Flashcards in The Hypothalamus-Pituitary Complex Deck (59)
What does the HP axis regulate?
The function of the thryoid, adrenal and reproductive glands
Growth, lactation, milk secretion and water metabolism
Where is the pituitary?
In a pocket of bone at the base of the brain below the hypothalamus close to the medial eminence
Connected to hypothalamus and is connected by a stalk containing nerve fibres and blood vessels
What are the key features of the anterior and posterior pituitary?
Anterior - unique blood supply where releasing factors are secreted into from the hypothalamus
Posterior - blood portal system takes hormones away with direct hormonal secretion from the posterior - not synthesised there however
What is the key role of the posterior pituitary?
body fluid homeostasis and reproductive function
Where are hormones synthesised for secretion by the posterior pituitary?
Magnocellular neurons secrete directly into pituitary
Paraventricular nuclei - oxytocin
Supraoptic nuclei - ADH
What are the pre-prohormones of the posterior pituitary?
What are Herring Bodies?
Axonal swellings due to the storage of secretory granules
What are the main functions of ADH?
Maintenance of normal osmolarity of body fluids
Normal blood volume
What are the primary targets of ADH?
Cells lining the distal renal tubule
Principle cells of the collecting ducts in the kidney
How does ADH act on the signalling pathways?
Binds to V2Rs on basal side of renal cells
V2R is linked to Gs-cAMP-PKA pathway
Stimulates insertion of Aquaporin 2 to apical membrane
Enhances trans-epithlial flow of water from lumen to renal interstitium
What happens in the presence of ADH?
Urine flow decreases
Urine Osmolality increases
What is ADH released in response to?
increased EC fluid osmolality
decreased blood volume and pressure
How do osmoreceptive neurons respond to changes in osmolality?
Respond to changes by shrinking or swelling
Innervate magnocellular neurons of PVN and SON
Increased osmolality stimulates ADH releases
How is blood volume regulated?
Cardiovascular volume receptors in the atria
What is the function of oxytocin?
stimulates the contraction of uterine smooth muscle during labour
What is the positive feedback mechanism related to oxytocin?
Stretching of the cervix stimulates oxytocin release which in turn facilitates labour further as a neuroendocrine reflex
After birth, stretching of cervix lessens, breaking the cycle
How is the anterior pituitary connected to the hypothalamus?
hypophyseal portal circulation
What kind of hormones are produced in the anterior pituitary?
What hormones does the anterior pituitary produce?
How are hormones released from the pituitary?
paravicellular neurosecretory cells secrete releasing factors into the capillaries of the pituitary portal system at the median eminance
What are the releasing factors associated with the hormones released?
CRH - ACTH
TRH - TSH
GHRH - GH
Somatostatin - inhibits GH
GnRH - FSH, LH
PRH - Prolactin
Dopamine - inhibits prolactin
What percentage of cells are corticotrophs and what are their target organs?
15-20% - adrenal glands, adipocytes and melanocytes
What percentage of cells are thryotrophs and what are their target organs?
3-5% - thyroid gland
What percentage of cells are gonadotrophs and what are their target organs?
10-15% - Gonads
What percentage of cells are somatotrophs and what are their target organs?
40-50% - All tissues, liver
What percentage of cells are lactotrophs and what are their target organs?
10-15% - breasts, gonads
What is the key feature of the feedback regulation in the HP-axis?
the ability of hypothalamic releasing hormones to provide negative feedback on the system
What is TSH?
a glycoprotein heterodimer composed of an a and b subunit
Where does TSH bind?
thyroid epithelial cells with TSH receptors