Olfaction & taste Flashcards
Where does olfaction take place?
Olfactory epithelium on the superior aspects of the nasal cavity.
What needs to happen to odorant molecules before they interact with the olfactory epithelium?
Dissolve into mucous over olfactory epithelium (and possibly associate with odorant binding protein)
How often do cells in the olfactory epithelium get renewed?
60 days
What is the series of events during olfactory transduction?
- Odorant binds to olfactory receptor, causing conformational change and activating it.
- Receptor molecule converts inactive Golf to active GOlf via G-protein exchange.
- αs subunit of GOlf stimulates adenylyl cyclase and synthesis of cAMP.
- Increased cAMP causes opening of cyclic nucleotide-gated cation channels (CNs).
- Influx of Na+ and Ca2+ through CNs depolarises receptor cell.
- Ca2+ also activates Cl- channels, causing outward Cl- current (due to Cl- being pumped into cell by NKCC), which amplifies depolarisation.
- Depolarisation causes generation of APs that propagate down axon to olfactory bulb.
What is the mechanism behind olfactory adaptation?
Ca2+ activates calmodulin, which desensitises CNs to cAMP.
What is the nature of neural connections in the olfactory bulb?
- Olfactory receptor axons synapse with mitral cells and tufted cells.
- Synpases form within specialised structures in olfactory bulb called olfactory glomeruli.
- Olfactory receptor cells expressing particular type of receptor synapses in only 2 glomeruli per olfactory bulb.
How does lateral inhibition occur in the olfactory bulb?
- When a mitral cell is stimulated, it stimulates the surrounding granule cells via the dendro-dendritic synapses.
- These granule cells then inhibit the adjacent mitral cells via lateral inhibition.
- Although all mitral cells have associated inhibitory granule cells, only the ones that are stimulate the most will produce the greatest stimulation of their respective granule cells, thus causing the greatest inhibitory effect on the neighbouring mitral cells.
What is the output from the olfactory bulb?
- Mitral cells send axons out of the olfactory bulb into the lateral olfactory tract.
- Fibres project into different nuclei from the lateral olfactory tract.
What are the projections of olfactory fibres?
- Anterior olfactory nucleus → Anterior commissure → Contralateral olfactory bulb: Takes part in inhibition of contralateral bulb (via granule cells).
- Olfactory tubercle → Medial dorsal nucleus of the thalamus → Orbitofrontal cortex: Conscious perception of smell.
- Pyriform cortex: Projects into other olfactory cortical regions.
- Amygdala → Reticular formation: Mediates arousal.
- Amygdala → Hypothalamus: Mediates autonomic responses.
- Entorhinal cortex → Hippocampus: Mediates motivation.
What type of receptors may be involved in detection of hormone pheromones?
Trace amine associated receptors (in olfactory epithelium)
What is the structure of the taste apparatus?
- Taste is mediated by taste receptors that are clustered into taste buds.
- Taste buds are embedded within papillae (foliate, fungiform, circumvallate).
What are the tastes?
- Sweet
- Sour
- Salt
- Bitter
- Umami
What are the types of cells involved with taste transduction?
- Receptor cells: Sweet, bitter, umami
- Pre-synaptic cells: Sour
- Glial-like cells: Salt
What types of receptor molecules are found in receptor cells?
Heterodimeric G-proteins:
- Sweet (T1R2 + T1R3)
- Umami (T1R1 + T1R3)
Monomeric G-proteins:
- Bitter (T2R receptors)
What is the transduction mechanism in receptor cells?
- Gβγ subunit stimulates PLC, which converts PIP2 to IP3 and DAG.
- IP3 stimulates Ca2+ release from intracellular stores (mainly ER).
- Ca2+ stimulates the opening of TRP5M channels, causing influx of Na+ ions and subsequent depolarisation of the receptor cell.
- Depolarisation coupled with Ca2+ results in activation of Panx1 which causes release of ATP, acting as a transmitter.
