Regulation of appetite Flashcards
State three triggers of thirst. Which is most potent?
- Increase in plasma osmolality (most potent). Hypothalamic response
- Decrease in blood volume
- Decrease in blood pressure
2.3 are peripheral baroreceptors, changes in blood volume and pressure.
Where are the osmoreceptors that trigger ADH release located?
OVLT - organum vasculosum
SFO - subfornical organ
These are found in the hypothalamus
How do the circumventricular organs detect changes in plasma osmolality?
they are NOT isolated by the BBB.
Osmoreceptive cells have cell bodies outside of the blood brain barrier that are bathed in the extracellular fluid (ECF). These cells are very sensitive to their local environment and quickly equilibrate with it. If the ECF varies from about 290 mOsm/kg, the cells will proportionately grow (in response to reduced osmolality) or shrink (in response to increased osmolality) by osmosis.
These changes will affect the firing rate of these cells, which will adjust the basal level of ADH being secreted. Increased ADH secretion will lead to water retention (and a decreased osmolality).
How do you maintain water balance?
Increased plasma osmolality:
Invokes drinking and ADH release.
Increased ADH stimulates kidney to conserve water.
Decreased plasma osmolality:
Thirst is suppressed and ADH release decreased
Absence of ADH the kidney excretes more water
How come thirst is temporarily relieved when you drink water but before it has any effect on plasma osmolality?
There are receptors in the mouth, pharynx and oesophagus that are involved in this temporary relief of thirst.Thirst is only quenched
Relief of thirst sensation via these receptors is short lived.
Thirst is only completely satisfied once plasma osmolality is decreased or blood volume or arterial pressure corrected. perception of thirst is no longer felt.
What major hormone is involved in regulating thirst? What does it do?
Angiotensin II - it stimulates fluid retention
It binds to receptors on vascular smooth muscle cells to stimulate vasoconstriction
It upregulates activity of the sympathetic nervous system, which largely promotes vasoconstriction
It stimulates aldosterone secretion, which increases sodium reabsorption in the nephron, which creates an osmotic gradient for water reabsorption
It directly influences sodium reabsorption, causing water reabsorption
It stimulates ADH release and stimulates thirst
Evokes the sensation of thirst
AII is increased when blood volume and pressure are reduced.
Activates SFO neurons
AII contributes to the homeostatic response to restore and maintain the body fluids at their normal level.
What neural and hormonal components have an effect on the hypothalamus in regulating hunger?
Neural - Vagal afferents
Hormonal - Ghrelin, PYY3-36, Leptin
Which part of the brain is heavily involved in regulating hunger and what does it communicate with?
Arcuate nucleus
It is located beneath the 3rd ventricle and communicates with the paraventricular nucleus (above the 3rd ventricle)
What are the two groups of neuronal populations in this part of the brain and what effect does this have on hunger?
Stimulatory - NPY/Agrp
Inhibitory – POMC
What feature of the arcuate nucleus makes this good for regulating hunger?
It has an incomplete blood-brain barrier thus allowing access to peripheral hormones e.g chemicals, nutrients and temperature etc
It integrates peripheral and central feeding controls.
Describe the melanocortin system and how it regulates hunger.
POMC can be broken down to produce an anorectic hormone called alpha-MSH that agonises the melanocortin-4 receptor and suppresses food intake.
Agrp, also produced in the arcuate nucleus, can antagonise the MC4R receptor and make you feel hungry.
What mutations or deficiencies within this system can cause obesity?
MC4R deficiency and POMC mutation
Where is leptin produced and what are its two roles?
Leptin is produced by adipocytes in white adipose tissue
Leptin tells the brain how much fat is stored and hence regulate food intake and regulates thermogenesis..
Leptin circulates in plasma in concentrations proportional to FAT MASS
Leptin binds to receptors in the hypothalamic circuits and stimulates anorexigenic behaviours (i.e. suppresses appetite).
State three possible mechanisms that would make leptin responsible for obesity.
Absence of leptin
Inability of the leptin stores to increase proportionally with increasing fat
Leptin resistance - partially responsible for obesity
What determines the amount of PYY secreted?
The size of the meal
Describe the action of PYY.
PYY travels to the arcuate nucleus and inhibits NPY release and stimulates POMC neurons so you DECREASE APPETITE.
Peptitde YY3-36 is the ‘satiety hormone’ and is secreted from cells in the ileum and colon in response to a meal. PYY3-36 binds to hypothalamic receptors and reduces the perception of hunger and the urge to eat.
PYY3-36 is a peptide hormone that regulates short-term appetite control by suppressing eating behaviours.
What structure in Ghrelin is necessary for its receptor activity?
Ghrelin has a fatty acid in the 3rd position which is required for receptor activity and it helps it to slip across membranes.
Describe the action of Ghrelin.
It stimulates NPY/Agrp and inhibits POMC thus INCREASING APPETITE.
Ghrelin is the ‘hunger hormone’ and is secreted from cells in the stomach in increaisngly higher quantities in the build up to a meal. This is because ghrelin binds to hypothalamic receptors and increases the perception of hunger and the urge to eat.
Ghrelin is a peptide hormone that regulates short-term appetite control by promoting eating behaviours.
What is the thrifty gene hypothesis?
It was evolutionarily sensible to put on some extra fat for use later on when food availability wasn’t guaranteed. So populations that are historically prone to starvation are the most obese.
What is the adaptive drift hypothesis?
Weight use to show a normal distribution curve with the heaviest people being preyed on. As we became smarter we stopped being preyed on so extra weight became a neutral change.
compare the sensitivity of the hypothalamic response to the peripheral barorecptors
The thresholds for responsive action are as follows:
A 2-3% increase in osmolality
A 10-15% decrease in volume and/or pressure
Physiologically, this is useful because corrective action can be initiated without any critical risk to the body.
