The pituitary gland (11) Flashcards
What is a hormone?
chemical messengers carried in the bloodstream to target organ
How are peptide hormones synthesised?
synthesised as prohormones- require further processing (e.g. cleavage) to be activated
How are peptide hormones stored?
stored in vesicles and only released in response to a signal (regulatory secretion)
How do peptide hormones work in terms of receptors?
bind to receptors on cell membrane and usually activate a secondary messenger system
How are steroid hormones synthesised?
made from precursor cholesterol in a series of reactions
How are steroid hormones stored?
released immediately (constitutive secretion)
How do steroid hormones work in terms of receptors?
bind to intracellular receptors- alter gene expression
Where is the pituitary gland located and what is its structure?
- pituitary gland hangs from infundibulum (stalk)
- sits in sella turcica of sphenoid bone
- anterior and posterior portions- posterior continuous with hypothalamus
- at front is optic chiasm
What are the 5 endocrine cell types in the anterior pituitary and what hormones do they produce?
- somatotrophs: produce growth hormone (somatotrophin)
- lactotrophs: produce prolactin
- thyrotrophs: produce thyroid stimulating hormone (TSH)
- gonadotrophs: produce luteinising hormone (LH) and follicle stimulating hormone (FSH)
- corticotrophs: produce adrenocorticotrophic hormone (ACTH)
How does the hypathalmo-pituitary portal system work?
- axon terminals of parvocellular neurons release hormones/factors (releasing/inhibitory) into median eminence- rich in blood vessels
- factors leak into portal system–> transported through blood stream into anterior pituitary
- factors bind to relevant anterior pituitary hormone cells (e.g. thyrotrophs)–> causes them to release their hormones into circulation
What hypothalamic factor regulates the production of prolactin?
dopamine- inhibiting, so less dopamine–> more prolactin
What 2 hypothalamic factors regulate the production of growth hormone?
growth hormone releasing hormone- releasing
AND
somatostatin- inhibiting
Where are the main target cells/receptors of growth hormone?
muscle tissue, bone, liver
Where are the main target cells of gonadotrophins (LH+FSH)?
ovaries and testes
Where are the main target cells of prolactin?
mammary glands
Where are the main target cells of corticotrophin?
adrenal gland- specifically adrenal cortex
Where are the main target cells of thyrotrophin?
thyroid gland
What happens when a pituitary tumour compresses the optic chiasm?
lose peripheral vision
‘bitemporal hemianopia’- common presentation
–> fibres from nasal retinae cross at the optic chiasm
–> tumour prevents transmission of sensory info from lateral visual fields to occipital lobe
What is the mechanism of milk production (a neuroendocrine reflex)?
- mechanical stimulation of nipple and surrounding area activates afferent pathways
- afferent signals integrated in the hypothalamus and inhibit dopamine release from dopaminergic neurons
- less dopamine in hypothalamic-pituitary portal system–> less inhibition of anterior pituitary lactotrophs
- more prolactin in plasma–> more milk secretion in mammary glands
What are the 2 ways in which growth hormone can stimulate growth?
GH binds to body tissues directly
BUT ALSO to GH receptors on liver–> causes production of insulin-like Growth Factor (IGF-1 and IGF-2)
so skeleton has IGF receptors and GH receptors
–> both leads to growth and development
What conditions are a result of too much growth hormone?
gigantism= children acromegaly= adults (can't get taller, but large jaw/prognathism, macroglossia, prominent nose, increased hand and foot size)
What are the posterior pituitary hormones?
arginine vasopressin (AVP/ADH) and oxytocin
How does regulation of the posterior pituitary gland work?
- 2 sets of distinct populations of hypothalamic neurosecretory cells producing AVP and oxytocin
- excitation of neuronal populations stimulates release of hormones–> transported into posterior pituitary
- diffuse into blood capillaries and leave via blood
What is the physiological action of vasopressin?
/Anti-Diuretic Hormone
*diuresis= production of urine
–> binds to V2 receptor in collecting duct–> stimulates intracellular cascade–> promotes Aquaporin-2 movement to membrane- then inserted–> water flows along conc. gradient (from urine) through collecting duct to Aquaporin 3–> water reabsorbed into plasma–> urine concentrated
What are hypothalamic magnocellular neurons?
- long
- originate in supraoptic (AVP) and paraventricular (oxytocin) hypothalamic nuclei
- flow down from hypothalamus–> through infundibulum–> posterior pituitary
N.B. posterior pituitary made of neural tissue- anatomically continuous w/ hypothalamus
What are the physiological actions of oxytocin?
milk ejection and delivery of baby
induces contraction of myometrial cells (delivery of baby) and myoepithelial cells (milk ejection)
What is the mechanism of milk ejection (a neuroendocrine reflex)?
- mechanical stimulation of nipple activates ascending sensory pathways
- afferent signals integrate into hypothalamus–> stimulate oxytocin neurons in posterior pituitary–> release oxytocin from nerve endings
- oxytocin secreted into bloodstream–> inc. milk ejection in mammary glands
What are hypothalamic parvocellular neurons?
- short, terminate on median eminence
- release hypothalamic releasing/inhibitory factors into capillary plexus in median eminence–> factors carried by portal circulation to anterior pituitary
What hypothalamic factor regulates the production of TSH?
Thyrotrophin releasing hormone
What hypothalamic factor regulates the production of both LH and FSH?
Gonadotrophin releasing hormone
What hypothalamic factor regulates the production of ACTH?
Corticotrophin-releasing hormone
