Olfactory mechanism Flashcards
What are the four main functions of odour in animal behavior?
- Kin recognition (e.g., mother sheep identifying their lambs)
- Danger signals (e.g., detecting smoke)
- Food identification (distinguishing edible vs. harmful)
- mate selection (pheromone-based mate choice)
How do mother sheep demonstrate the importance of scent for bonding?
• Mother sheep identify their lambs via scent
• Experiment: placing a mother’s lamb skin onto another lamb can make her accept the orphan
• Highlights strong role of odour cues in maternal behavior
What is an odorant?
• A low molecular weight, volatile compound that can be detected by the olfactory system
what is an odour
- either a single volatile compound or a complex mixture of other odourants
How do odours differ from tastes (gustation)?
• Odours are volatile, detected by the olfactory system
• Tastes are typically non-volatile, detected by taste receptors
Why are natural odours often considered complex mixtures?
• They can contain dozens or hundreds of compounds (e.g., rose scent with 30–400 components)
• The proportions and interactions of these compounds create unique olfactory “gestalts”
What two key features does the olfactory system encode?
1) Molecular identity – which compounds are present
2) Concentration – the amount of each compound
How does turbulence affect the distribution of odours in the environment?
- Odours travel in turbulent plumes, creating patchy, intermittent concentrations
- non linear relationship between scent and distance
- High concentration areas loacted next to low con areas
- in general high conc = closer to source
- not a simple conc gradient
What does gas chromatography reveal about odour composition?
- It separates volatile compounds, showing multiple peaks
- High-concentration compounds aren’t always the strongest perceived;
- receptor sensitivity matters
what is gestalt
- original percept that is more than the sum of its parts
- Mixtures of odour molecules can form olfactory gestalt
What was demonstrated by honeybee training studies (Guerrieri et al., 2005)?
- Bees trained on a specific odour (e.g., 1-hexanol) showed reduced recognition with changes in carbon chain length
- Suggests bees generalize odours based on molecular similarity
- small chemical difference can affect odour recognition
How do enantiomers illustrate odour specificity?
- R- and S-enantiomers of carvone smell different (spearmint vs. caraway)
- The mirror-image structure changes how molecules bind to olfactory receptors
- so even same molecular structure, but spatially imposed/orientation can lead to different smells
general representation of the olfactory system
- Mammalian OSNs: Found in the olfactory epithelium.
- Insect OSNs: Located on the antennae.
- C. elegans OSNs: Can express multiple receptor types within a single neuron (unlike mammals/insects).
What is the mammalian and insect olfactory pathway?
1) Odour detection by olfactory sensory neurons (OSNs) in the nasal epithelium or antennae
2) Signals relayed to the olfactory bulb or antennal lobe
3) Mitral cells (mammals) or projection neurons (insects) project to the olfactory cortex
4) Further processing by kenyon cells (mammals) or lateral horn cells (insects) in the thalamus and higher brain regions
name differences of the olfatcory pathway
- peripheral organs of olfactory system in mammals = nasal epithelium, in insects = antennae
- sensory neurons = OSNs in both
- 1st relay = antennal lobe (insects); olfactory bulb = mammals : all in brain
- 1st relay -> 2nd relay neurons in mammals = mitral cells, in insects projection neurons
- 2nd relay = olfactory cortex (mammals). mushroom body (insects): all in brain
- 2nd -> 3rd relay uses kenyon cells in mammals or lateral horn cells in insects
- 3rd = thalamus (mammals), lateral horn (insects)
How does the insect olfactory pathway differ from the mammalian pathway?
- Insects have OSNs on antennae, projecting to the antennal lobe
- Projection neurons then carry signals to mushroom bodies
- Further relays carried by projection neurons or lateral horn cells go to the lateral horn
In what way does C. elegans differ in its olfactory neuron setup?
• A single neuron can express multiple receptor types
• Mammals/insects usually have one receptor type per OSN
How is the olfactory periphery generally conserved across animal phyla?
- Both mice and Drosophila share similar mechanisms in their olfactory periphery
- One sensory neuron typically corresponds to one type of receptor
- Thousands of these specific receptor-neuron pairs exist, each specialized for particular compounds
What is the principle behind ‘one neuron, one receptor’ in olfactory systems?
- Each olfactory receptor neuron (ORN or OSN they are same thing) expresses only a single type of receptor
- ORNs that express the same receptor converge onto the same glomerulus in the brain
Why do all ORNs expressing the same receptor type converge to a single glomerulus?
- This convergence organizes sensory input by receptor type
- In mammals, they converge in glomerus in olfactory bulb; in insects, in the glomerulus of antennal lobe
- Each glomerulus receives input from one receptor type, streamlining early neural processing
What is the role of OSNs or ORNs in glomerular convergence and early neural processing?
