Smell Flashcards
How many odours can a receptor molecule detect?
Some detect only one, others multiple. USUALLY an OSN = its receptor type.
Exceptions: C. elegans, some Drosophila OSNs, MS4A Rs in mouse necklace cells, early vertebrate development
How does the brain encode odours?
Can be combinatorial code (where each odour can activate more than one receptor type and each receptor can be activated by more than one odour. Receptors are broadly tuned). OR Labelled line (rapid and specific response)
Example of odours acting as combinatorial code
Araneda et al (2000) - took one odour (octanal) and made variants of it. Rat receptor 17 seems to have receptive field i.e. really “likes” one thing but still responds to others, depends on chain length. Broadly tuned receptor.
Androstenone (human example)
Human detection involves single receptor (unusual): OR7D4. WM/WM and WM/RT variants are less sensitive (may be pleasant), RT/RT probably won’t like it.
Study looked at global distribution. RT form originated in Africa, WM came later (probably in SE Asia). Thought to be linked with pig domestication (would be an advantage if you’re WM as you would eat it).
Another study modelling the receptor showed that mutations on the inside of the TM region knocked down the response whereas outside didn’t.
How mouse OSNs respond
Non-linear response with concentration of odour vs current and firing rate (sigmoidal). Amplitude and frequency are affected by concentration.
Transduction pathways
Can have multiple transduction pathways EVEN IF CELL ONLY HAS ONE RECEPTOR TYPE.
Why is it so hard to determine receptor function? (7)
1) Some cells have multiple receptor types.
2) Some receptor types respond to multiple odours.
3) Can be multiple transduction pathways for one receptor type.
4) Concentration can affect response.
5) “Fuzzy coding” - some neurons will just not respond sometimes, e.g. only respond 50% of the time. Particularly if they’re at the edge of the receptive field.
6) Neurons talk to one another - response to one odour can change responses to other odours.
7) OSNs can adapt - constant stimulation can reduce responsiveness (shown in Drosophila ORs and not IRs)
Glomeruli in olfaction
OSN types are randomly distributed throughout nasal mucosa but converge on the same glomerulus.
Central coding
Mammalian (rodent) wiring: OSNs of same type –> glomerulus –> main olfactory bulb –> olfactory cortex. Network between/after glomeruli made up of periglomerular cells, mitral cells and granule cells.
Insect (Drosophila): “like” tends to be represented by one set of glomeruli, “dislike” by another set. Rs are on the antenna: antennal lobe –> mushroom body (associated with learning) or lateral horn (associated with innate response), both via projection neurons.
Insects also have receptors on their maxillary palps
Flour beetle
Not all insects are flies - there are differences. Tribolium castaneum have different brain structure to Drosophila.
Antenna and maxillary palps express not only ORs but IRs and GRs too.
Have a gnathal olfactory centre, unlike Drosophila. Both the ORs and GRs project here, whereas in Drosophila they are separated.
Mouse and rat olfactory bulb, bee
There is spatial distribution of glomeruli in the OB based on different classes of OSN. Different types of chemicals detected in different areas.
E.g. top part associated with spoiled food aversion.
Interconnections between glomeruli.
In rats, it has been shown that patterns of glomerular activity change with odour concentration (basis for detection of concentration). MORE glomeruli also activated at higher concentrations –> more overlap. However discrimination is better at higher concentrations?
Bee also has spatial coding. Patterns are associated with functional group and size of molecules. Study showed that the patterns of the glomeruli affect behavioural responses to odours. E.g. trained with odours to extend proboscis. Odours closely represented in the brain more likely to cause response.
Lateral inhibition and timing
Synapses around glomeruli inhibit and amplify signals (GABA and glutamate). This sharpens the code - stronger signals are favoured. Temporal sequence of glomerular activation determines which stimuli are transmitted - timing determines the pattern of inhibition.
Has been demonstrated in fly antennal lobe stimulated with odours. Looked at change in calcium levels with odours on their own or combined. Combined response is not just addition of the two individual odour responses. Timing is important.
Primacy code
First glomeruli to be activated by an odour are those with most sensitive receptors. More important in identification of odours.
Study used opto to block later-activated glomeruli, finding that the first 100ms is key to odour identification.
Stimulus duration
Some mitral/tufted cells only respond if stimulus is of particular duration (demonstrated in optogenetic study using mice - can’t detect the smell if neurons activated by different duration of time).
