Pituitary Gland and the Hypothalamus PDF

Pituitary Gland  Pituitary Gland and Its Relation to the Hypothalamus The pituitary gland, also called the hypophysis, is a small gland - about 1 centimeter in diameter and 0.5 to 1 gram in weight - that lies in the sella turcica, a bony cavity at the base of the brain, and is c...

Pituitary Gland  Pituitary Gland and Its Relation to the Hypothalamus The pituitary gland, also called the hypophysis, is a small gland - about 1 centimeter in diameter and 0.5 to 1 gram in weight - that lies in the sella turcica, a bony cavity at the base of the brain, and is connected to the hypothalamus by the pituitary (or hypophysial) stalk. Physiologically, the pituitary gland is divisible into two distinct portions: the anterior pituitary, also known as the adenohypophysis, and the posterior pituitary, also known as the neurohypophysis.  Six important peptide hormones plus several less important ones are secreted by the anterior pituitary, and two important peptide hormones are secreted by the posterior pituitary. The hormones of the anterior pituitary play major roles in the control of metabolic functions throughout the body Growth hormone: promotes growth of the entire body by affecting protein formation, cell multiplication, and cell differentiation Adrenocorticotropin (corticotropin): controls the secretion of some of the adrenocortical hormones, which affect the metabolism of glucose, proteins, and fats. Thyroid-stimulating hormone (thyrotropin) :controls the rate of secretion of thyroxine and triiodothyronine by the thyroid gland, and these hormones control the rates of most intracellular chemical reactions in the body. Prolactin: promotes mammary gland development and milk production. Two separate gonadotropic hormones, follicle stimulating hormone (FSH) and luteinizing hormone(LH) control growth of the ovaries and testes, as well as their hormonal and reproductive activities. The two hormones secreted by the posterior pituitary play other roles. Antidiuretic hormone (also called vasopressin) controls the rate of water excretion into the urine, thus helping to control the concentration of water in the body fluids. Oxytocin helps express milk from the glands of the breast to the nipples during suckling and helps in the delivery of the baby at the end of gestation.  Anterior Pituitary Gland Contains Several Different Cell Types That Synthesize and Secrete Hormones.  Usually, there is one cell type for each major hormone formed in the anterior pituitary gland. With special stains attached to high-affinity antibodies that bind with the distinctive hormones, at least five cell types can be differentiated 1. Somatotropes - human growth hormone (hGH) 2. Corticotropes - adrenocorticotropin (ACTH) 3. Thyrotropes - thyroid-stimulating hormone (TSH) 4. Gonadotropes - gonadotropic hormones, which include both luteinizing hormone (LH) and follicle stimulating hormone (FSH) 5. Lactotropes — prolactin (PRL) About 30 to 40 per cent of the anterior pituitary cells are somatotropes that secrete growth hormone, and about 20 per cent are corticotropes that secrete ACTH. Each of the other cell types accounts for only 3 to 5 per cent of the total; nevertheless, they secrete powerful hormones for controlling thyroid function, sexual functions, and milk secretion by the breasts. Posterior Pituitary Hormones Are Synthesized by Cell Bodies in the Hypothalamus. The bodies of the cells that secrete the posterior pituitary hormones are not located in the pituitary gland itself but are large neurons, called magnocellular neurons, located in the supraoptic and paraventricular nuclei of the hypothalamus. The hormones are then transported in the axoplasm of the neurons' nerve fibers passing from the hypothalamus to the posterior pituitary gland  Hypothalamus Controls Pituitary Secretion Almost all secretion by the pituitary is controlled by either hormonal or nervous signals from the hypothalamus. Indeed, when the pituitary gland is removed from its normal position beneath the hypothalamus and transplanted to some other part of the body, itsrates of secretion of the different hormones (except for prolactin) fall to very low levels. Secretion from the posterior pituitary is controlled by nerve signals that originate in the hypothalamus and terminate in the posterior pituitary. In contrast, secretion by the anterior pituitary is controlled by hormones called hypothalamic releasing and hypothalamic inhibitory hormones (or factors) secreted within the hypothalamus itself and then conducted,, to the anterior pituitary through minute blood vessels called hypothalamic-hypophysial portal vessels. In the anterior pituitary, these releasing and inhibitory hormones act on the glandular cells to control their secretion.. The hypothalamus receives signals from many sources in the nervous system. Thus, when a person is exposed to pain, a portion of the pain signal is transmitted into the hypothalamus. Likewise, when a person experiences some powerful depressing or exciting thought, a portion of the signal is transmitted into the hypothalamus. Hypothalamic releasing and inhibitory hormones Control anterior pituitary secretion 1. Thyrotropin-releasing hormone (TRH), which causes release of thyroid-stimulating hormone 2. Corticotropin-releasing hormone (CRH), which causes release of adrenocorticotropin 3. Growth hormone-releasing hormone (GHRH), which causes release of growth hormone, and growth hormone inhibitory hormone (GHIH), which inhibits release of growth hormone 4. Gonadotropin-releasing hormone (GnRH), which causes release of the two gonadotropic hormones, luteinizing hormone and follicle-stimulating hormone 5. Prolactin inhibitory hormone (PIH), which causes inhibition of prolactin secretion The hormones not under control of hypothalamus and pituitary gland are: 1. parathyriod H. 2. calcitonin H (thyroid). 