W1 L3: Hypothalamus, pituitary and feedback loops

Hypothalamic regulation of the anterior pituitary gland
  • Pituitary important ‘gateway’ b/w central & peripheral endocrine organs
  • Many (5) major endocrine cell types
    • each have regulatory control from hypothalamus via inhibitory &/or releasing factors
  • Many pituitary hormones regulate 2º (“downstream” endocrine organs)

 

Endocrine cells of the anterior pituitary gland

 

Hypothalamic “releasing factors”

  • Hypothalamic factors released into a highly vascularised region → median eminence

 

Median Eminence/Portal Circulation
  • Portal circulation = 2 capillary beds in series
  • Physiological portal systems → hepatic & pituitary
  • Pituitary portal system transfers releasing & inhibitory hormones from hypothalamus to anterior pituitary

 

Hypothalamic regulation of the posterior pituitary gland

 

Posterior pituitary hormones
  • Oxytocin
    • released during parturition
    • required for milk ejection in lactating mammary glands
    • both related to female reproduction
    • causes uterus & smooth muscle to contract & enables birth to take place
  • Arginine vasopressin (AVP)
    • a.k.a. antidiuretic hormone (ADH) → stops production of urine
    • contraction of blood vessels to ↑ blood pressure
    • secreted in response to ↑ blood osmolarity
    • stops production of urine → ↑ water absorption from kidneys
    • both have net effect of ↑ blood pressure

 

 

Feedback loops
  • Negative

   

  • imposes “brake” on system

  • important for homeostasis

  • common in physiology (inc. endocrinology)

    • Positive

   

  • amplifies original signal
  • by itself, not compatible w/ homeostasis
  • required additional control mechanisms
Negative feedback loops: Hypothalamo-Pituitary regulation

 

Positive feedback loops
  • Initiating stimulus causes more to be added
  • Rare in physiological systems as promote instability
  • Can be useful in certain circumstances when a large response is required
  • 2 notable examples in endocrinology
    • LH surge prior to ovulation
    • Oxytocin release during parturition → large inc. in oxytocin

The “LH surge”

  • Oestrogen usually inhibits LH secretion via -ve feedback
  • But, oestrogen surge occurs around D-12 of cycle
  • High oestrogen drives Δ from -ve → +ve feedback
  • +ve feedback causes LH surge → induces ovulation
  • After ovulation, ↓ oestrogen conc causes resumption of -ve feeback & thus LH/FSH secretion falls