Foundations of Psychology, Neuroscience & Behavior (PSYCH1XX3)
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PSYCH 1XX3 Chapter 8: Hunger and the Chemical Senses (Full Notes)
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Foundations of Psychology, Neuroscience & Behavior (PSYCH1XX3)
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McMaster University (mcmasteru)
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Chapter 8: Hunger and the Chemical Senses
Cooking: Using the energy in fire to dramatically alter the molecular Fat: Good way to store energy for extended periods of time
composition of food ● Slower to build up/break down than glycogen, so it’s less
● Uniquely human efficient to rely on day-to-day
● Kills harmful bacteria + breaks down complex molecules ● Stores more energy per gram than glycogen
into more easily digestible forms ○ Fat = 9 cal/g
● Allows us to take the energy once spent on digestion and ○ Glycogen = 4 cal/g
instead use it to fuel a bigger brain ● Great insulator = protects body against heat loss + cushions
● Resulted in our jaws and intestines shrinking + our skulls vital organs against physical traumas.
growing ● We store more energy as fat than we do as glycogen, so that
Digestion: The process of breaking down food into its component we have energy when we can’t eat for extended time periods
molecules which are then absorbed to power our body’s processes (resulting in our glycogen reserves running out)
● Three main, large molecules in the foods we eat are ○ Average 70 kg male can store a max of about 2000
digested/broken down into 3 smaller molecules which are calories as glycogen and at least 100,000 calories
absorbed by the small intestine: as fat stores
○ Carbohydrates → simple sugars (especially Insulin: Hormone released by the pancreas that mediates storage of
glucose) energy as both glycogen and fat
○ Fats → Fatty acids ● Simulates the muscles + liver to absorb glucose to make
○ Protein → Amino acids glycogen
Glucose: The preferred energy source for most cells in our body ● Tells the body’s fat cells to absorb glucose and turn it into
● Brain cells especially have a strong preference for glucose fat when there’s high enough glucose
● Transported throughout the body by the blood ● Also stimulates muscle cells to take up free amino acids
● Low blood-glucose → low energy → body’s metabolism which can be used to make new muscle + stimulates various
slows → eventually stops types of cell to take up fatty acids
● High blood-glucose → glucose sticks to sides of blood ● Person has carbs → digestion occurs → small intestine
vessels → blood vessels become stiff/narrow absorbs glucose from carbs → glucose levels start to rise →
● High blood-glucose → kills some types of cells directly Insulin is released → Insulin stimulates liver/muscles to
Glucose Storage: Body stores glucose absorbed from food outside of absorb glucose to turn into glycogen + stimulates fat cells to
the bloodstream + releases it into the blood slowly so blood-glucose absorb glucose to turn into fat → day proceeds → cells
levels are never too high/low break down glucose for energy as needed → blood-glucose
● Glucose is stored in 2 ways: a short-term reserve of falls → insulin level falls → liver turns stored glycogen
glycogen and long-term store of fat back into glucose + releases it into the blood → gives you
Glycogen: A bunch of glucose molecules forming long, branching energy until your next meal
chains Satiety: A psychological state in which an animal does not feel a
● Mainly stored in the liver and muscles + a little bit in the desire to eat food
brain ● 3 things signal that your body has enough energy
● Stores energy from food in the short-term (process of ○ Full glycogen stores (signal sent to brain through
making + breaking glycogen = quick and efficient) nerves running from liver to brain, that suppresses
● Easy to turn glucose into glycogen when energy levels are hunger + induces satiety)
high, + turn glycogen back into glucose when energy levels ○ Good blood-glucose levels, therefore good insulin
are low levels (insulin circulates through bloodstream +
● Person has meal → glucose from digested food is absorbed binds directly to neurons in brain to promote
by small intestine → liver turns glucose into glycogen → satiety)
liver turns glycogen back into glucose as needed + releases ○ Sizeable amount of energy stored as fat signal
glucose into bloodstream at just the right rate so blood ● Brain activated by liver + insulin to induce satiety =
glucose stays at an appropriate level/all the body’s cells hypothalamus (part of brain responsible for producing
receive the energy they need feelings of hunger and satiety)
● Overall blood-glucose level goes up following a meal + ○ Very low levels of fat can suppress both effects,
