Week 2 - Olfaction Flashcards
Olfaction
Olfaction from the L.olfacere (to smell)
Is one of our 2 chemical senses
Is classed an (ancient) sense (primitive to some)
Why is olfaction important?
For many animals olfaction is essential to survive (know friends, enimies, territories, find mates, reproduce, locate food etc
For humans there is a degree of survival and safety hoewver, smell is largely about novelty
Novelty is key in helping us recognise new smells
What are the differences in the olfactory brain areas of rats and humans ?
Rats have a considerably larger olfactory bulb and piriform (or olfactory) cortex compared to humans
Compare Dogs and Humans Olfaction
Dogs have 40 x the number of olfactory bulb neurons than humans and 1,000,000x more olfactory receptor neurons
What is the olfactory stimulus
Odorants
What are Odorants
= Volatile Chemical Compounds (meaning they’re predisposed to easily aerosolise
Sufficient quantity of a given odorant is required to elicit its smell
What is an Odour
Typically a mixture of several odorants
How do we code smells?
- Population coding
Several odourants can act on a population of neurons in the olfactory epithelium
These patterns of activity can generate smells
Odor Thresholds
Different odorants have a different detection threshold.
eg. Methyl Mercaptan has a very low detection threshold - meaning a very small amount of it can be detected
Ethyl ether and chloroform however are more difficult to detect as they have a higher detection threshold (more in the air is required to detect them)
Odor JND’s
In olfaction our JND’s are large. We are note good at discriminating odour intensities
Impact of Dementia on Olfactory JND’s - Hidalgo et al., 2011
JND aka the ability to discriminate between different odour intensities gets worse in people with dementia. this is one of the earliest signs of the condition
Steps in Olfaction
Is to take in the odourant with smell. In this process an odourant travels the nasal cavity to the olfactory epithelium
Odourant in the air –> Is inhaled through sniffing –> Places the odouran tin the nasal cavity
Once the odourant reaches the olfactory epithelium it is temporarily held in the mucus of this region allowing for binding to the receptors of the olfactory receptor neurons
The olfactory receptor neurons project cilia into the mucus membrane of the olfactory epithelium to detect inhaled odourants. This is the site of signal transduction
The olfactory receptor neurons project cilia into the mucus membrane of the olfactory epithelium to detect inhaled odourants. This is the site of signal transduction
Signal Transduction in Olfaction
- The odorant molecule binds to and activates an odorant receptor on the olfactory receptor neurons which initiates an electrical signal
There is type of receptor per olfactory receptor neuron, however these can multiple types of odorant
There is ~350 orn types (with different receptors)
As an odorant can bind multiple receptors, on therefore different ORN types, there is a population coding scheme, as multiple neurons can bind the same odorant and will fire and different intensities in response to this.
Anosmia and COVID
Anosmia = partial loss of smell
Present in 50% of COVID-19 cases and usually is temporary however 7% of COVID cases remain anosmic after 1 year
(most common in early and Delta strands)
Research shows this is not caused by a loss of olfactory receptor neurons but instead an attacking of the support cells byt eh virus.
As support cells are needed for trophic support, when impacted, neighbouring neurons are impacted and stop producing odorant binding proteins resulting in the loss of smell
Other factors can contribute too - eg. atrophy of regions of the cortex
Glomeruli in the Olfactory System
The glomerulus (plural glomeruli) is a spherical structure located in the olfactory bulb of the brain where synapses form between the terminals of the olfactory nerve and the dendrites of mitral, periglomerular and tufted cells.
Mitral cells
Mitral cells are neurons that are part of the olfactory system. They are located in the olfactory bulb in the mammalian central nervous system. They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomeruli.
