the heart and blood vessels are also involved. Endocrine System | Hypothalamic Regulation The intake of liquids is increased during eating (prandial drinking). The increase has been called a learned or habit response, but it has not been investigated in detail. One factor is an increase in plasma osmolality that occurs as food is absorbed. Another may be an action of one or more gastrointestinal hormones on the hypothalamus. Endocrine System | Hypothalamic Regulation When the sensation of thirst is obtunded, either by direct damage to the diencephalon or by depressed or altered states of consciousness, patients stop drinking adequate amounts of fluid. Dehydration results if appropriate measures are not instituted to maintain water balance. If the protein intake is high, the products of protein metabolism cause an osmotic diuresis , and the amounts of water required to maintain hydration are large. Endocrine System | Hypothalamic Regulation Most cases of hypernatremia are actually due to simple dehydration in patients with psychoses or hypothalamic disease who do not or cannot increase their water intake when their thirst mechanism is stimulated. Lesions of the anterior communicating artery can also obtund thirst because branches of this artery supply the hypothalamic areas concerned with thirst. Control Of Posterior Pituitary Secretion Endocrine System | Hypothalamic Regulation Control Of Posterior Pituitary Secretion: Vasopressin & Oxytocin In most mammals, the hormones secreted by the posterior pituitary gland are arginine vasopressin (AVP) and oxytocin. In hippopotami and most pigs, arginine in the vasopressin molecule is replaced by lysine to form lysine vasopressin. Endocrine System | Hypothalamic Regulation The posterior pituitaries of some species of pigs and marsupials contain a mixture of arginine and lysine vasopressin. The posterior lobe hormones are nanopeptides with a disulfide ring at one end. Endocrine System | Hypothalamic Regulation Arginine vasopressin and oxytocin. Endocrine System | Hypothalamic Regulation Biosynthesis, Intraneuronal Transport & Secretion: The hormones of the posterior pituitary gland are synthesized in the cell bodies of the magnocellular neurons in the supraoptic and paraventricular nuclei and transported down the axons of these neurons to their endings in the posterior lobe, where they are secreted in response to electrical activity in the endings. A Oxytocin and vasopressin are typical neural hormones, that is, hormones secreted into the circulation by nerve cells. Endocrine System | Hypothalamic Regulation Endocrine System | Hypothalamic Regulation Endocrine System | Hypothalamic Regulation Vasopressin & Oxytocin In Other Locations : Vasopressin-secreting neurons are found in the suprachiasmatic nuclei, and vasopressin and oxytocin are also found in the endings of neurons that project from the paraventricular nuclei to the brainstem and spinal cord. These neurons appear to be involved in cardiovascular control. In addition, vasopressin and oxytocin are synthesized in the gonads and the adrenal cortex, and oxytocin is present in the thymus. The functions of the peptides in these organs are unsettled. Endocrine System | Hypothalamic Regulation Vasopressin Receptors: There are at least three kinds of vasopressin receptors: V1A, V1B, and V2. All are G-protein–coupled. The V1A and V1B receptors act through phosphatidylinositol hydrolysis to increase intracellular Ca2+ concentrations. The V2 receptors act through Gs to increase cyclic adenosine monophosphate levels. Endocrine System | Hypothalamic Regulation Endocrine System | Hypothalamic Regulation Effects of Vasopressin : Because one of its principal physiologic effects is the retention of water by the kidney, vasopressin is often called the antidiuretic hormone (ADH). It increases the permeability of the collecting ducts of the kidney so that water enters the hypertonic interstitium of the renal pyramids. The urine becomes concentrated and its volume decreases. Endocrine System | Hypothalamic Regulation The overall effect is therefore retention of water in excess of solute; consequently, the effective osmotic pressure of the body fluids is decreased. In the absence of