Chemical senses

L20

What are the 5 basic tastes and the class of receptor that is used to detect them ?

• salty - ion channel, ENaC

• sour - ion channel

• sweet - GPCR

• bitter - GPCR

• umami - GPCR

What are the major anatomical structures of the upper gastrointestinal tract involved in taste?

primarily tongue, also cheeks, soft palate, pharynx, epiglottis with papillae taste buds

Where are receptors expressed allowing transduction of different stimuli?

membrane of taste cells

How do receptor potentials trigger action potentials in the primary afferent neurons of the gustatory system?

taste buds are triggered by chemicals/receptor potentials = initiation of AP = release of NT = activation of the gustatory afferent neurons = taste info passed onto brain

What are functions of the olfactory and gustatory systems?

• Identify food sources

• Avoid noxious substances

• Find a mate or mark territories

What general term do we use for the area in the CNS where information from different sensory systems is integrated?

association cortex

How does parallel processing of information from the gustatory and olfactory system function?

merged in the association cortex of the CNS

What is the oldest and most common form of sense?

gustatory

What is our preference towards salty food and the explanation for this?

high salt content is preferred (limit - not too salty)

salt required for many physiological processes like GI tract. AP etc

How is our liking of salt outdated in the modern world?

in wild, salt was needed and rarer so we sought it out // today readily available so we eat too much

Overall, what are the tastes that are craved and those avoided?

salt, sweet, umami // sour and bitter

What is our preference towards sour food and the explanation for this?

avoid bc sour indicates rotting food + can injure the GI tract

What is our preference towards sweet food and the explanation for this?

crave bc required for energy and growth

What is our preference towards bitter food and the explanation for this?

avoid bc associated with toxic/poisonous substances

What is our preference towards umami food and the explanation for this?

craved bc needed for protein synthesis, NTs etc

What do the salty and sour tastes relate to respectively?

salt is vital electrolytes // sour is acidity due to hydrogen content

Considering we generally avoid sour food, associated with rotting and GI tract injury, why do some people enjoy the flavour?

still not sure today!

What do the sweet, bitter and umami tastes relate to respectively?

sugars // diverse chemical structures (eg caffeine) // amino acids like glutamate

What are taste buds made up of?

~100 chemoreceptive taste cells per taste bud

What contributes to tasting food other than taste buds?

temperature, texture, smell

Where is the soft palate and what is it involved in?

towards throat on the roof of the mouth - taste organ

What is the function of a taste pore?

allows sensory transduction by microvilli, from saliva to taste cell down to the sensory ganglion

Where are taste buds found? How many on average are there?

lingual papillae, little ridges on tongue

2000 to 5000

Are taste buds specialised for a specific taste, allowing to taste all flavours of a food?

no, each taste bud contains all specialised cells for each taste so on bud can detect all

What distinguishes receptors involved in detecting sweet and umami tastes?

both are GPCRs with a T1R3 subunit, only difference is T1R2 in sweet vs T1R1 in umami

Essentially, how does the OTOP1 receptor function and which taste does it allow the detection of?

hydrogen ions pass through = detection of acidity of food so more ions = more acid, fewer = less

What does the transduction of different gustatory stimuli depend on?

the membranous receptors expressed on taste buds - so if sour one activated = sour taste

Which receptor detects bitterness?

T2Rs

Which receptor detects saltiness? Through which mechanism?

ENaC

more sodium through channel = more salty but downstream effects/why aversion to salt are still unknown

To how many different types of stimuli do taste cells // taste buds respond to?

one type // various stimuli

How can the labelled line code for taste be tricked? Eg?

by inserting synthetic taste receptors in a pathway usually dedicated to another taste

eg: pathway/specific cells are always sweet so when stimulated by chemicals from food then brain already recognises as sweet even if the stimulus acts on new receptors in the cell that respond to bitter and not sweet chemicals, still = tasting sweet

What are gustatory afferents? Are they distinct from taste cells?

sensory neurons that transmit taste info from taste buds to brain

yes ! different cells with different types of activation

What do gustatory afferents require to function?