These cells project their axons into the brain that synapse with neurons in the olfactory cortex
Synaptic Pathway in olfactory system
Olfactory Receptor Neurons –> Mitral cells (in olfactory bulb –> Olfactory Cortical Neurons in the olf cortex
Population Coding of the Olfactory System
Different populations of cells are active in each step of the olfactory system which helps to code different smells
- Theres a different recognition pattern across ORNs due to the different types of ORN and their relative activation will vary depending on the odourant
- The unique pattern across the mital cells as certain types of ORNS will synapse to certain types of Mitral cells
3) These Mitral cell pathways activate a specific pattern of olfactory cortical neurons
Each level of this pathway has its own computation
A population code emerges at each level and therefore there are different representations at every level
Olfactory Pathway from the Olfactory/Piriform Cortex
Olf cx has strong links to the temporal love (hippocampus and amygdala and the Orbitofrontal cortex)
–> Amygdala (emotion)
–> Hippocampal Formation (memory)
–> Thalamus (relay to/from other areas, attention
–> Orbitofrontal cortex (OFC) (emotion recognition, behavioural inhibition, hedonic assesment aka if something is pleasurable or not, decision making, taste
Odour Memories
= Emotional, autobiographical memories evoked by an odour
eg. Proust effect - the vivid reliving of event from the past thorugh sensory stimuli (smelt a biscuit and thought of an Aunt)
Proust Effect
Odors have the exceptional ability to instantaneously trigger vivid autobiographical memories—a phenomenon referred to as the Proust effect. While other stimuli can also (obviously) make us recall past memories, they are usually not as detailed, sudden, or vivid as those related to smells.
What regions of the brain attributes the Proust Effect
Odors have the exceptional ability to instantaneously trigger vivid autobiographical memories—a phenomenon referred to as the Proust effect. While other stimuli can also (obviously) make us recall past memories, they are usually not as detailed, sudden, or vivid as those related to smells.
–> Amygdala and Hippocampus. (emotion and memory)
Memmories are the strongest when they’re emotional and scent based.
= the vivid reliving of event from the past through sensory stimuli
Hedonic Assessment & Decision Making from Olfaction
People have strong hedonic assessments when it comes to smell (like digusting something or loving it)
This may be linked to avoidance of rancid food and pathogens needed for survival.
This phenomenon however is shaped by past experience, exposure, familiarity vs novelty, internal state (physically and psychologically) and our contextual expectation
This is due to processing in the orbitofrontal cortex - one of the projections of the olfactory pathway
BODS (Luizza et al., 2017/2018)
Is the Body Odour Disgust Scale. This suggests there are individual differences in disgust reaction to a variety of body odours.
- Certain personality traits that are related to pathogen avoidance (eg. Authoritarianism
Expectation in Olfactory Hedonic Assessment ; Herz and Von Clef (2001)
Again utilises the OFC
- Contextual expectation plays a role in this. (the nose smells what the eyes sees)
Herz and Von Clef introd ambiguous odours with either a positive or negative name (eg. violet leaf extract called cucumber or gathered mildew)
The hedonic assessment of smell was strongly affected by the label
4 reasons odour discrimination studies is hard
- Chemicals can change odour depending on physical structure
Eg. Isomers have different smells (R- carvone = spearmint but S+ carvone =caraway seeds . This can be controlled in a lab setting - Concentration-dependent changes in odour perception
Eg. Low concentration amyl butyrate is fruity whilst high concentration is pungent - Individual differences
- odour ID- semantic recall (memory, language issues)
- Culture/exposure ( eg ayabe-kanamura Et al.,1998 Japanese liquorice study)
- language - lacking terminology makes labelling a scent difficult
- mood, attention, wakefulness, motivation (bias)
- biology - Ecological validity
- smell presented are by one is not reflective of real world
- reflective of real life?
- lack of context or cross modal hints can occur when presented in isolation
Odour detection and discrimintion and age (Goty et al., 1984)
- There is high individualisation in odour detection and discrimination
- Smelling was tracked over age which showed at age 50-60 olfaction sense began to declined and rapidly dropped from 60 yrs old onwards
- There’s no clear data on why this is particularly, as it may be due to lexical access, atrophy of the olfactory cortex etc etc
Odour detection and discrimination - genetics
Genetics can code for olfactory receptor neuron proteins.
