Hypothalamus and Pituitary Gland PDF

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Endocrine Lecture 2,3 - Hypothalamus and pituitary gland and Endocrine glands and their disorders (James Wickham)...

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Hypothalamus and Pituitary Gland Describe the anatomy of the hypothalamus & pituitary gland & their relationship to each other The hypothalamus is the major control centre of endocrine system and ANS. It is shaped like a flattened funnel and forms the floor and walls of the 3rd ventricle of the brain. The pituitary gland is suspended from the floor of the hypothalamus by a stalk (infundibulum) and housed in a depression of the sphenoid bone (sella turcica). It is separated into two regions; anterior and posterior pituitary gland. The hypothalamus is situated superior to the pituitary gland. The pituitary gland is connected to hypothalamus by a stalk (infundibulum) which contains a blood portal system linkage between anterior pituitary and hypothalamus and a neural linkage (down growth of brain) between posterior pituitary and hypothalamus. List hormones produced by the hypothalamus, list target organ/s or tissue & state principal effects Hormone Target organs Principal effects Thyrotropin-releasing Regulates anterior pituitary Promotes secretion of hormone (TRH) thyroid-stimulating hormone (TSH) and prolactin (PRL) Corticotropin-releasing Regulates anterior pituitary Promotes secretion of hormone (CRH) adrenocorticotropic hormone (ACTH) Gonadotropin-releasing Regulates anterior pituitary Promotes secretion of hormone (GnRH) follicle-stimulating hormone (FSH) and luteinizing hormone (LH) Growth hormone-releasing Regulates anterior pituitary Promotes secretion of hormone (GHRH) growth hormone (GH) Inhibits secretion of Prolactin-releasing hormone Regulates anterior pituitary (PIH) prolactin Somatostatin Regulates anterior pituitary Inhibits secretion of growth hormone (GH) and thyroidstimulating hormone (TSH) Oxytocin (OT) Stored and released by Labor contractions, milk posterior pituitary release Antidiuretic hormone Stored and released by Water retention (ADH) posterior pituitary The first six hormones either stimulate or inhibit the anterior pituitary cells through the hypophyseal portal system to secrete or suppress other hormones found in the anterior pituitary gland. Oxytocin and ADH move down from the hypothalamus to the posterior pituitary gland via nerve fibres and are stored in the nerve endings of axons until a nerve signal coming down from these axons triggers their release. If a hormone has an RH at end, it’s from hypothalamus. List hormones produced by the anterior pituitary gland, list target organ/s or tissue & state principal effects Hormone Target organ/tissue Principal effects Follicle-stimulating Ovaries/testes F: growth of ovarian

hormone (FSH)

Luteinizing hormone (LH)

Ovaries/testes

Thyroid-stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH) Prolactin (PRL) Growth hormone (GH)

Thyroid gland Adrenal cortex Mammary glands Liver, bone cartilage, muscle, fat

follicles and secretion of estrogen M: sperm production F: ovulation, maintenance of corpus luteum M: testosterone secretion Growth of thyroid, secretion of thyroid hormone Growth of adrenal cortex, secretion of glucocorticoids Milk synthesis Widespread tissue growth, especially in stated tissues

List hormones stored by the posterior pituitary gland, list target organ/s or tissue & state principal effects Hormone Target organ/tissue Principal effects Antidiuretic hormone Kidneys Water retention (ADH)

Oxytocin (OT)

Uterus, mammary glands

Labor contractions, milk release; possibly involved in ejaculation, sperm transport, sexual affection, and mother-infant bonding

