Mapping of odor-related neuronal activity in the olfactory bulb by
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Transcript of The sense of smell Outline Main Olfactory System Odor Detection Odor Coding Accessory Olfactory...
The sense of smell
Outline
Main Olfactory System
Odor Detection
Odor Coding
Accessory Olfactory System
Pheromone Detection
Pheromone Coding
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Human experiment: How well do we taste without smell?
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Brief Anatomy of the Olfactory System
mucus
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Olfactory neurons respond to odors
Cineole Isoamyl Acetate Acetophenone
Firestein et al., J Physiol. 1993
neuron 1
neuron 2
neuron 3
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Models for Olfactory recognition
A. Vibrations of odor particles cause neurons to resonate at different frequencies
B. One receptor recognizes all odors with different affinities
C. One receptor recognizes a few odors; many different receptors to sense the different smells
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Olfactory Receptors are G Protein Coupled Receptors (GPCRs)
• largest family of receptors• hydrophobic/ hydrophilic
domains• seven transmembrane regions• Ligand-binding domain in plane
of membrane (TM3,5,6)• G protein binding domain in
loop 3 (btwn TM 5 and 6)
and C-terminus
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Humans have about 370 odorant receptorsMice have about 1000
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Each olfactory neuron contains one receptor
1. Each receptor is in 0.1% of cells2. Ten receptors are in 1% of cells3. Isolated olfactory neuron makes mRNA for only one receptor
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How is olfactory information mapped in the bulb?
Receptor map (labeled lines) Complex map (mixed lines)
In the Nose:Neurons express one receptorNeurons with the same receptor are in random locations
Topographic map?Odorant receptor map?Complex map?
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Neurons with the same receptor (blue) send axons to the same place
Epithelium to bulb Bulb cross-section
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Neurons with same receptor send axons to one glomerulus
Neurons with different receptors project to different glomeruli
The olfactory system uses labeled linesMap of odor receptors!!!
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• Fruit flies have 60 different receptors• Neurons with the same receptor project to one glomerulus
Glomerular structure is a general feature of olfactory systems
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Epithelium
Bulb
Basic Feature of Coding in the Olfactory Bulb
Olfactory neurons express 1 out of 1000 receptors
Neurons with the same receptorconverge on single glomeruliin olfactory bulb.
The glomeruli serve as modules, and are selectivelysensitive to particular odors
Model: Different odors activate different brain regions13
Response of Olfactory Bulbs to an Odorant Molecule, withthe Use of a Voltage Sensitive Dye.
Technologies: calcium-sensitive dyes, voltage-sensitive dyes and intrinsic signals (changes in blood flow, oxygen levels)
Results:•Odors activate a few glomeruli•Same glomeruli activated on repeated exposure•Different odors activate different glomeruli
Is there a map of different smells in the brain?
Rat olfactory bulb, olfactory imaging
Raw data C6 odor 2 times C3 odor then C6 odor
Plan: monitor neural activation in the brain
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How does an olfactory neuron choose only one receptor?
How do neurons find the right glomerulus?
How are odors processed by the brain?
Open questions in olfaction
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The receptor is involved in axon guidance
Schematics of the olfactory bulbReplace one receptor with another, targeting changes
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Signal Transduction in the Olfactory Epithelium
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Cellular Structure of Olfactory Bulbs
Lateral Olfactory Tract
Glomeruli
Olfactory Nerve
Periglomerular Cells
Granule Cells
Mitral/Tufted Cells
Receptor Cells
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Central Pathways of the Olfactory System
1. Olfactory sensory neurons project directly to the brain
2. From the olfactory bulb, information is sent to five different brain regions
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People sniff when they imagine pleasant odors
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Accessory Olfactory System
The Second Nose: the Vomeronasal organ
• detects pheromones (chemical cues secreted by animals)
• best evidence of pheromones in insects, many mammals
• triggers stereotyped behaviors (mating and fighting)
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The two noses in a mouse
(vomeronasal organ)22
How does the How does the VNO sense pheromones?
• Two large families of receptors (GPCRS)
• Not related to olfactory receptors
• Logic similar: one receptor per cell
• Projections different: neurons with same receptor project to many little glomeruli
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Anatomy in the Vomeronasal System
One receptor per neuronNeurons with same receptor project to many “glomeruli”
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Signal Transduction in the Vomeronasal System
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What happens to mice when their VNO doesn’t work?
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Do humans sense pheromones thru the VNO?
• There is a VNO• There is not an Accessory Olfactory Bulb• TRP channel is a pseudogene• VNO receptors are pseudogenes
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Main points about the olfactory system
1) Lots of receptors
2) One receptor per cell
3) Labeled lines in the olfactory bulb
4) Maps of different smells
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