To make an odourant receptor, you need DNA for that receptor as (DNA –> mRNA –> Proteins)
Without the gene for a given receptor, can result in a lack of the ability to smell that odour.
Genetic variation is common among olfactory receptor protein genes. Variation can result in a change in odourant perception. Simultaneously variation that leads to reduced receptor function = reduced odour intensity
Odour Detection and Discrimination - Genetics and Androstenone
Androstenone is a pheromone used in boars for mating. This was the first mammalian pheromone.
Has since been marketed for human use in sexual attraction. However, it does not act as a pheromone in humans.
As an odourant in humans, there is huge variation in response due to genetic differences (wysocki et al., 1984; 1989)
eg. One copy of the RT allele = ‘sweet, floral and pleasant’
two copies of the RT allele = sweaty, unpleasant, urine smell
As an odourant androstenone is one of the most common anosmias
- research (dorries et al., 1989) suggests detection of this scent is inversely related to sexual maturation in humans as children smell this most strongly and this reduces by 50% in adolescence
- Those who initially are anosmic to androstenone, can acquire a perception for this. Initially it was perceived 30-50% of anosmics, however, it is more likely to be like 2%. This was shown in a signal detection experiment which accounts for bias. It showed (bremner et al., 2003) that there is low confidence in responses for this scent, due to the pressure for a forced choice, detection responses were impacted
- Plasticity is also involved in the ability to become able to perceive androstenone. As exposure increases, the neural signals from the olfactory epithelium and the olfactory cortex increase as detection threshold decreases (example of rate coding)
Odour Discrimination and detection - Exposure - Dalton and Wysocki, 1996)
When exposed to a smell we do become desensitised to it.
This is hard to control
This research showed than there was reduced sensitivity to an odoured still evident 2 weeks after the last exposure
All humans are ___ to some odorants
anosmic
Problems with odour detection paradigms - individual differences
- smell detection is hugely variable according to age, sex, health status, as these factors influence olfactory receptor neurons and therefore bottom up processing
- nose anatomy - nasal airflow, or obstruction
- top down factors like emotional state or memory can impact this too - eg oldactory memory
Odour discrimination
Looks at the quality of a smell not the intensity.
So asks not whether a smell is present but instead the quality of the snell, eg. I’d it enjoyable , what is it? Etc
Seeks to explore how we discriminate and distinguish 1 trillion scents
Our ability to distinguish smells is often influenced by our cognition (memory & language skills).
Functions of Olfaction
- Detection of a stimulus in the environment
- Identification of traits or conditions of a stimuli
- Communication between con-specifics
Role of Olfactory detection
Smell helps us detect;
1. dangers
2. things in the environment
3. if we’re in a good or bad place (familiarity vs novelty)
Pheromones
Greek ; Bringer of excitement (pheromones were originally believed to be part of sex signalling but this is not always the case)
- Pheromones are non-volatile chemicals that can elicit specific responses in conspecifics (members of the same species)
- Pheromone based signalling is considered automatic and thus not influenced much but other factors
Pheromones should potentially have a broader definition
Pheromone Signalling Path
Pheromones (as non-volitile chemicals) are only passed in close proximity.
Pheromone signalling is thought to involve the vomeronasal organ which is part of the accessory olfactory system.
Pathway
the VNO is believed to be enriched in pheromone receptor cells which send projections along (CN 0) to the accessory olfactory bulb (or the comeronasal bulb) and from there to the amygdala and hypothalamus to release homrones associated with the response
THE VNO (Vomeronasal Organ)
Is the site of the pheromone receptor cells and considered part of the accessory olfactory system
Is an anatomical mystery as whilst it was first discovered in humans some people question its existance as they shrink (or get lost) as we get older, so can be hard to find
Is located in the anteroinferior portion of the nasal septum just above the roof of the mouth.
