Week 2 - HPG axis Flashcards
What is the adrenohypophysis/pars distalis?
-Anterior portion of pituitary
What is the neurohypophysis/pars nervosa?
-Posterior portion of pituitary
Which side of the pituitary stains darker?
-Anterior
What type of tissue composes the posterior pituitary?
-Nervous tissue
What is the function of the posterior pituitary?
-It is a neuro-secretory gland producing ADH and oxytocin
What is the median eminence?
-Upper portion of neural stalk which projects into hypothalmus
What tissue is the anterior pituitary composed of?
-Glandular cells
How is the posterior pituitary connected to the hypothalamus?
-Hypothalamic neurones
How is the anterior pituitary connected to the hypothalmus?
-Superior hypophyseal artery
What 6 hormones are produced from the anterior pituitary?
- TSH
- GH
- ACTH
- Prolactin
- FSH
- LH
How does the hypothalmus regulate the anterior pituitary?
-Releases Hypothalamic Releasing Hormones into the hypothalamic pituitary portal which act on specific trophic cells in the anterior pituitary to control secretion of their hormones
How is the whole hypothalamic-pituitary-gonadal axis controlled?
-Negative feedback from trophic hormones
How are hypothalamic releasing hormones characterisically secreted?
-Pulsatile release synchronised by external signals eg light/dark
How do hypothalamic releasing hormones act on target cells?
-Bind to specific membrane receptors and transduce signals via secondary messangers which stimulate the release f stored pituitary hormones and synthesis of new ones
Name the hypothalamic releasing hormones and their corresponding effects
- Corticotrophin releasing hormone -> Stimulates ACTH secretion
- Thyrotropin releasing hormone -> Stimulates TSH and prolactin secretion
- Growth hormone releasing hormone -> Stimulates GH secretion
- Somatostatin -> inhibits GH secretion
- Gonadotropin releasing hormone -> Stimulates FSH/LH secretion
- Prolactin releasing hormone -> stimulates PRL secretion
- Dopamine -> inhibits prolactin secretion
How is GnRH characteristically released?
-Pulsatile -> once an hour
What type of receptors does GnRH stimulate on gonadotrophic cells of pituitary?
-GPCR
How do FSH and LH exert their effects on the gonads?
-Bind to GsPCR -> Gas -> Increase Adenylate Cyclase activity -> Increased cAMP -> Activation of PKA
Which cells specifically do FSH and LH act on?
- Ovarian granulosa cells and theca interna in females
- FSH -> Sertoli cells and LH ->leydig cells in males
What are the two major effects of FSH and LH regarding reproduction?
- Stimulate sex steroid synthesis
- Control gamete production
What effect does testosterone have on GnRH, FSH/LH?
Negative feeback on the hypothalmus and pituitary causes:
- Reduces GnRH secretion
- Reduced FSH/LH
What effect does high levels of oestrogen alone have on GnRH, FSH/LH?
Positive feeback on the hypothalmus and pituitary causes:
-Promotes GnRH secretion
-Promotes FSH/LH secretion
(LH surge)
What effect do moderate levels of oestrogen have on GnRH, FSH/LH?
Negative feeback on the hypothalmus and pituitary causes:
- Reduced GnRH secretion
- Reduced FSH/LH secretion
How does Progesterone influence the effect which oestrogen has on GnRH? What effect does this have on LH?
- Increases the inhibitory effects of moderate oestrogen -> Minimal FSH/LH production
- Prevents positive feedback of high oestrogen -> no LH surge
What is different between the way which oestrogen and progesterone reduce GnRH secretion?
- Oestrogen reduced the amplitude of GnRH pulse
- Progesterone reduced the frequency of GnRH pulse
What is the function of inhibin?
-Selectively reduces FSH secretion
From where is inhibin produced?
-Gonads
Which cells in the male produce testosterone?
-Leydig cells
Which cells in the male produces androgen binding globulin and inhibin?
-Sertoli cells
What is androgen binding globulin?
-A protein which binds to testosterone and keeps in within the seminiferous tubules
What is the function of inhibin in males?
-Supports spermatogenesis and inhibits the production of FSH, LH and GnRH
What are the functions of LH in the male?
- Promote testosterone secretion
- Promote spermatogenesis
What happens, with regards to controlling the system,if testosterone reaches a high level in the male? What is the advantage of this
- Negative feedback on hypothalmus and pituitary causing decreased GnRH, LH and FSH which causing a reduction in testosterone secretion ->returns to normal
- Produces a constant level of testosterone and spermatogenesis
What happens if spermatogenesis speeds up?
-more inhibin secreted -> reduces FSH secretion -> returns to normal
In what type of rhythm is testosterone produced?
-Long term testosterone levels are constant but is under circadian rhythm so is highest in the morning (also effected by environmental stimuli)
What drives secondary sexual characteristics in male puberty?
-Testosterone
What is the function of inhibin in females?
- Inhibition of activin which stimulates GnRH
- Selective inhibition of FSH allowing LH surge
How is the pituitary gland connected to the hypothalamus?
-Stalk containing nerve fibres and blood vessels
