Olfaction Flashcards
What is the importance of the chemical senses?
Allows organisms to recognise, evaluate and distinguish between food resources and dangerous substances.
What do olfactory systems detect?
Airborne inorganic and organic chemicals (odors).
What do odors mediate?
Innate and learned behaviours
e.g. aversion, attraction, mating, foraging, feeding, escape from predators.
What are the 4 principles that olfactory systems use for odor discrimination?
- Large number of odorant receptors dedicated to olfaction
- Each olfactory sensory neuron usually only expresses one OR / each OR has distinct odor ligand profile
- OSNs expressing same receptor extend axons that converge on same glomerulus
- Each odor is encoded combinatorially
(one odor activates multiple ORs / each OR can respond to multiple odors)
Mammalian Olfactory Receptors (ORs). Structure and Function?
7 TMS GPCRs
Binding results in GPCR signalling + neuoronal depolarisation.
Insect ORs. Structure and Function?
Evolutionary distinct from mammalian.
Thought to be odor-gated cation channels, may be modulated by G-protein signalling.
OSNs anatomically similar to mammalian.
How are odors encoded?
Combinatorially
= one odor can activate multiple ORs
= each OR can respond to many odors
How is odor information encoded?
Spatially encoded
= produces topographical olfactory map in the brain.
What does each olfactroy sensory neuron (OSN) express?
Usually only a single OR / each OR has a distinct odor ligand profile.
OSNs expressing same receptor extend axons that converge on the same glomerulus in the brain (spatial mapping).
How are synapses organised in olfactory systems?
Into spherical neuropils.
Olfactory glomeruli = connect sensory input with output neurons and local modulatory interneurons.
What problem do olfactory glomeruli solve?
The problem of mapping discontinuous chemical space onto the brain.
What smells are flies attracted to?
Vinegar, yeast, rotting fruit, each other.
What was the first chemosensory behaviour mutant studied?
acj6
Obaid Siddiqi , 1980s
= found to be key transcription factor in regulation of a subset of odorant receptor genes.
How many ORs does fly genome have? How have they been analysed?
62 ORs , encoded by 60 genes.
Analysed at level of:
= expression pattern on olfactory sensory neurons
= functional response to odors
= anatomical mapping to antennal lobe (higher brain centre)
What is the structure of the Drosophila Olfactory Organs?
What is the distribution of Drosophila Odorant Receptors?
What is the distribution of Odorant Binding Proteins?
What is the organisation of Drosophila Odor Coding?
Axons expressing same OR converge on the glomeruli in the antennal lobe.
Excitatory projection neuron sends dendrites to single glomerulus.
Inhibitory neurons project to multiple glomeruli.
What are exceptions to the Drosophila odor coding rule?
Co-convergence.
Co-expression.
What are the parallels in olfactory circuit wiring in mammals and insects?
Structure of the drosophila olfactory circuit?
MB = mushroom bodies
LH = lateral horn
SOG = suboesophageal ganglia
AL = antennal lobe
What are the 2 stages of Drosophila Odor Discrimation?
Decoder Stage = used to influence behaviour.
Encoder Stage = receives input from antenna / maxillary palp.