Is enriched in pheromone receptor cells
- It sends projections along Nerve 0 to the accessory olfactory bulb (aka vomeronasal bulb)
- From there to the amygdala and hypothalamus (for a release of hormones)
Candidate Pheromones
are chemicals that elicit or influence human behaviour like mating behaviour. They influence behaviour in a top down fashion
Do seem to affect Mate selection in humans
Receptors for candidate human pheromones all seem to have their receptors in the olfactory mucosa (not the VNO)
McClintock Effect
= Synchronisation of menstrual cycles
Is supposedly driven by pheromones, as demonstrated using ‘sniff pads’ from women’s underarms collected after 8hrs use at various points of menstrual cycle - wipe upper lip of sniffer daily
This data was controversial however, and there have been many failed replication studies.
1/3 of cycles didn’t change or even became out of sync and there were clear outliers biasing the data.
not to mention there still isn’t an identified human pheromone pathway
Olfactometry
is the standardised method for measuring odour detection
- A dilution instrument (olfactometer) presents and odour sample diluted with odour-free air at precise ratios
The Odour units (ou/m3) is numerically equivalent to the dilution factor necessary to reach the odour threshold for a given odourant
In olfactometry, the amount of odourant is increased until there is a physiological response, however, it’s hard to determine what’s suitable here (eg. eeg reading or subjective report?)
Problems with Odour Detection methods
- Detection is hugely variable according to age, sex, health status (all affect ORNS - thus affecting bottom-up processing)
- Noise anatomy (nasal obstruction?)
- Top down factors too like emotional state, or memory etc
- Can we account for these biases? Yes - Signal detection theory
Bushdid et al 2014
Bushdid et al., 2014 - Humans can discriminate more than 1 trillion olfactory stimuli (theoretically calculated off of the number of ORNS we have)
Laska et al., 2000 - Humans can outperform many species, even dogs, at identifying certain odours. Humans are in fact (Macrosmats) -aka excellence smells but this is only for certain smells (true for all animals
Russel 1976 - Your smell is well detected by yourself.75% of the time you’d be able to pick out a shirt that’s use out of a selection
McClintock et al., 2002 - women prefer the odor of males to whome they’re genetically similar to but not identical
Olsson et al.,2014 - Within hours of activating the immune system, humans elicit and aversive body odour (to trigger avoidance or as a help signal)
de Groot et al., 2012 - fear chemosignals generate fearful facial expressions and sensory acquisition (increased sniff magnitude and eye scanning); digust chemosignals evoke disgusted facial expressions and sensory rejection (decreased sniff magnitude and eye scanning)
Wallace 1977 - Humans can identify male vs femmale humans by smell with 75% accuracy
Mitro et al., 2012 - Humans can identify younger/middle ages and older people by smell (old people are said to smell nicer)
Laska et al 2000
Laska et al., 2000 - Humans can outperform many species, even dogs, at identifying certain odours. Humans are in fact (Macrosmats) -aka excellence smells but this is only for certain smells (true for all animals
Russel 1976 - Your smell is well detected by yourself.75% of the time you’d be able to pick out a shirt that’s use out of a selection
McClintock et al., 2002 - women prefer the odor of males to whome they’re genetically similar to but not identical
Olsson et al.,2014 - Within hours of activating the immune system, humans elicit and aversive body odour (to trigger avoidance or as a help signal)
de Groot et al., 2012 - fear chemosignals generate fearful facial expressions and sensory acquisition (increased sniff magnitude and eye scanning); digust chemosignals evoke disgusted facial expressions and sensory rejection (decreased sniff magnitude and eye scanning)
Wallace 1977 - Humans can identify male vs femmale humans by smell with 75% accuracy
Mitro et al., 2012 - Humans can identify younger/middle ages and older people by smell (old people are said to smell nicer)
Russel 1976
Russel 1976 - Your smell is well detected by yourself.75% of the time you’d be able to pick out a shirt that’s use out of a selection