- OSNs are same as ORNs (they are the same thing, used interchangeable)expressing the same receptor type converge onto a single glomerulus
- Glomeruli contain local interneurons and projection neurons
- In insects, a ‘bottleneck’ occurs: many sensory neurons feed into fewer projection neurons, refining odour signals before higher processing
what is glomerulus
- Glomerulus is a structure in the brain which contains lots of dendrites
- organization where is receives information from sensory neurons (also 1-1 with receptor type)
- contains projection neurons which relay information to mushroom body or olfactory cortex
- also contain interneuorns which communicate
what is the bottleneck effect of convergence in processing sensory information
- In insects, a ‘bottleneck’ occurs: many sensory neurons feed into fewer projection neurons, refining odour signals before higher processing
What role do projection neurons play in the insect olfactory pathway?
- They receive input from ORNs or OSNs in glomeruli in the antennal lobe
- They transmit processed signals to higher centers like the mushroom body and lateral horn (in insects)
- This step helps integrate and refine odour information
what olfactory systems do mice have
- they have a more general olfactory system which recognizes scent and food smalle
- they have another specialised vomeronasal system (olfactory system) which can recognize and detect olfactory sexual signals
What is the vomeronasal system in mice, and how does it differ from the general olfactory system?
- It is a specialized olfactory system for detecting sexual signals
- Located in a specialized area of the palate which detects only sexual olfactory signals and projects to the accessory olfactory bulb (AOB)
- Unlike the general system, its signals are sent only to the amygdala for emotional/sexual behavior processing
How does the general olfactory system in mice project information through the brain?
- OSNs → Olfactory bulb → Various brain regions (hippocampus, thalamus, amygdala, cortex)
- Uniquely, the olfactory bulb directly projects to the piriform cortex (bypassing the thalamus first)
- This direct route allows quick memory recall and rapid behavioral responses
in mice, what areas does the projection neurons or mitral cells project informations to higher regions of the brain
- hippocampus
- thalamus
- amydaloid: emotion
- cortex
- these are areas used for spatial learning, memory, secretion of hormone and fear -> meaning olfaction can influence and have connections w these functions
what is special about the 3rd relay projection in mice
- mitral cells or neurons can surpass thalamus and send signal directly to cortex (cannot dop this for other senses like gustation, they have to go thru thalamus first)
- cortex used for memories -> direct projection allow quick recall of memory and reminder of that scent (and associated memories)
- quick decision abt danger avoidance
Why is the direct projection from the olfactory bulb to the cortex considered an ‘ancient’ pathway?
• Olfaction is one of the earliest evolved senses in animals
• Early organisms needed to detect volatile compounds for survival (e.g., finding food, avoiding danger)
• The direct pathway to the cortex supports rapid recognition and memory association of scents
Where are olfactory sensory neurons (OSNs) located in insects, and how do they detect odours?
- OSNs are housed within sensilla (specialized hair-like structures) (surface, a bit like the nasal region in mammals)
- Odour molecules enter through pores and bind to carrier proteins
- These proteins then bind to receptors on the OSN dendrites
What specialized region in insect antennae processes pheromones?
- Male and female insects often have a dedicated portion of the antennal lobe for pheromone detection
- In honeybees, this region handles alarm and kin pheromones
- In moths, this region can detect sex pheromones that respond to sex pheromone compounds
how do moths detect sex pheromones
- specialised region of antennal lobe devoted to detecting pheromones
- male moth sex pheromones are deetcted using macroglomerular complex
- macroglomerular complex is a very specialised, and only sensory neurons that detect sex pheromones can repond to sex pheromone compounds
- they can detect + respond to very low con of sex pheromones cuz macroglomerular complex functions to amplify it
what is macroglomerular complex
- in male moths
- macroglomerular complex is a very specialised, and only sensory neurons that detect sex pheromones can repond to sex pheromone compounds
- they can detect + respond to very low con of sex pheromones cuz macroglomerular complex functions to amplify it
How do insect OSNs typically transmit signals to higher brain centers?
• A ‘bottleneck’ occurs where many OSNs feed into fewer projection neurons
• These projection neurons then send information to the mushroom body (learning/memory) and lateral horn (innate responses)
• This structure refines and integrates odour information before it reaches higher centers
how are signals projected from periphery olfactory organ to the brain
- periphery organs have olfactory receptor neurons (ORNs)
- they are excited by receptor binidng compounds and cause action potential or gated potential (G-protein in mammals; ligan gated ion channels in insects)
- action potential or gated potentials sends signal to glomerulus in olfactory bulb (or equivalent)
What are the signal transduction mechanisms in mammals?
