Hunger and the Chemical Senses

Early feeding behaviours

Centred around the constant need to obtain energy and nutrients essential to survival

Present feeding behaviours

Not as driven as calories now come cheaply and easily

Blood Glucose

Important for keeping the body’s functions operational; primary energy source for the brain — low glucose = hungry

Glycogen

How glucose is stored so the brain can release it in between meals to keep a constant supply of energy going

Where is glycogen mainly stored?

Liver and muscles

When does glycogen get converted back into glucose?

When blood glucose levels are low

Insulin

Hormone in the pancreas that is secreted to promote the uptake of glucose for immediate use — mediates storage for excess glucose in its glycogen form

What happens when blood glucose levels get too low?

The liver begins to break down the stored glycogen into glucose, releasing it back into blood circulation

Neuropeptide Y (NPY)

Potential appetite stimulant; a hunger cue — high levels of this in the hypothalamus are associated with an increase in appetite

Stretch receptors

Activated in the stomach as it fills with water or food which signals the brain directly through the vagus nerve that connects the gut with the brain stem, signalling satiation

Digestive hormones

Secreted by the gastrointestinal tract that sends signals to the brain for satiation

Cholecystokinin (CCK)

A hormone produced by the small intestine that triggers short-term satiety

Adipose tissue

Endocrine organ; a form of fat where long-term energy storage takes place

Why would excess energy be stored in fat if glycogen is a quicker transference source of energy?

Fat has more than twice the energy density and fat is found all over the body

Leptin

Hormone secreted by adipose tissue that act on receptors in the hypothalamus, when high enough to reduce appetite, maintaining long-term energy storage

OB gene

Controls leptin production

Leptin resistance

Beyond certain level of body fat, effect of leptin on appetite is reduced; obese people with already normal leptin levels won’t be affected if given more leptin

Leptin in human history

High levels would be rare since calories were a limited resource

Endogenous opioids

Naturally occurring chemical substances that have morphine-like analgesic (pain-killing) actions on the body — contributes to palatability (tastiness) and reward-driven feeding

Blocking endogenous opioids

Using naloxone, this would reduce intake of rewarding foods, i.e. sugar

Universal taste preferences

Shaped by cultural influences which in turn are shaped by local food availability

Sweet

Energy-rich, i.e. fruits

Salt

Electrolytes — needed for action potentials (Na)

Sour & Bitter

Warning of harmful or spoiled or poisonous foods

Umami

Detects amino acids glutamate and aspartate

Gustatory cortex

Holds specific neurons that respond to each taste; connects to many other areas, helping combine taste with other information

Gustatory cortex to primary somatosensory

Combines taste with texture

Gustatory cortex to orbital cortex

Combines taste with smell

Retronasal smell

The process that allows for the ability to sense flavour due to the interaction between taste and smell that occurs in the nasopharynx at the back of the throat

How is smell processed in the brain?

Olfactory receptor cells activate unique patterns of firing across multiple receptors, thereby responding to a range of stimuli — unique in that it has a direct link to the cortex

Odourants getting processed into neuronal signals — step 1

Odourant binds to olfactory receptors

Odourants getting processed into neuronal signals — step 2

Receptors fire action potentials which travel down the receptors’ axons to the olfactory bulb

Odourants getting processed into neuronal signals — step 3

Axons synapse with dendrites of glomeruli cells that receive the input from thousands of olfactory receptors

Odourants getting processed into neuronal signals — step 4

Glomeruli send action potentials along their axons to higher brain regions

Ghrelin

A hormone produced by cells lining the stomach that stimulates hunger