The Chemical Senses Flashcards
How do we perceive flavour?
Flavour is the junction between taste, smell and touch (texture and temperature)
What are the organs of taste?
Taste is primarily a function of the tongue – however, other areas of the mouth, throat and nasal passages also have important roles in taste.
Palate- Roof of mouth separating oral and nasal cavities – taste buds present in palate
Epiglottis- Leaf shaped cartilage covering laryngeal inlet – taste buds present in epiglottis
Pharynx and nasal cavity- Odours can pass, via the pharynx, to the nasal cavity to be detected by olfactory receptors
What are the different papillae and taste buds on the tongue?
Papillae
The surface of the tongue contains small projections called papillae:
- Ridge-shaped (foliate)
- Pimple-shaped (vallate)
- Mushroom-shaped (fungiform)
Taste buds
The papillae contain taste buds:
- Taste buds contains taste receptor cells ranging from 1 to 100s and each one has 100s of taste receptor cells
- Taste buds are surrounded by basal cells (precursors of taste cells) and gustatory afferent axons
What are taste receptor cells?
Taste receptors cells express different types of taste receptors – it has been shown that most taste receptor cells respond primarily (or even exclusively) to one of the five basic tastes.
Three taste receptor cells sequentially exposed to salt, bitter, sour and sweet stimuli – membrane potential recorded
Taste receptor cells display different sensitivities
Taste receptor cells form synapses with gustatory afferent axons to transmit this gustatory information
How are different tastes transduce?
The five different tastes are transduced via different mechanisms Ion channel mechanisms: - Saltiness - Sourness GPCR mechanisms via T1 and T2 taste receptors: - Bitterness - Sweetness - Umami
How is saltiness detected?
The prototypical salty chemical is table salt (NaCl) - taste of salt is mostly the taste of the cation sodium (Na+).
Mechanism:
- Na+ passes through Na+ selective channels, down its concentration gradient
- This depolarises the taste cell, activating voltage-gated Ca2+ channels (VGCCs)
- Vesicular release of neurotransmitter is elicited, and gustatory afferents activated
Special Na+ selective channel (amiloride sensitive) used to detect low concentrations of salt – insensitive to voltage and generally stays open
How is sourness detected?
Protons (H+) are the determinants of acidity and sourness.
Mechanism:
- H+ may affective sensitive taste receptors in several ways – although these processes are not well understood
- However, it is likely that H+ can pass through proton channels and bind to and block K+ selective channels
- This leads to depolarisation of the taste cell, activating VGSC and VGCCs
- Vesicular release of neurotransmitter is elicited, and gustatory afferents activated
What are the taste receptor proteins?
Transduction mechanisms underlying bitter, sweet and umami tastes rely on two families of related taste receptor proteins – T1Rs and T2Rs.
T1Rs and T2Rs are G-protein coupled receptors (GPCRs) and are Gq coupled – evidence suggests that they form dimers
- Bitter substances are detected by approximately 25 T2Rs (because bitter is associated with poison so must be detected even in a low amount)
- Sweet substances are detected by one receptor – T1R2 and T1R3 proteins
- Umami substances are detected by one receptor – T1R1 and T1R3 proteins
How is bitterness detected?
Bitter tastants binds to T2R, which is coupled to the G-protein Gq
This stimulates the enzyme phospholipase C (PLC), leading to the production of inositol triphosphate (IP3)
IP3 intracellularly activates a special type of Na+ ion channel and releases Ca2+ from intracellular storage sites
Both these actions depolarise the taste cell – release of ATP is elicited, and gustatory afferents are activated
How is sweetness detected?
Sweet tastants binds to dimer receptor formed from T1R2 and T1R3, which is coupled to the G-protein Gq
The same signal transduction mechanism as bitterness occurs
Why do we not confuse bitter and sweet tastes?
Taste cells express either bitter or sweet receptors – not both
In turn, bitter and sweet taste cells connect to different gustatory axons
How is umami detected?
- Umami tastants bind to dimer receptor formed from T1R1and T1R3, which is coupled to the G-protein Gq
- The same signal transduction mechanism as bitterness and sweetness occurs
- Shares T1R3 protein with sweetness – T1R subunit determines specificity to umami
Why do we not confuse bitter, sweet and umami tastes?
Taste cells express either bitter, sweet or umami receptors
In turn, bitter, sweet and umami taste cells connect to different gustatory axons
What are the central gustatory pathways?
What is the flow of taste information to the CNS?
The main flow of taste information is from taste cells to gustatory axons, into the gustatory nucleus (medulla), up to the ventral posterior medial nucleus (thalamus) and to the gustatory cortex
Three cranial nerves carry gustatory axons and bring taste information to the brain:
Anterior tongue sends gustatory axons to a branch of cranial nerve VII
The posterior tongue to a branch of nerve IX
And the epiglottis to a branch of cranial nerve X
What are the olfactory epithelium and its three main cell types?
We do not smell with our nose we smell with the olfactory epithelium, a small thin sheet of cells high up in the nasal cavity
It has three main cell types-
Olfactory receptor cells:
- Site of transduction – genuine neurons unlike taste receptor cells
- Genuine neurons with axons that penetrate into the CNS
Supporting cells:
- Function to produce mucus – odorants dissolve in mucus layer before contacting cilia of olfactory receptor cells
Basal cells:
- Immature olfactory receptor cells able to differentiate into mature olfactory receptor cells – olfactory receptor cells continuously grow, degenerate and regenerate
What is the mechanism of olfactory transduction?
Mechanism:
- Odorant molecules bind to odorant receptor proteins on the cilia - Olfactory-specific G-protein (Golf) is activated which is coupled to adenylyl cyclase - Adenylyl cyclase activation increases cAMP formation - cAMP-activated channels open, allowing Na+ and Ca2+ influx - Ca2+ activated chloride channels open enabling Cl- efflux - Causes membrane depolarisation of the olfactory neuron
What is the flow of smell information to the CNS?
Action potentials propagate continuously along the olfactory nerve axon
Receptor potential propagates along the dendrite and triggers a series of action potentials within the olfactory receptor cell soma
Odorants generate a slow receptor potential in the cilia
Olfactory receptor cells send axons into the olfactory bulb
Olfactory receptor cells expressing the same receptor proteins project to the same glomeruli in the olfactory bulb
Signals are relayed in the glomeruli and transmitted to higher regions of the brain