3. Aldesterone H ( adrenal cortex). 4. insulin and glucagon H( pancrease). Protein in nature Similar to prolactin Rapidly metabolized in the liver  1-Effect on growth A- Effect on bone and cartilage (skeletal growth):  Imporatnat note the effect of growth hormone is mediated by somatomedin which is polypeptide secreted from the liver under effect of GH. It mediates the action of GH on the bone, and it’s called sulphation factor because it incoportae sulphate into the bone. Somatomedin C is the most common, which is called insulin like growth factor, because it has insulin like effect in contrast to GH it self. 1-Effect on growth GH stimulates growth of bone in length( at epiphysial cartilage before union of epiphysis) and thickness, this is done by stimulation of: A-widening of the epiphysial cartilage and lying down more bone matrix. B-mineralization of the bone: by stimulation of deposition of Ca and sulphate in bone.  B- soft tissue growth: It stimulate soft tissue growth secondary to it’s anabolic effect increase bulk of soft tissue.  2-Metabolic effect 1 On protein: - ( anabolic ) increase protein synthesis. - Increase the transport of amino acid across the cell membrane. - Positive nitrogen and phosphorus balance. - Stimulate erythropioesis.  2-Metabolic effect 2- on carbohydrate: (diabetogenic ) hyperglycemic due to: - Stimulation of gluconeogenesis, -Inhibits glucose uptake by the tissue (anti-insulin effect) -Stimulation of glucogenolysis by stimulation the alpha cells of pancreatic cell to release the glucagon.  2-Metabolic effect 3- on fat : -Ketogenic (lipolytic) due to: -Mobilization of the fatty acid causing lipaemia. -Stimulation of ketone body formation by the liver. - preferentially metabolized 4-on electrolytes: -Increase the GIT absorption of Ca, and decrease the Na and K excretion.  3- lactogenic effect Is similar in the structure and function to the prolactin , so it increases the milk formation  Control of GH secretion Hypothalamic control Feed back control GHRH: Sleep GH is ,stress, exercise, inhibited by: GHIH: hypoglycemia, sex Blood level of hyperglycemia, hormones,aragini GH cortisone, FFA e Blood level of somatomedin Excessive GH  1-gigantism(giantism) The condition results from over production of GH in the re adult life (before union of epiphysis). Characteristic feature: 1-over production of all bones( 2 meter long ) growth of bone is disproportional because of the effect of GH on the long bones of the limbs so: A- the span will be more than the height. B-the distance from symphysis pubis to sole is more than that from the crown to the symphysis. 2 -growth of soft tissue: A-viscero-megaly :spleen- cardiac. B-muscle in early stage are strong , later on , the muscle become weak(because muscle growth doesn’t go parallel to skeletal growth. Result to muscle weakness. 3-hypogonadism 4-hyperglycemia. 5- 4%of female develop lactation in absence of pregnancy. 6-if over production of growth hormone continuous in adult life signs of acromegaly develop.  acromegaly The condition result from overproduction of GH in the adult life (after union of epiphysis). Charactarestic feature: 1- overproduction of terminal portion of the skeleton. After union of epiphysis the effetct of GH on bone growth is: Disproportional growth of thickness of long bones especially small bones of hands and feet result in thick and broad hand(spade like hand) and feet. B-continued growth of membranous bones( that have no epiphysis to unite) as skull. Jaws. And vertebrae leading to: 1- skull (ape like face) Excessive development of supra orbital ridge. Enlargement of the nose may be doubled normal size. 2- mandibule :broad teeth separation thick forward protrusion. 3- vertebrea: enlargement kyphosis  Gigantisms Acromegaly closure of the Before union After union epiphysis Bones and soft tissue Over production of all Over production of the long bones terminal of the Viscromagaly : spleen skeleton -cardiac Same Gonads hypogondisms Same Glucose in blood Hyperglycemia Same DECREASE GH  Pituitary dwarfism Causes: 1 organic lesion or surgical removal of the pituitary. 2 inherited growth hormone deficiency (autosomal recessive gene). 3 secretion of inactive GH. 4 lack of organ to response to GH due to due to failure to release somatomedin(loran dwarfism) most important cause.  characters 1- symetrical retardation of growth: -Height doesn’t exceed 120cm. -There is rapid union of epiphysis, but ossification centers appears normal. -Dentition is not delayed 2- symmetrical retardation of growth of soft tissue, however, birth weight is normal. 3- mentally normal: but dwarf emotional unstable. 4- sexual activity : is normal.  Pituitary infantilism Dwarfism accompanied by hypogonadism. It’s due deficiency of both GH and gonadotropins. -The secondary sexual organ remain infantile. -The secondary sexual characters does not appear.