- a loss of water causes cell shrinkage, as turgor is lost
- Osmoreceptors use their own shrinkage as a cue to both initiate hormonally controlled
water conservation measures (release of antidiuretic hormone which decreases urine
production in the kidneys), and to motivate the onset of drinking behavior.
-This is an example of a negative feedback system: when there is a lack of
something needed (body fluid), behavior occurs to increase the levels (drinking).
- Other than simple physiological needs to response there are other cues that can be used to
trigger behaviors (in this case drinking)
-Both rats and pigeons will increase their drinking levels in response to a rise in air
temperature even though their internal fluid balance has not in fact changed. There can
therefore be a behavioral component to homeostasis, both in response to current need and
in anticipation of a future requirement. This could imply that animals learn to associate
warmer conditions with a dehydration risk on the basis of past experience.
The motivation to eat
-Hunger: response of the organism to a hormonal signal. High levels of leptin = no
secretion of ghrelin and no motivation to eat. Decreased levels of leptin = secretion of
ghrelin (high levels of ghrelin) and motivation to eat.
-leptin is released by fat storage cells. Thus increased fat storage results in higher
leptin levels and indicate the adequacy of the body’s stored energy reserves. Levels of leptin
are measured by areas of the hypothalamus.
-If leptin levels are high appetite is suppressed.
- If leptin levels are low the stomach secretes the hormone ghrelin. This acts
upon the same three areas of the brain but its effect to stimulate appetite to signal
-fish experiment. 1 group not fed for 40 days, other for 20. Predicted the 1st group to have a
greater intensity of hyperphagia (eat more, more quickly) or (eat more, for longer).
-Results: starvation resulted in a strong motivation to feed in both groups, but that the
result of their higher energy deficit the very hungry fish were motivated to feed at higher
levels for a longer period (eat more for longer).