Explain how pituitary secretion is controlled by the hypothalamus and target organs and exemplify using multiple examples The control of pituitary secretion is regulated by hypothalamus, other brain centres and feedback from target organs. Feedback from peripheral target organ can be negative or positive and hence inhibit or enhance secretion respectively of hormones from hypothalamus and pituitary gland. Hypothalamic control enables the brain to monitor conditions within and outside the body and to stimulate or inhibit the release of pituitary hormones appropriately. For example; Stress -> CRH (Corticotropic releasing hormone)) -> Adrenocorticotropic hormone secretion from anterior pit gland -> Secretion of cortisol from adrenal gland and subsequent mobilization of materials needed for tissue repair. Neuroendocrine reflex – Hormone release from posterior pituitary gland in response to nervous system signals like positive feedback cycle with oxytocin and childbirth. Suckling infant -> Stimulates nerve ending in nipple -> Hypothalamus -> Posterior lobe -> Oxytocin -> Milk ejection. Negative feedback inhibition of the pituitary gland by the thyroid gland (steps 5,6): 1-The hypothalamus secretes thyrotropinreleasing hormone (TRH). 2-TRH stimulates the anterior pituitary to secrete thyroidstimulating hormone (TSH). 3-TSH stimulates the thyroid gland to secrete thyroid hormone (TH). 4-TH stimulates the metabolism of most cells throughout the body. 5-Th also inhibits release of TSH by pituitary. 6-To a lesser extent, TH also inhibits release of TRH by hypothalamus.

Describe the effects of growth hormone Growth hormone is not targeted to just one or a few organs like other pituitary hormones, but has widespread effects on the body especially on cartilage, bone, liver, muscle and fat. GH induces liver to produce growth stimulants called insulin like growth factors (IGF-I and IGF-

II) or somatomedins which stimulate target cells in diverse tissues. One effect of IGF is to prolong the action of GH. The mechanism of GH-IGF include;  Protein synthesis – is increased which boosts transcription of DNA, production of mRNA, amino acid uptake into cells, suppresses protein catabolism.  Lipid metabolism – is increased, GH stimulated adipocytes to catabolize fat and release fatty acids and glycerol into blood (protein-sparing effect) which provides energy for growing tissues.  Carbohydrate metabolism – GH also has a glucose-sparing effect, stimulating glucose synthesis by liver.  Electrolyte balance – GH promotes Na+, K+, and Cl- retention in kidneys and enhances Ca2+ absorption by the small intestine and makes these electrolytes available to the growing tissues. Most prominent effects of GH happen during childbirth and adolescence. IGF-1 accelerates bone growth at the epiphyseal plates, in adulthood it stimulates osteoblast activity and the appositional growth of bone (continues to influence bone thickening and remodeling). Describe the effects of growth hormone hypo & hypersecretion Hypersecretion (increase) of Growth hormone in adults causes acromegaly (thickening of bone and soft tissues especially visible in hands, face, and feet). Hypersecretion of GH in childhood or adolescence cause gigantism (excessive growth of bones, very tall but otherwise normal). Hyposecretion (decrease) of GH in childhood produces pituitary dwarfism as epiphyseal plates close early. Endocrine Glands and Disorders Pineal gland; state its location, list the hormone/s it produces and their effects The pineal gland has a pine cone shape and is attached to roof of 3rd ventricle of the brain, beneath the posterior end of the corpus callosum. It produces the hormone melatonin which contributes to the setting of the body’s biological clock hence sleep/wake cycle. Melatonin may influence mood and sexual maturation, regulates reproduction and seasonal breeding in most animals but the role in human reproduction is unclear. Thyroid gland; state its location, describe its structure, list hormones produced & their effects & describe its disorders The thyroid gland is the largest adult gland to have a purely endocrine function. It lies adjacent to the trachea, immediately below the larynx and is shaped like a butterfly; wraps around trachea with 2 winglike lobes usually joins inferiorly by a narrow bridge of tissue (isthmus). It microscopically consists of sacs called thyroid follicles with each lined by follicular cells (Which secrete T3 AND T4) that surround a protein rich colloid. Between the follicles is the parafollicular C cell (which secrete calcitonin). Thyroid gland secretes 3 hormones; thyroxine (T4), triiodothyronine (T3) and calcitonin. T4 and T3 targets most tissues and has the effect of elevating metabolic rate and heat production. Calcitonin targets bone and stimulates bone deposition, mainly in children.