McClintock et al., 2002 - women prefer the odor of males to whome they’re genetically similar to but not identical
Olsson et al.,2014 - Within hours of activating the immune system, humans elicit and aversive body odour (to trigger avoidance or as a help signal)
de Groot et al., 2012 - fear chemosignals generate fearful facial expressions and sensory acquisition (increased sniff magnitude and eye scanning); digust chemosignals evoke disgusted facial expressions and sensory rejection (decreased sniff magnitude and eye scanning)
Wallace 1977 - Humans can identify male vs femmale humans by smell with 75% accuracy
Mitro et al., 2012 - Humans can identify younger/middle ages and older people by smell (old people are said to smell nicer)
Mclintock et al 2002
McClintock et al., 2002 - women prefer the odor of males to whome they’re genetically similar to but not identical
Olsson et al.,2014 - Within hours of activating the immune system, humans elicit and aversive body odour (to trigger avoidance or as a help signal)
de Groot et al., 2012 - fear chemosignals generate fearful facial expressions and sensory acquisition (increased sniff magnitude and eye scanning); digust chemosignals evoke disgusted facial expressions and sensory rejection (decreased sniff magnitude and eye scanning)
Wallace 1977 - Humans can identify male vs femmale humans by smell with 75% accuracy
Mitro et al., 2012 - Humans can identify younger/middle ages and older people by smell (old people are said to smell nicer)
Wallace 1977
Wallace 1977 - Humans can identify male vs femmale humans by smell with 75% accuracy
Mitro et al., 2012 - Humans can identify younger/middle ages and older people by smell (old people are said to smell nicer)
Mitro et al 2012
Mitro et al., 2012 - Humans can identify younger/middle ages and older people by smell (old people are said to smell nicer)
Identification as a function of the olfactory system studies
Bushdid et al., 2014 - Humans can discriminate more than 1 trillion olfactory stimuli (theoretically calculated off of the number of ORNS we have)
Laska et al., 2000 - Humans can outperform many species, even dogs, at identifying certain odours. Humans are in fact (Macrosmats) -aka excellence smells but this is only for certain smells (true for all animals
Russel 1976 - Your smell is well detected by yourself.75% of the time you’d be able to pick out a shirt that’s use out of a selection
McClintock et al., 2002 - women prefer the odor of males to whome they’re genetically similar to but not identical
Olsson et al.,2014 - Within hours of activating the immune system, humans elicit and aversive body odour (to trigger avoidance or as a help signal)
de Groot et al., 2012 - fear chemosignals generate fearful facial expressions and sensory acquisition (increased sniff magnitude and eye scanning); digust chemosignals evoke disgusted facial expressions and sensory rejection (decreased sniff magnitude and eye scanning)
Wallace 1977 - Humans can identify male vs femmale humans by smell with 75% accuracy
Mitro et al., 2012 - Humans can identify younger/middle ages and older people by smell (old people are said to smell nicer)
Odour discrimination Paradigms
Looks at identification (quality not intensity)
- Asks how we distinguish 1 trillion scents
- Distinguishing scents often is a test of memory recall than on olfactory ability - tip of the nose phenomenon
Olson eg al 2014
Olsen et al.,2014 - Within hours of activating the immune system, humans elicit and aversive body odour (to trigger avoidance or as a help signal)
de Groot et al., 2012 - fear chemosignals generate fearful facial expressions and sensory acquisition (increased sniff magnitude and eye scanning); digust chemosignals evoke disgusted facial expressions and sensory rejection (decreased sniff magnitude and eye scanning)
Wallace 1977 - Humans can identify male vs femmale humans by smell with 75% accuracy
Mitro et al., 2012 - Humans can identify younger/middle ages and older people by smell (old people are said to smell nicer)
Ambient Scenting
takes advantage of the detection value of smell
Is a marketing tool used by supermarkets, real estate agents in which a good smelling stimuli is placed strategically to promote a certain feeling.
