Taste and Smell Flashcards
How many taste buds do humans have?
4000
Where are taste buds found?
- Tongue
- Cheeks
- Soft palate
- Pharynx
- Epiglottis
Types of lingual papillae
- Filiform
- Foliate
- Fungiform
- Vallate
Filiform papillae
- Spiked
- No taste buds
- Most abundant
Foliate papillae
- Least abundant
- Gone by 2-3 years
Fungiform papillae
- ~3 apical taste buds
- Especially at tip and sides of tongue
Vallate papillae
- Large
- Back of tongue
- Contain about half of all taste buds (approx 200 buds each)
Taste transduction - salty
- Salt-sensitive taste cells have selective Na+ channels (not voltage gated) which are open all the time
- When you taste something salty, extracellular Na+ concentration rises and so the gradient across the membrane is made steeper
- Na+ moves into the cell causing membrane depolarisation
Anions effect on salt taste
The larger the anion the less salty the food will taste as it inhibits the salt taste of the cation
Taste transduction - sour
- High in acidity
- H+ permeates the Na+ channel, inward H+ flow and depolarisation
- H+ binds to and blocks K+ sensitive channels, depolarises the cell
Taste transduction - sweet
- Some molecules become sweet when they bind to specific receptor sites and activate cascade of second messengers
- GPCR triggers formation of cAMP within cytoplasm, activates protein kinase A which phosphorylates a K+ selective channel causing a blockade, this depolarises the receptor cell (similar to activation of noradrenaline receptor in some neurons)
- Some sweet stimuli activate pathway which involves IP3
- May also be mechanism which doesn’t involve second messengers, set of cation channels gated by sugars
Taste transduction - bitter
- Bitter taste receptors are often poison receptors
- Some bitter substances bind directly to K+ channels and block them
- Also specific membrane receptor proteins for bitter substances which activate GPCR IP3 mechanisms
- IP3 mechanism modulates transmitter release without changing the membrane potential
- Another bitterness mechanism reduces cAMP by activating enzymes which break down cAMP
Taste transduction - umami
- Comes from glutamate or aspartate
- Glutamate directly activates an ion channel that is permeable to the cations Na+ and Ca2+, causing depolarisation, opening voltage-gated Ca2+ channels which triggers transmitter release
- Glutamate binds to GPCRs which decrease cAMP levels which in turn modify some unknown channel
Olfactory system
- Sensitive
- Detect 2000-4000 odours
- Sensory cells are neurons
- Molecule binds receptor, opens Na+ channels, second receptor cAMP
- Adapt quickly, synaptic inhibition
What do we smell with?
- Olfactory epithelium
- Small, thin sheet of cells high up in the nasal cavity
Cell types in olfactory epithelium
- Olfactory receptor cells
- Supporting cells
- Basal cells
Olfactory receptor cells
Site of transduction
Supporting cells
Similar to glia, help produce mucus
Basal cells
Source of new receptor cells
Life cycle of olfactory receptors
4-8 weeks
Olfactory transduction
- Odorants
- Bind to membrane odourant receptor proteins
- G-protein stimulation
- Adenyl cyclase activated
- cAMP formed
- cAMP binds to cation channel
- Cation channel opens, influx of Na+ or Ca2+
- Opening of Ca2+-activated Cl- channels
- Current flow, membrane depolarisation