Thyroid gland disorders include; Hypothyroidism - can be caused by lack of iodine, since iodine is needed to make hormone and causes stunted bone growth (dwarfism), severe mental retardation, thickened facial features (characteristic round face, thick nose, large tongue) and lethargy if present at birth (Congenital). It can also decrease metabolic rate (low energy), lethargy, weight gain, low body temperature, sensitivity to cold and pronounced facial edema producing puffy face if occurring in adults. This disorder can be treated with an oral thyroid hormone. Goitre – is the enlargement of thyroid gland due to excessive stimulation of gland. Iodine deficiency: Insufficient T3/T4 produced so decreased negative feedback to TRH/TSH stimulate growth in gland. No thyroid hormone in the circulation to feedback to brain to not produce more thyroid hormone, so there is a stimulation still being produced to release hormones. High levels of TRH/TSH also stimulates growth of glands. This disorder can be treated by antithyroid agent and hormone medication or removal of thyroid gland. Parathyroid gland; state its location, describe its structure, list hormones produced & their effects & describe its disorders Parathyroid glands are ovoid glands (usually 4 in number) partially embedded in the posterior surface of the thyroid. They are separated from the thyroid follicles by a thin fibrous capsule and adipose tissue. Parathyroid gland secretes parathyroid hormone (PTH) which regulates blood calcium levels. Unlike the thyroid gland, the parathyroids are not regulated by the pituitary, but directly monitor blood composition and secrete PTH when the calcium levels dip too low. PTH raises calcium level by stimulating calcium reabsorption from the bones and reducing calcium loss in the urine. Parathyroid gland disorders include; Hypoparathyroidism - leads to deficiency of blood calcium which causes neurons and muscle fibres to depolarise and produce action potentials spontaneously leading to tetany of skeletal muscle, and can be caused by accidental removal during thyroid surgery.

Hyperparathyroidism - Leads to bones becoming soft, deformed and fragile because the hormone secreted is the parathyroid hormones which stimulate increase of blood calcium levels. The calcium is taken from our bones resulting in soft fragile bones. Lots of calcium in

blood that gets into the urine which is calcium urea and phosphate also in the urine. They combine and create kidney stones. Adrenal glands; state their location, describe its structure, list hormones produced & their effects & describe its disorders The adrenal glands sit like a cap on the superior pole of each kidney. They are retroperitoneal, lying outside of the peritoneal cavity between the peritoneum and posterior body wall. It consists of two discrete regions; a smaller inner core called the adrenal medulla and a thicker outer region called the adrenal cortex. The adrenal cortex is further divided into 3 zones; Zona Glomerulosa (most superficial), Zona Fasciculata (Middle zone), Zona Reticularis (Deepest). The inner region of the adrenal glands, the adrenal medulla, consists of modified postganglionic neurons (chromaffin cells) of the sympathetic NS that secrete hormones instead of neurotransmitters. Hormones produced by adrenal medulla include adrenaline (epinephrine) & noradrenaline (norepinephrine), their effects are generally similar to results of mass activation of sympathetic nerves, but longer lasting. The effects include; increase cardiac output, dilation of blood vessels in heart, lungs, brain and muscle, Vasoconstriction in vessels of the skin and viscera, Increase blood glucose (from glycogen stores), Increase respiratory and metabolism, Increase fatty acid mobilisation from adipose tissue (basically augments the fight or flight response). The adrenal cortex surrounds the medulla on all sides and produces more than 25 steroid hormones known collectively as corticosteroids or corticoids. Hormones produced by each region include; Mineralocorticoids (zona glomerulosa) – mainly aldosterone, which have effects such as; maintaining extracellular fluid volume (blood pressure) by regulating salt & water balance, and stimulate kidneys to retain NaCl & water while excreting K+ in urine. Glucocorticoids (zona fasciculata) – mainly cortisol which has a variety of effects; stimulated by ACTH, glucocorticoids stimulate fat and protein catabolism, stimulate gluconeogenesis, increase blood glucose and fatty acids levels in the bloodstream, glucose sparing effect, suppress immune response with long-term secretion, anti-inflammatory effect (medical use), and generally helps the body adapt to stress. Gonadocorticoids (zona reticularis) – weak androgens and cortisol. Weak androgens such as dehydroepiandrosterone (DHEA) & some estradiol supplement the functions of the sex steroids produced by the gonads. Adrenal cortex disorders include; Addison’s disease - a hyposecretion of cortisol and aldosterone, Symptom’s incudes hypoglycaemia, mental lethargy, weight loss and muscle weakness (decreased cortisol). Loss of aldosterone leads to Na+/K+ imbalance, dehydration, hypotension and even cardiac arrest. The skin takes on a bronzed appearance often mistaken from a suntan. This disorder can be controlled by corticosteroids and increasing sodium in the diet. Cushing’s disease – a hypersecretion of cortisol with symptoms including protein catabolism, redistribution of body fat with distended abdominal and spindly arms and legs, a round moon face, a buffalo hump on the back, and hyperglycaemia. This disorder may be caused by over secretion of ACTH or a tumor of the adrenal gland. The pancreatic islets; state their location, describe their structure, list hormones produced & their effects & describe its disorders The pancreatic islets (islets of Langerhans) are the endocrine portions of the pancreases which is located in the curve of the duodenum. Endocrine islets of Langerhans consist of three different cell types that each produce and secrete a hormone; Alpha cells secrete glucagon- raise blood sugar,

