chemical senses Flashcards
where are chemoreceptors located and what do they measure?
in the arteries of the neck, measure co2 and o2 levels in the blood
how are we warned of chemical irritants?
nerve endings in skin/mucous membranes
how do we detect acidity?
sensory nerve endings in muscle respond to acidity
what is gustation?
taste
what is smell?
olfaction
how do we perceive flavour?
smell, taste and touch
texture and temperature
name some organs of taste other than the tongue
- palate
- epiglottis
- pharynx and nasal cavity
palate
Roof of mouth separating oral and nasal cavities
where are taste buds present?
palate and epiglottis
epiglottis
Leaf shaped cartilage covering laryngeal inlet upon swallowing
pharynx and nasal cavity
odours can pass via the pharynx to the nasal cavity
-here, they are detected by olfactory receptors
what are papillae?
structures on the tongue
different papillae shapes?
fungiform = mushroom shaped foliate = ridged shaped vallate = pimple shaped
cross section of a papillae?
taste buds embedded into them
- chemically sensitive end of a taste bud is a taste pore
- taste cells are within this, connect and synapse with the gustatory afferent axons, which transmit taste information to the brain
what is a taste pore?
chemically sensitive end of a taste bud
what are taste cells?
they are within the taste pores
they connect and synapse with the gustatory afferent axons, which transmit taste information to the brain
what do taste receptor cells express?
express different types of taste receptors
- display different sensitivities to sweet, bitter, sour, salt
- can be measured in AP discharge
five different tastes are transduced via?
different mechanisms
saltiness and sourness transduced via?
ion channel mechanisms
saltiness taste transduction mechanism
- Na+ passes through Na+ selective channels down its conc gradient
- depolarises the taste cell and activates VGCCs
- vesicular release of neurotransmitter
- gustatory afferents activated
what is the special Na+ selective channel and its role?
it is amiloride sensitive, and used to detect low concentrations of salt – insensitive to voltage and generally stays open
amiloride – diuretic drug that antagonises this paticular receptor
sourness taste transduction mechanism
- H+ passes through Na+ selective channels down its conc gradient
- H+ also binds to and blocks K+ selective channels
- Both these actions depolarise the taste cell, activating VGCCs
- vesicular release of neurotransmitter
- gustatory afferents activated
bitterness, sweetness and umami transduced via?
T1 and T2 taste receptors
-T1 family and T2 family
what are T1Rs and T2Rs?
They are 2 families of related taste receptor proteins
G-protein coupled receptors, specifically Gq coupled – form dimers
bitter substances are detected by which receptors?
2 T2Rs
Sweet substances are detected by?
one receptor – T1R2 and T1R3 proteins
Umami substances are detected by?
one receptor – T1R1 and T1R3 proteins
bitterness transduction mechanism?
- Bitter tastants binds to T2 receptor - Gq coupled
- Stimulates phospholipase C which converts PIP2 to IP3 and DAG
- IP3 intracellularly activates a special type of Ca+ ion channel, releasing Ca2+ from the ER
Taste cell depolarised, release of ATP, gustatory afferents activated
sweetness transduction mechanism?
Sweet tastants binds to dimer receptor (T1R2 and T1R3) - Gq coupled
same signal transduction mechanism as bitterness
Why do we not confuse bitter, sweet and umami tastes if the mechanisms are the same?
because taste cells express either bitter, sweet or umami receptors – not all three
bitter, sweet and umami taste cells connect to different gustatory axons
umami transduction mechanism?
Umami tastants bind to dimer receptor (T1R1/T1R3) - Gq coupled
same signal transduction mechanism as bitterness and sweetness occurs
what do umami and sweetness have in common?
both have T1R3 protein - so the other T1R subunit determines specificity to sweetness or umami
What is the flow of taste information to the CNS?
anterior tongue/posterior tongue/epiglottis –> gustatory nucleus –> ventral posterior medial nucleus –> gustatory cortex
damage at any point within these pathways, it would result in people not being able to taste certain things
are taste and smell innate or learnt?
some taste/smell preferences are innate
-however experience can strongly modify our innate preferences
what do we smell with?
the olfactory epithelium
what is the the olfactory epithelium?
small, thin sheet of cells high up in the nasal cavity
-dendrites of olfactory cells that protrude from olfactory bulbs through the holes in our skull (cribiform plate), into the top of THE nasal passage
- olfactory receptor cells - site of transduction
Each of the olfactory cells expresses a type of olfactory receptor protein - supporting cells - help produce mucus
- basal cells - immature olfactory receptor cells – source of new olfactory receptor cells
role of the cilia (protrusions from olfactory cells)
cilia are covered in a mucus layer containing sugars, variety of enzymes and odorant binding proteins
what do the odorant binding proteins in the mucus layer do?
catch the smell particles and present them to the cilia of the olfactory receptor cells
what is it important that the mucus layer contains antibodies?
because certain viruses/bacteria commonly enter our bodies through our nasal passage
olfactory transduction mechanism
- Odorant molecules bind to odorant receptor proteins on the cilia, activate Golf
- AC activated, cAMP formation increased
- cAMP-activated channels open, Na+ and Ca2+ influx
- Ca2+ activated chloride channels, Cl- efflux
- cell membrane depolarisation
after depolarisation what happens to AP’s generated?
- odorants generate a slow receptor potential in the cilia
- receptor potential propagates along the dendrite and triggers a AP’s in olfactory receptor cell soma
- propagated along the olfactory nerve axon to the brain
where do the olfactory nerve axons go?
into the olfactory bulb
-signals relayed in the glomeruli and transmitted to higher regions of the brain
explain convergence that occurs?
olfactory receptor cells expressing the same receptor proteins project to the same glomeruli in the olfactory bulb - convergence
which higher regions of the brain are signals transmitted to?
frontal cortex - conscious perception of smell
hypothalamus and amygdala - motivational/emotional aspects of smell
hippocampus - memory
Population coding for gustation and olfaction
axons in the brain responds broadly to smell, so combinations of patterns are used so they can detect which smell it is (different response patterns)
