Hypothalamus and Pituitary Flashcards
The pituitary gland is located right below the optic chiasm in the sella of the brain
How is the pituitary gland connected to the hypothalamus?
the infundibulum
What is the posterior lobe (neurohypophysis) of the piutitary gland derived from?
neuroectoderm (left)Right: adenohypophysis (derived from the oral ectoderm (Rathke pouch))
Hormones of the posterior lobe?
ADH and oxytocin
How is the hypothalmus connected to the posterior pituitary? What are Herring bodies?
The posterior pituitary is actually a collection of nerve axons whose cell bodies are in the supraoptic and paraventricular nuclei of thehypothalamus that terminate near capillaries.Thus ADH and oxyotcin are actually neuropeptides, that is, they are peptides released from (magnocellular)neuronsHerring Bodies:axonal swellings packed with hormonal granules from the hypothalamic nucleiPitucytes:supportive glial cells
Although both posterior hormones are synthesized in both nuclei of the hypothalamus, ADH is primarily associated with the _____ nuclei and oxyotcin is primarily associated with _______ nuclei.
ADH-supraopticOxytocin- paraventricular
Describe the transmission ans secretionof ADH and oxytocin once synthesized.
The hormones are transported down the axons of the hypothalmic-hypophyseal tractin neurosecretory vesicles and stored in bulbous nerve terminalsin the posterior pituitary. Once stimulated, the neurosecretory vesicles are exocytosed and the secreted hormones enter nearby fenestrated capillariesoriginating from the capillary plexus of the infundibular processNOTE: Thecapillary plexus of the infundibular process originates from the inferiorhypophysial artery(from the internal carotid artery) and empties into the posteriorhypophysial veins. (to the internal jugular vein).NOTE: a small portion of the hypophysial veins connect to the anterior pituitary so traces of these hormones may be seen there
How are the posterior pituitary hormones transported in blood?
This venous blood enters circulation and Oxytocin is transported in free form along sideNeurophysin I and ADH is carried withNeurophysin II(these are peptides so no carriers)NOTE:The peptide precursor for ADH is prepropressophysinand the precursor for oxytocin is prepro-oxyphysin.In the Golgi, the signal peptides of each of these are cleaved to form prohormones, which are packaged into secretory vesicles to travel down the axon through the hypothalamic-hypophysial tract, during which the neurophysins are cleaved from the prohormones
Where does the anterior lobe of the pituitary originatefrom?
oral ectoderm (Rathke pouch)
What are the anterior pituitary hormones?
FSH, LH, ACTH, TSH, Prolactin, and Growth hormoneFLAT PiG
The hypothalamus and the anterior pituitary, unlike the posterior lobe, are related both neurallyAND in an endocrine fashion.How are they physically connected?
Directly by the hypothalamic-hypophysial portal blood vessels
Describe thehypothalamic-hypophysial portal blood vessels
Arterial blood is delivered to the hypothalamus via the superior hypophysial arterieswhich distribute the blood into a capillary network in the median eminence,called the primary capillaryplexus(thus, in this set of capillaries, hormones from the hypothalamus enter).These capillaries converge to form long hypophysial portal vessels,which travel down the infundibulum to the anterior pituitary. In the lobe,another set of capillaries, the secondary capillary plexusarises from the inferior hypophysial artery (this is where the hormones of the anterior pituitary enter) to be drained to the internal jugular veinNOTE: in contrast to most organs, the anterior pituitary receives primarily venous blood
How are hormones from thehypothalamus transported to the anterior pituitary?
Hypothalamic-releasing hormones are synthesized in the cell bodies of hypothalamic neurons in the ventral hypothalamus and travel down the axons of these neurons to the median eminence of the hypothalamus. Upon stimulation, the hormones are secreted into surrounding hypothalamic tissue and enter thenearly capillary plexus to be delivered to the anterior lobe
TSH is secreted by ____.FSH and LH aresecreted by ____.ACTH is secreted by ____.GH is secreted by _____.Prolactin is secreted by _____.
TSH- thyrotrophs (5%)FSH and LH- gonadotrophs (15%) (co-secretion)ACTH- corticotrophs (15%)GH- somatotrophs (20%)Prolactin- lactotrophs (15%) (higher in females and in pregnancy)NOTE: These hormones are synthesized in the anterior lobe in the standardpreprohormone, prohormone, and maturation process
The hormones of the anterior pituitary are organized into ‘families’ based on structural similarities. What are these families?