declines as time passes but level change is much smaller (causing feeling of hunger even if insulin +
than it would be without the glycogen-storing function of glycogen levels = high)
the liver
● Muscle + brain glycogen = not involved in this process/not
released into the bloodstream between meals
○ Instead, these glycogen stores function as on-site
energy reserves in case of emergencies
●
Hunger: An unpleasant psychological state designed to motivate an
animal to seek/consume food
●
, Chapter 8: Hunger and the Chemical Senses
■ Ghrelin = binds directly to NPY neurons
to make them release more
Satiety Cues: To stop you from eating too much
● Stomach Stretching: How much physical space in the
stomach food is taking up, produces sensation of fullness
(stomach couldn’t possibly fit another bite)
○ Neurons in stomach detect high levels of stretch
→ send signals to hypothalamus → NPY neurons’
activity decreases → satiety = promoted
○ High levels of ghrelin can inhibit these
stretch-sensitive neurons from firing (ie you’re
less sensitive to a stretching stomach if you were
● particularly hungry before you started eating)
● Ghrelin: “Hungry hormone”, produced by cells lining the ○ Stretch-sensitive neurons also exist in the small
stomach, increases in bloodstream as time since last meal, intestine (may be more important than stomach
binds directly to neurons in hunger centre of the neurons in producing feeling of satiety)
hypothalamus making you feel hungry ● Cholecystokinin (CCK): As food passes from your stomach
○ Also involved in meal anticipation + part of the to your small intestine, cells within the intestine release 3
reason that skipping a meal can cause hormones that promote feelings of satiety, one of which is
immediate/intense hunger CCK
○ In addition to acting on the brain, ghrelin also acts ○ Acts locally to slow the movement of food from
in the stomach, where it increases gastric motility stomach to small intestine, ensuring small
(frequency of involuntary contractions by muscles intestine has time to fully digest what’s already
in stomach, painful/hard-to-ignore signals of there before more gets added
hunger) ○ Also acts on hypothalamus:
■ Motile/contracting stomach = better ■ Directly by circulating in the
prepared to digest food bloodstream + binding there
○ Has a positive effect on learning by binding to ■ Indirectly through acting on nerves that
neurons in hippocampus (part of brain highly run from the gut to the hypothalamus to
involved in forming new memories + facilitates suppress hunger and promote satiety
those neurons making new synapses/connections ○ Exclusively regulates how much is eaten within an
with other neurons) individual meal, but does not strongly affect total
■ Can help a hungry person learn which daily food consumption
cues in the environment are linked to ■ If you give rats extra CCK during their
food acquisition meals, they’ll eat less in each meal but
● Neuropeptide Y (NPY): “Hungry neurotransmitter”, eat more meals in a day so their overall
released by a population of neurons in the hypothalamus, energy balance stays the same
essential for producing sensation of hunger ■ Smaller meals = less rise in
○ NPY’s importance in hunger demonstrated by rats blood-glucose + liver glycogen, leading
in which NPY is injected into their bloodstream: rat to feel hungry again sooner
■ Eat much more when food is put in front ● Portal Vein Circulation: Presence of nutrients in portal vein
of them induces post-meal satiety
■ More willing to press a button many, ○ Portal Vein: Connects small intestine to liver, all
many times in exchange for a small food small molecules/nutrients absorbed from digested
reward (control rats will give up far food by the small intestine must travel through the
earlier, when they judge the amount of portal vein + liver, before they can reach the rest
food to not be worth the effort) of the body, lined by special neurons sensitive to
■ More willing than normal rats to eat concentrations of glucose/fatty acids/amino acids
food laced with quinine (compound that ○ When neurons detect these molecules in the portal
rats + humans perceive as incredibly vein, they fire signals to the hypothalamus,
bitter) inhibiting NPY firing + promoting satiety
■ Rats consumed increased carbs/sugars ○ Glucose in portal vein causes insulin to rise + as
(high calorie), diff baseline preferences glucose passes through liver, much of it gets
for carbs resulted in higher/lower turned into glycogen (rise in insulin + glycogen =
preferences for carbs during experiment, further promotes satiety)
showing genetic predisposition towards
carbs
○ Acting on NPY-expressing neurons stimulates
hunger
■ High levels of insulin + glycogen =
inhibit NPY release
■
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