Alimentary mechanisms Flashcards
Membrane transport; Absorption; Hydration; Appetite
What is appetite?
Intrinsic desire to consume food
How is circulating gut and appetite hormones detected by the brain?
Hypothalamus has an incomplete blood brain barrier so it can detect increasing and decreasing concentrations in the blood
What are features of the arcuate nucleus?
Has an incomplete blood brain barrier which allows peripheral signals to directly activate its circuitry
Two neuronal populations
Where is the arcuate nucleus?
Located at the base of the brain
Adjacent to 3rd ventricle and median eminence
Part of the hypothalamus
What are the two neuronal populations in the arcuate nucleus?
NPY/Agrp(Neuropeptide Y/Agouti-related peptide)
POMC (proopiomelanocortin)
What do NPY/Agrp neurones do and where are they located?
Stimulate food intake
Located medially in the arcuate nucleus
What do POMC neurones do and where are they located?
Inhibit food intake
Located more laterally in the arcuate nucleus
Where do the axons from NPY/Agrp neurones project for appetite?
Project to many locations in the brain
Key site is the paraventricular nucleus
How do POMC neurones reduce food intake?
α-MSH (alpha melanocyte stimulating hormone) from POMC neurones binds to MC4R of paraventricular nucleus
Agonises the receptor and reduces food intake
How do NPY/Agrp neurones increase food intake?
Agrp binds to MC4R of paraventricular nucleus
Antagonises receptor
Maintains food intake behaviours
What does MC4R stand for and where is it located?
Melanocortin 4 receptor in the paraventricular nucleus
What mutations in the hypothalamic appetite circuit influence energy balance?
No known mutations in NPY/Agrp that effect appetite
POMC deficiency and MC4R mutation caues morbid obesity
What are the 3 hormones that regulate food intake?
Leptin
Ghrelin
PeptideYY₃-₃₆
What is the purpose of leptin and where is it secreted?
Long term appetite regulation
Stimulates anorexigenic behaviours
Secreted by cells in white adipose tissue
What is the leptin feedback mechanism and how could it be disrupted?
well nourished adult will accumulate body fat
Increases leptin secretion ∴ suppresses appetite
Leptin resistance is associated with obesity
What is ghrelin and where is it secreted?
Peptide hormone that regulates short term appetite control by promoting eating behaviours
Secreted from stomach in increasingly higher quantities in the build up to a meal
Why is ghrelin the hunger hormone?
Binds to hypothalamic receptors and increases the perception of hunger and the urge to eat
What is PeptideYY₃-₃₆ and where is it secreted?
Peptide hormone that regulates short term appetite control by suppressing eating behaviours
Satiety hormone
Secreted from cells in ileum and colon in response to a meal
Why is PeptideYY₃-₃₆ the satiety hormone?
Binds to hypothalamic receptors and reduces the perception of hunger and the urge to eat
Why is the osmolarity of the blood tightly regulated?
Maintain :
- solute gradients
- Suitable hematocrit
- Ideal viscosity
What are the sensing organs for solute concentration and where are they found?
Osmoreceptors
Found in brain adjacent to structures with an incomplete blood brain barrier - hypothalamus within OVLT and SFO regions
What are baroreceptors?
Respond to changes in blood volume and pressure
What are the thresholds for responsive action by the hypothalamus and why are they useful?
2-3% increase in osmolarity
10-15% decrease in volume and/or pressure
Corrective action can be initiated without any critical risk to the body
What is vasopressin and what are its primary functions?
Antidiuretic hormone produced in the hypothalamus and released from the posterior pituitary gland
Insert aquaporin-2 channels into the collecting duct of renal nephrons to increase water absorption
Stimulate vasoconstriction
Stimulate thirst to lead to increased fluid intake