- Mammalian ORNs use G protein-coupled receptors (GPCRs)
- Odour binding triggers a cAMP cascade
- Calcium influx leads to action potentials and signal amplification
- leads to channel openings and signal sent to glomerulus then sent to higher brain regions
How do insect olfactory receptors differ from mammalian GPCR-based receptors?
- Insect receptors are ligand-gated ion channels with two subunits
- One subunit (e.g., OR83b) allows fast ion flow
- Another subunit (ORX) functions as a slower G protein-coupled component, affecting calcium levels
- they use heteromeric receptor complexes
What is temporal coding in olfaction, and how do ORNs use it?
• Temporal coding refers to how neurons fire over time in response to odours
• ORNs can be excited or inhibited by specific odours
• The pattern and frequency of spiking encodes odour identity and intensity
How does spiking frequency relate to odour concentration?
- Higher concentration of an odour → Higher firing rate
- Once the neuron reaches maximum capacity, the firing rate plateaus
- This relationship allows the ORN to signal both presence and intensity of an odour
- this spiking happens at the ORN 1st relay
What did the de Bruine et al. (2001) study on Drosophila ORNs reveal?
- A tested ORN responded strongly to the aldehyde E-2-hexenal
- As concentration increased, spiking frequency increased until it plateaued
- Different odours showed varying sensitivities, illustrating how neurons encode both identity and intensity
- spiking happens at ORNs 1st relay
What is the three-stage olfactory pathway?
1) First Relay: OSNs in nasal epithelium (mammals) or antennae (insects)
- Bind odour molecules, generate action potentials
2) Second Relay: Antennal lobe (insects) or olfactory bulb (mammals)
- ORNs wuth same receptor type converge onto a single glomeurli (both mammals and insects)
- Glomeruli organize and refine odour signals send to higher brain centres via projector neurons or mitral cells
3) Higher Centers:
- Mammals → Cortex, amygdala, hippocampus, thalamus
- Insects → Mushroom body (learning/memory), lateral horn (innate responses)
function of glomeruli
- receive input from ORNs expressing same receptor type (1-to-1 relation)
- organise and refine signal to send to higher brain centres
What is the role of local interneurons in glomeruli?
• They can be inhibitory (spanning multiple glomeruli) or excitatory (often targeting specific glomeruli)
• Inhibitory interneurons help prevent sensory overload by gating certain signals
• Excitatory interneurons can enhance or sharpen specific odour signals
Why is inhibition of interneurons at the glomeruli (antennal lobe orolfactory bulb) critical for odour discrimination?
- Inhibitory neurons (type of interneurons at olfactory bulb or antennal lobe) filter and refine sensory input, preventing all neurons from firing at once
- combining inhibitions w excitations this creates an output
- This selective gating allows the system to distinguish between similar odours
- Without inhibition, oscillatory patterns weaken, leading to impaired odour discrimination
what produces oscillatory patterns
- each projection neurons -> higher brain centres spikes differently over time, can produce temporal code by varying firing time and temporal code and hence creating oscillation patterns
- LFP recordings reveal that odour stimulation changes amplitude of neural activities
How do projection neurons contribute to temporal coding in the olfactory system?
- Projection neurons relay the processed signals from the olfactory bulb/antennal lobe to higher brain centers
- They exhibit complex firing patterns over time (temporal code) depends on
- Different projection neurons may respond at different times or rates to the same odour, creating a rich, time-varying signal
flow chart for compiles process of mammals olfatcory system
Odour Molecules
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Olfactory Epithelium:
- Odour molecules bind to receptors on ORNs (each ORN expresses one receptor type)
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Action Potential Generation in ORNs
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Glomerular Convergence in the Olfactory Bulb:
- ORNs expressing the same receptor converge onto a single glomerulus
- Local interneurons (inhibitory & excitatory) modulate the signal:
* Inhibitory interneurons: Provide lateral inhibition to sharpen signal
* Excitatory interneurons: Enhance specific signals
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Temporal Coding & Oscillations in the Olfactory Bulb:
- Mitral/Tufted cells (projection neurons) fire in patterns modulated by inhibition
- Oscillatory activity and synchrony encode odour identity and intensity
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Projection to Higher Brain Centers:
- Direct projection to the Piriform Cortex (unique to olfaction)
- Additional projections to:
* Amygdala (emotional/pheromonal processing)
* Hippocampus (memory)
* Thalamus (further sensory integration)
flow chart compiled olfactory system of insects
Odour Molecules
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Sensilla on Antennae:
- Odour molecules enter through pores
- Bind to carrier proteins which deliver them to receptors on OSNs
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Action Potential Generation in OSNs
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Glomerular Convergence in the Antennal Lobe:
- OSNs with the same receptor converge onto the same glomerulus
- Local interneurons (inhibitory & excitatory) shape the incoming signal:
* Inhibitory interneurons: Create gating and prevent sensory overload
* Excitatory interneurons: Amplify specific odour signals
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Temporal Coding & Oscillations in the Antennal Lobe:
- Projection neurons exhibit complex, time-varying spiking patterns
- Synchronized oscillations encode both odour identity and concentration
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Projection to Higher Brain Centers:
- Signals are relayed via projection neurons to:
* Mushroom Body (learning/memory)
* Lateral Horn (innate behavioral responses)
what do recordings from projection neurons show in response to odours
- they show complex patten of activity
- each different neuron has a different response output to the odor, such as temporal firings differ (they fire at different times) showing the temporal code
- produces a unique output to brain for odour discrimination
What did Gilles Laurent’s lab discover about oscillatory patterns in olfactory coding?
