Anterior Pituitary Flashcards
what does hypophysectomy cause
stops growth and lactation
atrophy (thyroid/adrenal cortex/gonads)
disruption to (salt and water balance, carb and protein metabolism)
anatomy of the pituitary
adenohypophysis/anterior - glandular
neurohypophysis/posterior - neuronal processes in SON/PVN
intermediate lobe - forms MSH, not in adults (in animals/babies)
posterior lobe formation
forebrain (diencephalon) invagination
anterior lobe formation
buccal cavity outgrowth (rathke’s pouch)
formation steps
1) outgrowth of tissue forms at diencephalon (FB becomes thalamus and hypothalamus)
2) outgrowths combine
3) anterior and posterior lobes begin to form (Rathke’s pouch removal)
4) posterior lobe (neural tissue) anterior lobe (non-neural tissue) sella turcica (skull) protects pituitary gland
posterior vs anterior pituitary connections
posterior pituitary has neural connections
anterior pituitary has no neural connections - connected to median eminence via pituitary portal system
blood supply
blood passes capillary loops through long/short hypophyseal portal vessels (specific to anterior pituitary)
superior hypophyseal artery supplies ME and stalk
inferior hypophyseal artery supplies posterior lobe directly / indirectly to the anterior lobe via short portal veins
inferior and superior artery connected via trabecular artery
A and P lobes drain into venous sinuses
2 features of the anterior pituitary
hypothalamic hypophysiotropic hormones (very diluted released into wider circulation)
feedback by target gland hormones
where are hypothalamic hypophysiotropic hormones synthesised
in the cell body and transported to axon terminal to be stored until release in adjacent capillary
control of hypothalamic hypophysiotropic hormone release
1) 1 or more neurons in neural pathway converge on 1 neurosecretory hormone
2) direct synapse on soma/dendrite of neurosecretory hormone
3) axo-axonic synapse on nerve ending of neurosecretory hormone
4) neuron releases NT into portal vessels which modifies action of neurohormone
5 cells in the anterior pituitary
gonadotroph cells
corticotroph cells
somatotroph cells
lactotroph cells
thyrotroph cells
tropic vs trophic
tropic = hormone release
trophic = stimulates growth of downstream tissue
hormone release
TSH/LH/FSH/ACTH - tropic
GH - stimulates growth of liver hormones/somatomedians - trophic
PRL - not trophic/tropic
episodic hormone secretion
not secreted at a constant rate
secreted in a pulsatile pattern
GH secretion
spontaneous surges throughout the day
occurs 3-4hrs (undetectable inbetween)
GHRH and somatostatin determine GH rhythm (peak during sleep)
what causes pulsatile secretion
intrinsic rhythm generators in hypothalamus
why is episodic secretion important
desensitisation of hormone receptors when challenged with partner hormone (maintains sensitivity of system)
negative feedback system
y (circulating hormone concentration) is sensed and compared to set point
difference in feedback is sensed and produces an error signal
hormone maintained in a narrow range
CNS negative feedback
specific signal e.g. stress
CRH release acts on corticotroph cells in anterior pituitary cells
ACTH release
ACTH acts on adrenal glands to release cortisol
negative feedback to hypothalamus
high glucocorticoids = CRH and ACTH low (vice versa)
positive feedback
occurs when increase in a hormone’s levels leads to enhancement of its effect by stimulating further release
oestrogen release
normally uses negative feedback
high oestrogen levels use positive feedback, stimulates gonadotropin release from pituitary, rapid surge of FSH and LH (needed for ovulation)
circadian rhythm
endogenous oscillations across a period of ~ 24hrs
synchronised precisely 24hrs rhythm by environmental signals L/D
anterior pituitary hormones and CR
GH/PRL/ACTH pulses are larger amplitude/more frequent during sleep
larger TSH peaks in morning/peak in males
CRH/TRH/GnRH show CRs
SCN
suprachiasmatic nucleus
in anterior hypothalamus
retina sends daylight info to SCN via optic nerve and optic chaism (retinohypothalamic tract)
SCN projections
medial hypothalamus
ARC
ME
parvocellular portion of PVN
melatonin secretion from pineal gland
NA nerves from superior cervical ganglia project to pineal gland
NA release regulated by SCN controls melatonin release
melatonin synthesised and released during darkness
light onto retina inhibits output from SCN
what controls oestrous and menstrual cycles
GnRH release
modulated by steroid feedback
what are seasonal rhythms
supression of rhythm generators for discrete periods of the year - suppression related to changes in daylength (photoperiod)
what characteristics does daylength affect
coat colour
food intake
fattening
hibernation
seasonal changes driven by photoperiod
no of hours of light/darkness
driven by changes in melatonin release