NT release across the synaptic cleft (not stimulus/chemicals from food)

Name the major anatomical structures of the nose involved in smell. Describe the structure of the olfactory pathway from the olfactory bulb to olfactory receptor cells

olfactory bulb, cortex and epithelium

• olfactory bulb containing glomeruli, latter attach to

• afferent olfactory cells that make up the olfactory epithelium

• and pass through the cribriform plate

• dendrites of afferent olfactory cells go onto specific regions of brain

Describe the transduction mechanism in olfactory receptor cells

• odorant molecule binds to odorant GPCR in the cilia

• g protein activated = increased adenylyl cyclase activity, making ATP into cAMP second messenger

• opens cyclic nucleotide gated ion channels = depolarisation

• calcium gated chloride channels open = further depolarisation (bc of unusual high chloride within cilia)

How do receptor potentials trigger action potentials in the primary afferent neurons of the olfactory system?

receptor potential will reach threshold for action potential to be fired so Intense stimulus = large receptor potential = increased action potential firing rate

What’s the organisation of cells within the glomerulus?

• receives input from axons of olfactory sensory neurons

• contains: dendrites from mitral, tufted, and periglomerular cells = first layer of the olfactory bulb where initial odour processing occurs

How small can odorants be while still getting detected by our olfactory system?

a few parts per trillion

How large is the human olfactory system? And the dog’s?

10 cm squared vs 170 cm squared

What structures make up the olfactory system?

olfactory bulb, cortex and epithelium

What types of neurons are olfactory receptor cells?

bipolar chemoreceptive

How do odorants reach olfactory receptor cells?

must first dissolve in the mucus layer then go through transduction at the end of cilia

What are primary olfactory afferent neurons?

axon of the olfactory receptor cell

Describe the axons of olfactory receptor cells ie the primary afferent neurons?

thin and unmyelinated

Why are olfactory receptor cells regularly replaced?

cilia go out into the external env. so in contact with bacteria etc can get damaged - if not replaced then loss of sense of smell forever

How many odorant receptor proteins exist in humans? In rats?

around 350 in humans // 1000s in rats

How many different odorant receptors can olfactory receptor cells express? How many odorants can they recognise?

only one type of receptor per cell!

one odorant receptor can recognise multiple odorants so cells can too

What does a combination of odorant receptors/olfactory receptor cells allow?

to distinguish a specific odour

How much of the human genome is dedicated to receptor proteins for odour?

3% !

What type of receptor protein are odorant receptor proteins?

GPCRs with slightly different subunits/structures

What does the image show?

shows that different combination of odorant receptors = picking up different odorants = specific odour is detected

What downstream pathway is used by every odorant receptor?

• G_{olf (olfactory)}

• Adenylyl cyclase

• cAMP

• Cyclic nucleotide gated ion channels

• Depolarisation

• Ca²⁺-gated Cl^- channels 

• Further depolarisation

What is unusual about the ion concertation in olfactory cells? What consequence does this have?

higher chloride inside than outside !

chloride moves out of cell through GABA ie GABA = excitation of the cell as it becomes more positive with negative ions moving out

How is the stimulus for olfactory afferents made into neural activity?

receptor potential = action potential when the threshold for its firing is reached

How would a large stimulus for olfactory afferents impact olfactory neuron action potential firing rate?

Intense stimulus = large receptor potential = increased action potential firing rate

What are glomeruli of the olfactory bulb? From what do they receive input form?

contain first synapse of olfactory pathway and transmit this info to other neurons in the bulb

receive input from only one type of olfactory receptors

Considering glomeruli of the olfactory bulb only receive input from one type of olfactory receptor, how is this information processed together later on?

through convergence on second order neurons

formation of a spatial map of odorant info: pattern of activation in the olfactory bulb, where odorants = distinct combinations of active glomeruli.

Where do second order neurons/olfactory projections carry information from the glomeruli to?

various regions of the brain like the olfactory cortex, hippocampus, hypothalamus, amygdala