Appetite regulation Flashcards
When do we percieve thirst?
- Body fluid osmolarity is increased.
- Blood volume is reduced.
- Blood pressure is reduced.
Where are osmoreceptors found?
Hypothalamus, OVLT (Organum Vasculosum) and SFO (Subfornical Organ)
What is special about osmoreceptors?
- Have an incomplete blood-brain barrier so they can detect changes in the blood
- Cells shrink or swell in response to osmolarity and thus send signals to the ADH cells in the hypothalamus to alter ADH release
Why is thirst is decreased by drinking even before sufficient water has been absorbed by the GI-tract to correct the osmolarity of the blood?
- Due to receptors in the mouth, pharynx and oesophagus the are involved.
- Relief due to these receptors is short-lived (thirst only satisfied once osmolarity corrected).
What does angiotensin 2 do?
Induces thirst when blood volume and pressure is reduced
Acts by activating the SFO neurones.
What controls our appetite
The hypothalamus
- You can modify the hypothalamic output voluntarily (like with breathing) but at times when you are not, the hypothalamus takes over
- Recieves many inputs from letptin, ghrelin, gut hormones and normal input from other brain regions
- It organises food intake and energy expenditure
Which 2 nuclei are involved in central control of appetite?
arcuate nucles and paraventricular nucleus
Arcuate nucleus
- Located at the base of the brain, and has an incomplete blood brain barrier
- Has two neuronal populations NPY/Agrp neurones and POMC neurones
What do NPY/Agrp neurones and POMC neurones do and where are they located?
NPY/Agrp neurons stimulate food intake and are located medially. POMC neurons inhibit food intake and are located more laterally.
How do POMC neurones inhibit food intake?
The MC4R (Melanocortin 4 Receptor) exists in the paraventricular nucleus and when stimulated, decreases food intake.
- So POMC stimulates MC4R to reduce feeding.
- POMC is broken down to alpha-MSH which acts on MC4R
Mutations affecting POMC and NPY/Agrp neurones
No NPY or Agrp mutations have been associated with appetite regulation in humans.
POMC deficiency and MC4R mutations cause morbid obesity
What are some higher brain centres involved in appetite?
- Higher centres, the amygdala, vagus nerve and other parts of the hypothalamus can all
- Dieting is an example of a higher brain function limiting appetite.
Leptin
- A circulating hormone produced by white adipose tissue that is detected by the hypothalamus
- Leptin binds to receptors in the hypothalamic circuits and stimulates anorexigenic behaviours (i.e. suppresses appetite)
- Awell-nourished adult will accumulate body fat, which in turn increases leptin secretion and suppressing appetite
- This feedback mechanism can become dysfunctional as leptin resistance is associated with obesity
- The hormone is present but doesn’t signal effectively
Why does the hypothalamus alter neuropeptides?
- Regulate food appetite (intake).
- Alter thermogenesis (expenditure).
Congenital leptin deficiency
- Very rare and occurs due to mutations in the ob gene
- Results in people that are severely hyperphagic and obese
- These people however can be given leptin (unlike the leptin resistant obese)