Regulation of appetite Flashcards
When does an individual perceive thirst
Body fluid osmolality is increased.
Blood volume is reduced.
Blood pressure is reduced.
Plasma osmolality increased is the more potent stimulus – change of 2-3% induces strong desire to drink.
Decrease of 10-15% in blood volume or arterial pressure required to produce the same response.
Describe the action of ADH/ vasopressin
Acts on the kidneys to regulate the volume and osmolality of urine.
When plasma ADH is low a large volume of urine is excreted (water diuresis)
When plasma ADH is high a small volume of urine is excreted (anti diuresis).
Describe osmoreceptors and ADH release
§ Osmoreceptors are found in the hypothalamus, OVLT (Organum Vasculosum) and SFO (Subfornical Organ).
o These have an incomplete blood-brain barrier (BBB) so they can detect changes in the blood.
o Cells shrink or swell in response to osmolarity and thus send signals to the ADH cells in the hypothalamus to alter ADH release.
The same regions seem to regulate thirst
What happens when plasma osmolality is increased
Invokes drinking and ADH release.
Increased ADH stimulates kidney to conserve water
What happens when plasma osmolality is decreased
Thirst is suppressed and ADH release decreased
Absence of ADH the kidney excretes more water.
Describe the sensation of thirst
Thirst is decreased by drinking even before sufficient water has been absorbed by the GI tract to correct plasma osmolality.
Receptors in mouth, pharynx, oesophagus seem to be involved.
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.
Describe the 5 primary effects of Ang 2 in the hormonal control of thirst
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 thirs
Summarise the hormonal control of 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.
Describe the two ways in which thirst can be satisfied
Firstly, even the presence of water in the GI tract can quench thirst, for a short period of time at least. This is thought to results from receptors in the upper GI tract (mouth to oesophagus) - short term feedback
Secondly, once the original stimulus (osmolality or blood pressure) has been corrected, then the perception of thirst is no longer felt - long term feedback
These mechanisms are not fully understood.
Summarise osmoreceptors
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).
Describe the importance of controlling plasma osmolality
The osmolality (concentration) of blood is very tightly regulated (285-295 mOsm/kg) to maintain solute gradients, a suitable haematocrit and the ideal viscosity. To achieve this, the kidneys can produce urine of wildly different osmolalities (50-1200 mOsm/kg). For comparison, the desert rat can produce urine of 5000 mOsm/kg.
Compare the sensitivity of osmoreceptors to that of baroreceptors
The sensing organs for solute concentration are osmoreceptors, which are found in the brain adjacent to structures with an incomplete blood brain barrier. Principally the hypothalamus, within the OVLT and SFO regions.
During (even mild) dehydration, the hypothalamic response is considerably more sensitive than peripheral baroreceptors (which respond to changes in blood volume and pressure). 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.
Why is it important that the hypothalamus has an incomplete blood brain barrier
The hypothalamus can combine peripheral signals to elicit a coordinate central response. To achieve this, many regions of the hypothalamus have an incomplete blood brain barrier, which allows sensing organs in hypothalamus to detect increasing and decreasing concentration of circulating gut and appetite regulation hormones.
Summarise the role of the hypothalamus in appetite
§ The hypothalamus modulates the inputs.
§ You can modify the hypothalamic output voluntarily (like with breathing) but at times when you are not, the hypothalamus takes over.
§ There are LOTS of inputs that are sorted in the hypothalamus.
Inputs include:
Neural input from the periphery and other brain regions
Leptin
GHRELIN, PYY & other gut hormones
Hypothalamus integrates these inputs to control energy expenditure and food intake
Describe the important features of the arcuate nucleus
Importantly located at the base of the brain, and has an incomplete blood brain barrier, which allows peripheral signals (nutrients, hormones, concentrations, temperature etc.) to directly activate its circuitry.
Describe the neuronal populations in the arcuate nucleus
The arcuate nucleus has two neuronal populations NPY/Agrp (Neuropeptide Y/Agouti-related peptide) and POMC (proopiomelanocortin). NPY/Agrp neurons stimulate food intake and are located medially. POMC neurons inhibit food intake and are located more laterally. POMC is a long peptide that can be cleaved many ways for many different purposes. The axons from these neurons (whose cell bodies lie in the arcuate nucelus) project to many locations in the brain, but for appetite, a key site is the paraventricular nucelus.