- Recording from projection neurons showed increased synchrony and oscillation during odour presentation, and showed temporal code for spiking patterns
- After odour offset, activity returned to baseline but remained altered
- Blocking inhibition disrupted these oscillations, impairing odour discrimination
- showing inhibition and excitation produce odor presentation
What is the relationship between temporal coding and oscillations?
• Neurons fire in rhythmic patterns when an odour is present
• Synchrony across neurons encodes odour identity and intensity
• Inhibition is key to generating these oscillations
How do mice exhibit similar temporal coding phenomena as insects?
• Mitral cells in the olfactory bulb show oscillatory firing modulated by inhibitory interneurons
• Local Field Potential (LFP) recordings reveal increased amplitude during odour presentation
• Higher odour concentrations produce stronger or longer oscillations
How can odour identity and concentration be visualized using population activity?
• Different odours evoke distinct spiking patterns across multiple projection neurons
• Principal Component Analysis (PCA) can cluster these patterns by odour identity and concentration
• Suggests that the collective firing across the antennal lobe or olfactory bulb encodes odour information
What is sparse coding in higher brain centers?
• A coding strategy where only a small subset of neurons (e.g., Kenyon cells) become active for a given odour
• Requires simultaneous input from multiple projection neurons to overcome strong inhibition
• Prevents the brain from being flooded with unnecessary signals
what are kenyon cells/cortical neurons?
- kenyon cells (insects) located in mushroom body
- cortical neuron (mammals) located in cortex
- they receive input and spikes from projection neurons
- kenyon cells are under strong inhibition by lateral horn cells, and only when they are receive multiple spikes/input from many projection neurons simultaneously (summation) do they fire
why are kenyon cells strongly inhibited
- they are strongly inhibited by lateral horn cells
- so that they only spike when they receive a lot of spontaneous spikes/firings from projection neurons
- this is to prevent the brain from being overwhelmed by redundant information
How does inhibition shape sparse coding in Kenyon cells or olfactory cortex neurons?
- Lateral horn cells (in insects) or local circuits (in mammals) strongly inhibit kenyon cells/cortical neurons
- Kenyon/cortical cells stay silent unless multiple inputs simultaneous and multiple
- Ensures only selective response to meaningful odours
what is the sparse code
- selective activation of Kenyon cell/cortical neuron
- so that they are only activated when they receive multiple spikes spontaneously from projection neurons (when they reach a threshold)
- prevents overwhelming the brain with redundant information
- there fore they only coherent input across multiple projection neurons triggers a response
What are the key points in the overall summary of olfactory processing?
• Odour importance: kin recognition, danger signals, food identification, sexual signaling
• OSNs & receptor specificity: one receptor type per neuron, converging onto glomeruli
• Temporal coding & oscillations: synchronized firing patterns crucial for discrimination
• Sparse coding: higher centers require simultaneous input, ensuring distinct odour representations
What are some potential essay questions to explore based on these topics?
1) What is an odour, and what are its relevant features?
2) How do vertebrate and invertebrate olfactory systems compare, and what special systems exist?
3) Describe the process of odour encoding at each phase of the olfactory pathway.
4) How are odours encoded and represented at each stage in the olfactory system.
- Breaking down how each phase shows odour recognition
1) At ORN in antennae/periphery = the pattern and rate of spiking reveals information of concentration, but the spatial location doesn’t reveal much about the odour (unlike in gustation the position of the togue reveals some information about the taste) - each ORN binds to target odour not in specialised regions
2) At the antennal lobe/olfactory bub = pattern/rate of spiking by the projection neurons show temporal code, and the increase in amplitude of oscillation also reveal whether odour is detected and what type of odour. Spatial location/map= which glomeruli it converges to is unique and differentiates the odour (1 type of receptor to 1 glomeruli)
3) At the mushroom body = the activation of Kenyon cells/cortical cells show that the stimulus/odour is robust, but the spatial location doesn’t reveal any information (i.e being at the mushroom body doesn’t mean it a certain compound is detected)