vasopressin, the urine is hypotonic to plasma, urine volume is increased, and there is a net water loss. Consequently, the osmolality of the body fluid rises. Endocrine System | Hypothalamic Regulation Endocrine System | Hypothalamic Regulation Effects of Oxytocin : In humans, oxytocin acts primarily on the breasts and uterus, though it appears to be involved in luteolysis as well. A G-protein–coupled oxytocin receptor has been identified in human myometrium, and a similar or identical receptor is found in mammary tissue and the ovary. It triggers increases in intracellular Ca2+ levels. Endocrine System | Hypothalamic Regulation Anterior Pituitary Hormones Endocrine System | Hypothalamic Regulation Control of anterior pituitary secretion: Anterior Pituitary Hormones The anterior pituitary secretes six hormones: 1) Adrenocorticotropic hormone (corticotropin, ACTH) 2) Thyroid-stimulating hormone (thyrotropin, TSH) 3) Growth hormone 4) Follicle stimulating hormone (FSH) 5) Luteinizing Hormone (LH) 6) Prolactin (PRL). An additional polypeptide, β-lipotropin (β-LPH), is secreted with ACTH, but its physiologic role is unknown. Endocrine System | Hypothalamic Regulation Endocrine System | Hypothalamic Regulation Nature Of Hypothalamic Control : Anterior pituitary secretion is controlled by chemical agents carried in the portal hypophysial vessels from the hypothalamus to the pituitary. These substances used to be called releasing and inhibiting factors, but now they are commonly called hypophysiotropic hormones. The latter term seems appropriate since they are secreted into the bloodstream and act at a distance from their site of origin. Small amounts escape into the general circulation, but they are at their highest concentration in portal hypophysial blood. Endocrine System | Hypothalamic Regulation Hypophysiotropic Hormones : There are six established hypothalamic releasing and inhibiting hormones: 1. Corticotropin-releasing hormone (CRH). 2. Thyrotropin-releasing hormone (TRH). 3. Growth hormone–releasing hormone (GRH). 4. Growth hormone– inhibiting hormone (GIH, now generally called somatostatin). 5. Luteinizing hormone–releasing hormone (LHRH, now generally known as gonadotropin-releasing hormone [GnRH]). 6. Prolactin-inhibiting hormone (PIH). Endocrine System | Hypothalamic Regulation Endocrine System | Hypothalamic Regulation In addition, hypothalamic extracts contain prolactin-releasing activity, and a prolactin-releasing hormone (PRH) has been postulated to exist. TRH, VIP, and several other polypeptides found in the hypothalamus stimulate prolactin secretion. Receptors for most of the hypophysiotropic hormones are coupled to G-proteins. Endocrine System | Hypothalamic Regulation Endocrine System | Hypothalamic Regulation There are two human CRH receptors: hCRH-RI and hCRH-RII. The physiologic role of hCRH-RII is unsettled, though it is found in many parts of the brain. In addition, a CRH-binding protein in the peripheral circulation inactivates CRH. It is also found in the cytoplasm of corticotropes in the anterior pituitary, and in this location it might play a role in receptor internalization. However, the exact physiologic role of this protein is unknown. Other hypophysiotropic hormones do not have known binding proteins. Chapter Summary Endocrine System | Hypothalamic Regulation Chapter Summary : Neural connections run between the hypothalamus and the posterior lobe of the pituitary gland, and vascular connections between the hypothalamus and the anterior lobe of the pituitary. In most mammals, the hormones secreted by the posterior pituitary gland are vasopressin and oxytocin. Vasopressin increases the permeability of the collecting ducts of the kidney to water, thus concentrating the urine. Endocrine System | Hypothalamic Regulation Oxytocin acts on the breasts (lactation) and the uterus (contraction). The anterior pituitary secretes six hormones: adrenocorticotropic hormone (corticotropin, ACTH), thyroid stimulating hormone (thyrotropin, TSH), growth hormone, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL). Other complex autonomic mechanisms that maintain the chemical constancy and temperature of the internal environment are integrated in the hypothalamus.