Beta cells secrete Insulin lower blood sugar, Delta cells secret somatostatin Contain endocrine and exocrine, so have ducts and hormones. Insulin – (anabolic hormone) is the hormone of nutrient abundance, secreted during and immediately following a meal when blood nutrient levels are rising. Insulin increases glucose uptake and synthesis of energy stores (glycogen and fat), stimulates amino acid uptake & protein synthesis. Glucagon – is secreted between meals when blood glucose concentration falls below 100mg/dL. Glucagon exerts 2 actions on the liver; glycogenolysis which is the breakdown of glycogen into glucose and gluconeogenesis which is the synthesis of glucose from fats and proteins. These effects lead to the release of glucose into circulation by using energy reserves. Somatostatin – growth hormone-inhibiting hormone, works with amylin to limit the secretion of stomach acid. Also inhibits the secretion of insulin and glucagon and slows absorption from GIT. Disorders in pancreatic islet include; Type 1 diabetes – the absolute deficiency of insulin, usually genetic happens from birth. Usually juvenile onset, high blood glucose (hyperglycemia) and sugar in urine (glucosuria) and increased breakdown of fat reserves leads to acidosis. It is treated with exogenous insulin. Type 2 diabetes – more common than type one, happens when insulin is not working properly (usually sufficient or excess insulin) and result in high blood glucose concentration and decrease in target cell receptors for insulin. This condition is usually controlled by healthy weight, diet and exercise. The gonads; state their location, describe their structure, list hormones produced and their effects The gonads include ovaries and testes and are both endocrine and exocrine. The ovaries are paired oval bodies located in the female pelvic cavity, consisting of follicles which surround egg cells. They produce hormones; estradiol (main estrogen) targets many tissues which regulates female reproductive cycle and stimulates secondary sex characteristics, progesterone targets uterus and mammary glands which increases thickness of endometrium preparing for lactation also regulates menstrual cycle, and inhibin targets many tissues and inhibits FSH secretion. The testes are paired oval glands in each scrotum consisting of microscopic seminiferous tubules that produce sperm with interstitial cells (Leydig cells) nestled between them that produce testosterone. Testosterone targets many tissues and stimulates fetal and adolescent reproductive development, musculoskeletal growth, sperm production, and libido. Testes also secrete inhibin targeting many tissues which inhibits FSH secretion....