eg. coffee in the start of a coffee shop gets you craving treats
Olfaction in Communication
Mammals send and receive odour messages in a variety of scenarios;
- defnece mechanisms
- territory marking
- emotional state
- sex-related (fertility/reproductive compaitibility/interest)
- Disease/immune status
All of these displays are highly influenced by context, mood, attention, memory etc (aka Top down processing)
How to scientifically investigate smell
Two basic variables of interest;
- odour detection (intensity) - absolute threshold and just noticable difference
These sorts of studies are difficult to achieve as sensitivity to smell differs depending on the odourant and gaining ecological validity is challenging too. JND can also vary considerably from 5 - 33%
- odour discrimination (quality)
Skunk water
Takes advantage of the olfactory detection system
- Is a malodorant used by the military or law enforcement to disperse crowds as it smells bad
What’s unique about th ePheromone System in Humans?
Despite animals having a clear path for Pheomone signalling along Nerve 0 this in humans cannot be found
In Summary how does the Vomeronasal Organ work?
- It detects non-volatile chemical cues that is picked up through direct physical contact with the source of the pheromone
- Uniquely there is no evidence of functional pheromone receptor cells in adult humans
Odour Detection Experiments
Absolute threshold experiments can simply ask participants “can you smell that?” whilst presenting different odorant concentrations (Fechners constant stimuli method?)
De Groote et al 2012
de Groot et al., 2012 - fear chemosignals generate fearful facial expressions and sensory acquisition (increased sniff magnitude and eye scanning); digust chemosignals evoke disgusted facial expressions and sensory rejection (decreased sniff magnitude and eye scanning)
Wallace 1977 - Humans can identify male vs femmale humans by smell with 75% accuracy
Mitro et al., 2012 - Humans can identify younger/middle ages and older people by smell (old people are said to smell nicer)
Why is Olfaction Important
For many animals olfaction is essential to survive (know friends, enimies, territories, find mates, reproduce, locate food etc
For humans there is a degree of survival and safety hoewver, smell is largely about novelty
Novelty is key in helping us recognise new smells
Signal Transduction in Olfaction
- The odorant molecule binds to and activates an odorant receptor on the olfactory receptor neurons which initiates an electrical signal
There is type of receptor per olfactory receptor neuron, however these can multiple types of odorant
There is ~350 orn types (with different receptors)
As an odorant can bind multiple receptors, on therefore different ORN types, there is a population coding scheme, as multiple neurons can bind the same odorant and will fire and different intensities in response to this.
Olfactory Pathway from the Olfactory/Piriform Cortex
Olf cx has strong links to the temporal love (hippocampus and amygdala and the Orbitofrontal cortex)
–> Amygdala (emotion)
–> Hippocampal Formation (memory)
–> Thalamus (relay to/from other areas, attention
–> Orbitofrontal cortex (OFC) (emotion recognition, behavioural inhibition, hedonic assesment aka if something is pleasurable or not, decision making, taste
Hedonic Assessment & Decision Making from Olfaction
People have strong hedonic assessments when it comes to smell (like digusting something or loving it)
This may be linked to avoidance of rancid food and pathogens needed for survival.
This phenomenon however is shaped by past experience, exposure, familiarity vs novelty, internal state (physically and psychologically) and our contextual expectation
This is due to processing in the orbitofrontal cortex - one of the projections of the olfactory pathway
Has a nature and nurture component
BODS (Luizza et al., 2017/2018)
Is the Body Odour Disgust Scale. This suggests there are individual differences in disgust reaction to a variety of body odours.
- Certain personality traits that are related to pathogen avoidance (eg. Authoritarianism
Odor Thresholds
Different odorants have a different detection threshold.
eg. Methyl Mercaptan has a very low detection threshold - meaning a very small amount of it can be detected
Ethyl ether and chloroform however are more difficult to detect as they have a higher detection threshold (more in the air is required to detect them)
What regions of the brain attributes the Proust Effect
Odors have the exceptional ability to instantaneously trigger vivid autobiographical memories—a phenomenon referred to as the Proust effect. While other stimuli can also (obviously) make us recall past memories, they are usually not as detailed, sudden, or vivid as those related to smells.