TSH, FSH, and LH- family IACTH- family II (found in the middle of the lobe)GH and prolactin- family III (acidophils) found in the peripheral of the anterior lobe (Peripheral hormones -especially GH- are more likely to be impacted by injury)
How are the posterior pituiatry hormones relased?
An action potential is transmitted from the cell body in the hypothalamus and the secretory vesicle, with the hormone and their respective neurophysin, is exocytosed to a nearby capillary when Ca2+ levels rise
What arethe main roles of oxytocin?
Produces milk ejection (aka ‘letdown’)from the lactating breast by stimulating contraction of myoepithelial cells lining the milk ductsPromotes uterine contraction- can be used to induce laborand reduce postpartum bleeding
What factors induce secretion of oxytocin from the posterior pituitary?
suckling; the sight, smell, and sound of an infant; orgasm;and dilation of the cervix
T or F. Oxytocin primarily utilizes negative feedback mechanisms
F. It is actually one hormone that works commonly via positive feedback
Why are women in labor so response to oxytocin but others are not?
receptors are upregulated via estrogen
What stimulates ADH release?
small increases in serum osmolarity causeslarger decreases in ECF volume (10+%)Angio IIPain and NauseaHypoglycemiaNicotine, Opiates
How does ADHwork?
1) binds to V2receptors on the principle cells of thedistal tubules of the CDwhich is coupled to a Gs protein to increase water reabsorption by inserting more aquaporin 2 channels in the luminal membranes.Thus, urine will be more concentrated2)Causes smooth muscle contraction by binding the V1 receptorwhich is coupled to a Gq proteinand acts via phospholipase C to cause increased BP
Inhibitors to ADH release?
Ethanola-adrenergic agonistsDecreased serum osmolarityANP
Describe how ADHis stimulated to be released via increased osmolarity?
Increases in serum osmolarity is sensed by osmoreceptors in the anterior hypothalamus.
Describe how ADHis stimulated to be released by depleted ECF volume
For ECF depletion to stimulate ADH release, there must be a 10+% drop, which is sensed by peripheral baroreceptors and is then transmitted via the vagus nerve to the hypothalamus
NOTE: Drops in ECF override osmolarity changes (i.e. if ECF is low and osmolarity is low, ADH will still be secreted)
What is central diabetes insipidus?
Failure of the posterior pituitary to secreteADH, causing serum osmolarity to rise and large volume of dilute urine to be produced
What is nephrogenic diabetes insipidus?
Failure of ADH to bind to the V2 receptor of the principle cells inthe collecting duct (or a defect in the Gs protein of adenylyl cyclase). This differs from central DI in that serum ADH will still be high even though serum osmolarity will be high and urine will be dilute
How is nephrogenic DI treated? Why?
thiazide diuretics work by inhibiting Na+ reabsorption in the easly distal tubule
How is SIADH treated?
demeclocycline or water restriction
Why would vasopressin be upregulated in heart failure?
circulating blood decreases to activate ADH (and additional renin release causes systemic vasoconstriction which further increasing afterload of the heart)
Normal microscopic appearance of anterior lobe hormones
Describe the TSH, FSH, and LH family of anterior pituitary hormones
These are all glycoproteinsconsisting ofidenticalalpha subunits and unique beta subunits (similarly, human chorionic gonadtropin of the placentashares these features)
Describe ACTH family derivation
All derived from pro-opomelanocortin (POMC) which is then manipulated to form ACTH, y- and b-lipotropin/endorphin, or melanocyte-stimulating hormone (MSH)
What is the effect of both POMC and ACTH containing some MSH activity?
Conditions that are associated with over-expression of these products (i.e. Addison disease) can produce skin pigmentation
Generation of ACTH is inhibited by what?
cortisol
Overview of GH production
Produced by somatotrophs when stimulated by GHRH and is important in body growth and metabolism. Secretion starts at childbirth and increases vastly at pubertyand then gradually decreases from that point in life until death and is only secreted during Stage III/IV Sleep (much more so in younger people)NOTE: Insulin like-growth factor I and thyroid hormones can act as inhibitors for release of GH andGHRH
Describe Prolactin regulation
Released from the anterior pituitary and is under tonic inhibition by dopaminebut when a stimulus is sensed, this inhibition is released due to neural input into the hypothalamus.
The release of prolactin is augmented by what?
oxytocin, AVP, and thyroid releasing hormone
What happens when the stimulus for prolactin goes away?
prolactin itself provides a negative feedback to produce more dopamine
