Hunger / Chemical Senses

Blood Glucose

• Low levels allow you to feel hungry

• Keeps body functions operational (preffered source of fuel)

• Your body stores glycogen to regulate blood glucose levels

  • Stored in muscles/liver

• When levels decrease, liver breaks down glycogen as glucose to put into blood

  • As glycogen reserves deplete, a signal of hunger gets sent to brain

Insulin

• Mediates glucose/glycogen balance

• Made in pancreas

• Promotes uptake of glucose by cells for immediate use/storage

NPY

• Drives hunger

• Drives high level of activity in hypothalamus associated with increases appetite and food-seeking behaviour

Brain Signals: Satiety

Stomach Stretch Receptors

• Stretch receptors in the stomach activated as it fills

• the vagus nerve connects to the gut and brain transferring that signal

Gastrointestinal Hormones

• Digestive hormones in the gastrointestinal tract send signals of satiety

Liver:

• Liver sends brain signals to trigger hunger/satiety

• It moniters glucose stores and blood-sugar levels to regulate hunger

  • If you inject a dog with glucose in a vein leading to the liver, it stops eating

  • If you inject glucose in any other vein, it will keep eating

  • low glucose = low glycogen = hunger

CCK

• Produced in the small intestine

• Brain receptors detect this and serves as signal to stop eating

• It’s a short-term signal for ending a meal

Long-term Energy Storage

• Fat is the ideal form instead of glycogen

• It contains twice the energy density and can be found anywhere on the body

Leptin

• Secreted by the adipose tissue

• Involved in longterm energy balance correlating with fat mass

• They act on hypthalamus receptors to reduce appetite and feeding

• It’s production is controlled by the OB gene

  • Mice lacking OB gene = no leptin = fat mouse

• When humans are given high enough amount of leptin, they become leptin resistant

Evolutionary Leptin Role

• The primary adaptive function of leptin was to serve as an indicator of low energy stores

  • instead of reducing food intake

• Low leptin = more foraging effort OR minimize activity to conserve energy

• It was rare for people to have high leptin/adipose tissue back then bc harder to get food

Leptin/NPY

• Leptin acts to inhibit the actions of NPY

• The mediated increase of NPY is prevented by leptin to decrease appetite and energy consumption

• They interact to regulate your weight to optimal levels

NPY and Carbs Consumption

• NPYurgic neurons can affect reward driven feeding for high calorie foods

• Experiment:

  • Satieted rats were injected with NPY in the brain

• Results:

  • Increased intake of sucrose

  • They worked harder for a cue associated with sucrose

  • They increased saccharine consumption

  • They chose carbs over protein or fat diet

• NPY action promoted conditional/unconditional behaviours leading to inc carb consumption

Endogenous Opioids

• Naturally occuring chemical substance with morphine-like analgesic actions in the body

• Contributes to palatability and reward-driven feeding

• Naloxone reduces intake of saccharin, sucrose and saline

• Proof: Mice who lack the opioid receptors have lower preference for saccharin and control mice

• Overeating in people may be reflective of a maladative opioid mediated reward driven feeding mechanism

  • Overeating may be caused by a dysfunctional reward system in the brain, driven by opioids.

Weight Regulating Hormones/Molecules Summary

Taste

• Indication of nutritional quality

• Bitter/Sour: Poison

• This skill is innate as all babies have the same response to those foods

  • It’s an adaptive mechanism as it’s controlled by lower regions of the brain

• Umami: Amino Acids (Glutamate and Aspartate)

• Salty: Electrolytes

• Sweet: Glucose

Personal Taste Sensitivity

• Foods you enjoy as an adult is learned by experience

• High sensitive tasters have more taste buds on average

• Females usually are more sensitive to sweet and bitter

  • Increases during pregnancy first trimester which is when the fetus is most sensitive to toxins

Detecting Taste

• Taste buds have 50-150 taste receptor cells

• 2/3 of taste buds are your tongue while the rest are on the soft palate and throat opening

• Taste receptors fire an action potential sent from the main gustatory nerve to the medulla in the brain stem

  • Info may act locally in response, ex. gagging even before being aware

• Info goes from medulla to thalamus and then to several higher brain regions

  • Ex. Gustatory Cortex

Gustatory Cortex

• Lets you percieve taste with specific neurons responding to each of the five basic tastes

• Connects with many other brain areas, combining taste with other info

• Primary somatosensory cortex combines taste with feel and texture

• Orbital Cortex combines taste with small info to provide flavour

Smell in Flavour/Tasting

• Taste receptors detect sweetness and categorize the sublte differences broadly under “sweet”

• Small allows you to detect flavour when it interacts with taste in the nasal pharynx

Spice

• Not a taste

• Results from capsasin binding to heat and pain receptors in the mouth

• Causes action potentials to fire, making you feel pain

Human Smell Mechanism

• Has direct link to the cortex without going through thalamus

• It’s important for alerting you of specific env changes/events

• Airborne molecules can communicate info from long distances

• Molecules enter the nasal cavity and dissolve in the mucus

• they interact with the Olfactory cilia, where 10-20 of them give input to one olfactory receptor cell

• A specific smell activates a unique pattern of firing across multiple receptos allowing you to detect a large range of stimuli

Summary:

• Odourants bind to receptors

• Olfactory receptors are activated and send signals to the olfactory bulb

• Signals relayed in the glomeruli recieving input from thousands of olfactory receptors

• The signals are transmitted to higher brain regions

Higher Brain Area Processing of Smell

• Signal goes from glomeruli to:

  • hypothalamus

  • areas of the limbic system

  • primary olfactory cortex in the temporal lobe

  • secondary olfactory cortex in frontal lobe

Short-term/Long-term

Shorterm:

• Hunger regulated by glucose/glycogen levels

• Hunger regulated by CCK and NPY

Longterm:

• Leptin regulated body weight through actions on NPY in hypothalamus