Lecture 2 - Olfaction

Question 1

Why is it important for olfactory systems to discriminate between odours?

Answer 1

Odors mediate innate and learned behaviours such as aversion and attraction, mating, feeding, and escape from predators.

Question 2

What are four principles that olfactory systems use for odour discrimination?

1. Large numbers of … dedicated to olfaction.
2. Each … usually expresses only a single .. and each .. has a distinct … (it will bind to a number of different chemicals/odors).
3. … expressing the same .. extend axons that converge on the same … in the brain.
4. Each odour is encoded … So, one odour can activate … and each OR can respond to multiple odours.

Answer 2

1. Large numbers of odorant receptor (OR) genes dedicated to olfaction.
2. Each olfactory sensory/receptor neuron (OSN/ORN) usually expresses only a single OR and each OR has a distinct odor ligand profile (it will bind to a number of different chemicals/odors).
3. ORNs expressing the same OR (responding to the same odor ligand profile) extend axons that converge on the same glomerulus in the brain.
4. Each odor is encoded combinatorially – one odor can activate multiple ORs and each OR can respond to multiple odors.

Question 3

How is odour information spatially encoded to produce a topographical olfactory map?

Answer 3

Each ORN expresses a single OR and each OR has a distinct odour ligand profile. ORNs expressing the same OR extend axons that converge on the same glomerulus in the brain.

Question 4

Olfactory receptors on ORNs are GCPRs in mammalians. Insect ORNs are anatomically similar to mammalian ORNs but the GCPRs are thought to be … that may be modulated by … signalling.

Answer 4

Insect ORNs are anatomically similar to mammalian ORNs but the GCPRs are thought to be odour-gated cation channels that may be modulated by g-protein signalling.

Question 5

How do olfactory glomeruli solve the problem of mapping discontinuous chemical space onto the brain? Think of convergence.

–How do olfactory glomeruli help in brain mapping–

Answer 5

Olfactory circuits are wired according to the convergence of axons from sensory neurons expressing the same OR. The synapses are organised into olfactory glomeruli.

Question 6

Sensilla are located on olfaction organs and have pores so that the odours can get to the dendrites of the ORN. Where are the olfaction organs located in flies?

Answer 6

Olfaction organs are located on antenna and maxillary palp.

Question 7

What are the three types of sensilla (basiconic, trichoid and coelonic) specialised in?

Answer 7

Basiconic: food odours

Trichoid: pheromones

Coelonic: food odours

Question 8

What is the co-receptor of ORs called in flies?

Answer 8

Or83b. It is necessary and sufficient to mediate both the ciliary targeting and functional expression of ORs.

Question 9

Flies have taste receptors in mouth, legs and wings. Their … is used for feeding. The gustatory receptor neurons project to … , where taste is processed.

Answer 9

Their prob​oscis is used for feeding. The gustatory receptor neurons project to sub-esophageal ganglion (SOG), where taste is processed.

Question 10

Explain the exceptions of Drosophila odour coding: co-convergence and co-expression.

Answer 10

Co-convergence = different odours going to the same glomerulus.

Co-expression = combining two smells and connecting in one glomerulus.

Question 11

Describe the olfactory circuit. Use ORNs, MBs, LH, etc.

Answer 11

(1) ORNs expressing the same OR converge into the same glomeruli.
(2) The information is conveyed to local interneurons (LNs) and projection neurons (PNs).
(3) From PNs information is sent to higher processing centers in the brain (mushroom bodies and lateral horn).

Question 12

What is a consequence of the glomerulus receiving input from ORNs i.e. odour information?

Answer 12

Odour information is constrained to the distinct ligands of the ORs.

Question 13

Why is information being sent to local interneurons (LNs)?

Answer 13

PNs display more complex firing patterns, so cross-talk has to happen in AL: horizontal connections among glomeruli through LNs.

Major role of LN network = synchronise PN activity.

Question 14

How is the olfactory circuit different in mammals compared to insects?

Answer 14

Mammals: olfactory epithelium –> olfactory bulb –> piriform context, olfactory tubercle and entorhinal cortex.

Insects: antenna, maxillary palp –> antennal lobe (glomeruli) –> MB calyx and LH.

Question 15

Name some functions of the MBs and LH.

Answer 15

MBs are integrative centres controlling various functions: olfactory learning, locomotor activity, male courtship behaviour, and sleep.

The LH seems to be involved in experience independent odour recognition.

Question 16

What are Kenyon cells?

Answer 16

PNs synapse with MB neurons, called Kenyon cells.

Question 17

Topographic map of olfactory information is retained in higher olfactory centres, but the character of the map differs from the AL: How are PNs mapped in MB calyx and LH?

Answer 17

In MB calyx: concentrically in target zones deriving from specific AL glomeruli.

In LH: in axes.

Question 18

How are MBs capable of olfactory learning? (Think of: PN zones, interpretation of PN activity, etc.)

Answer 18

Dendrites of Kenyon cells collectively cover the concentric PN zones in the brain. These neurons may interpret combined PN activity, which means that MBs are capable of integrating a wide range of odour information. This is consistent with their role in olfactory learning.

Question 19

What role does LH play in direct behavioural responses to odours? (Think of: PN target areas, etc.)

Answer 19

3rd order neurons in LH are constrained within a particular PN target area. These areas are linked with different brain areas. That’s how distinct subsets of glomeruli are associated with particular brain regions that ultimately control odour-driven behaviour.

Question 20

Name a big difference between adult olfactory circuit and larval olfactory circuit.

Answer 20

1. Larval olfactory circuit lacks convergent and divergent connectivity.
2. Larval olfactory circuit contains less ORNs.
3. These mentioned factors reduce olfaction sensitivity in larvae.

Question 21

Which two major genes are expressed in a very large number of overlapping gustation receptor neurons (GRNs)?

Answer 21

Gr5a and Gr66a.

Question 22

What are the consequences of lacking Gr5a and Gr66a?

Answer 22

Lacking Gr5a: not attracted to sugar and low salt concentrations.

Lacking Gr66a: failure to avoid bitterness and high concentrations of salt.

Question 23

GRNs located in different organs and body all target the SOG. Fill in the topomap:

… dorsal SOG = internal taste organs.

Central and … SOG = proboscis and leg.

Answer 23

Anterior dorsal SOG = internal taste organs.

Central and posterior SOG = proboscis and leg.

Question 24

Gr5a projections are sent to … side of SOG. Gr66a projections converge in …. web in … SOG.

Answer 24

Gr5a projections are sent to ipsilateral side of SOG. Gr66a projections converge in ring-like web in medial SOG.