JD - Olfactory Flashcards
What are Chemoreceptors?
Receptors that generate a signal when they bind to chemicals in the external environment
Olfaction vs Gustation
Olfaction: Information about airborne molecules
Gustation: Information about ingested substances
How do lower and higher organisms differ in terms of detecting olfactory senses?
Lower organisms
- Seeking and avoidance behaviour
Higher organisms:
- Stimulating the GI system
- Quality of food source
- Nutritionally beneficial or toxic
What is olfaction
Olfaction is a reflection of the pattern of different cells that are activated by the different molecules (across-fibre pattern coding) interpreted at higher centres in the CNS
Where are sensory receptors located?
Sensory receptors are located in the olfactory epithelium
Where do olfactory afferent fibres project?
To the olfactory bulb in the central nervous system
What are 6 features of the olfactory epithelium?
- Olfactory neurons are bipolar
- Unmyelinated sensory afferent
- Specialised cilia embedded within a mucus layer
- Mucus concentrates the chemicals and brings them into contact with the cilia
- The Bowman’s gland produces a mucus layer
- Olfactory neurons are prone to damage – last 6-8 weeks
How are odours transduced?
Experiments were performed to determine the location of the odorant receptors (odour applied to cilia and soma
* Cilia are sensitive to the odour
What molecules serve as chemoreceptors?
GPCRs serve as chemoreceptors
- The role of GPCRs as olfactory receptors is evolutionarily conserved
- Large amount of sequence variability – allows different GPCRs to detect different chemicals
Contain variable amino acids which enhance our ability to detect different odours
Molecular mechanisms of odorant transduction (5)
- Olfactory g-protein (G-olf) binds to GTP and becomes activated
- Adenyl cyclase III activated and second messenger cAMP activated
- cAMP binds to cAMP-gated channel
- Influx of Na+ and Ca2+
- Causes depolarisation caused by signaling cascade
General features of transduction in olfact. receptors (6)
- The nasal epithelium is the structure that houses the bipolar olfactory receptor neurones
- Olfactory receptor proteins are concentrated in the cilia of olfactory receptor neurones
- Individual olfactory receptors genes appear to have a selective distribution in the nasal epithelium
- Odorant molecules bind to GPCRs and this leads to the activation of a cyclic nucleotide-gated (CNG) channel
- A graded receptor potential passively spreads to the initial segment of the afferent axon – threshold – Action potential
- The olfactory receptor processes pass through the cribriform plate and enter the olfactory bulb
What does the mammalian olfactory bulb look like?
Layer of mitral cells (olfactory tract)
Layer of Glomeruli (convergence and amplification)
- Synaptic connections made between mitral cells and olfactory neurons
Convergence of signals on glomeruli for amplification (3)
Experiments have shown that olfactory neurons expressing the same odorant receptor send their projections to the same bilateral glomeruli
- A single glomerulus can contain dendrites from up to 25 mitral cells and 25,000 olfactory cells
- The axons of the mitral cells project from the olfactory bulb to accessory olfactory nuclei
What do different activations of olfactory receptors activate?
Different activations of olfactory receptors will activate different regions of glomeruli in the olfactory bulb
E.g.
- The red olfactory receptor will all project back to red glomeruli
- The blue olfactory receptor will all project back to blue glomeruli
Different odours and chemicals will activate a unique spatial pattern in the olfactory bulb depending upon their chemical composition
What regions does the olfactory bulb target (4)?
Olfactory bulb targets the:
1) Piriform cortex (major target in humans)
2) Olfactory tubercle
3) Amygdala
→ Orbitofrontal cortex <-> Thalamus
→ Hypothalamus
4) Entorhinal cortex
→ Hippocampal formation