–> Amygdala and Hippocampus. (emotion and memory)
Memmories are the strongest when they’re emotional and scent based.
= the vivid reliving of event from the past through sensory stimuli
What are the differences in the olfactory brain areas of rats and humans ?
Rats have a considerably larger olfactory bulb and piriform (or olfactory) cortex compared to humans
Impact of Dementia on Olfactory JND’s - Hidalgo et al., 2011
JND aka the ability to discriminate between different odour intensities gets worse in people with dementia. this is one of the earliest signs of the condition
Step two in Olfaction; the smelling process
Once the odourant reaches the olfactory epithelium it is temporarily held in the mucus of this region allowing for binding to the receptors of the olfactory receptor neurons
The olfactory receptor neurons project cilia into the mucus membrane of the olfactory epithelium to detect inhaled odourants. This is the site of signal transduction
Olfactory Epithelium Cell Types
- Basal Cells - are stem cells that divide to give supporting of orn cells
- Supporting Cells - there for trophic support (somewhat similar to glia)
- Olfactory Receptor Neurons (electrically active cells involved in signal transduction by expressing odorant binding proteins)
Proust Effect
Odors have the exceptional ability to instantaneously trigger vivid autobiographical memories—a phenomenon referred to as the Proust effect. While other stimuli can also (obviously) make us recall past memories, they are usually not as detailed, sudden, or vivid as those related to smells.
Scented Studying?
Utilises the Proust Effect
- State dependent effect
Can you provide the scent needed for recall?
Expectation in Olfactory Hedonic Assessment ; Herz and Von Clef (2001)
Again utilises the OFC
- Contextual expectation plays a role in this. (the nose smells what the eyes sees)
Herz and Von Clef introd ambiguous odours with either a positive or negative name (eg. violet leaf extract called cucumber or gathered mildew)
The hedonic assessment of smell was strongly affected by the label
What is an Odour
Typically a mixture of several odorants
What is the olfactory stimulus
Odorants
What are Odorants
= Volatile Chemical Compounds (meaning they’re predisposed to easily aerosolise
Sufficient quantity of a given odorant is required to elicit its smell
How do we code smells?
- Population coding
Several odourants can act on a population of neurons in the olfactory epithelium
These patterns of activity can generate smells
Odor JND’s
In olfaction our JND’s are large. We are note good at discriminating odour intensities
Anosmia and COVID
Anosmia = partial loss of smell
Present in 50% of COVID-19 cases and usually is temporary however 7% of COVID cases remain anosmic after 1 year
(most common in early and Delta strands)
Research shows this is not caused by a loss of olfactory receptor neurons but instead an attacking of the support cells byt eh virus.
As support cells are needed for trophic support, when impacted, neighbouring neurons are impacted and stop producing odorant binding proteins resulting in the loss of smell
Other factors can contribute too - eg. atrophy of regions of the cortex
Glomeruli in the Olfactory System
The glomerulus (plural glomeruli) is a spherical structure located in the olfactory bulb of the brain where synapses form between the terminals of the olfactory nerve and the dendrites of mitral, periglomerular and tufted cells.
Mitral cells
Mitral cells are neurons that are part of the olfactory system. They are located in the olfactory bulb in the mammalian central nervous system. They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomeruli.
These cells project their axons into the brain that synapse with neurons in the olfactory cortex
Population Coding of the Olfactory System
Different populations of cells are active in each step of the olfactory system which helps to code different smells
- Theres a different recognition pattern across ORNs due to the different types of ORN and their relative activation will vary depending on the odourant
- The unique pattern across the mital cells as certain types of ORNS will synapse to certain types of Mitral cells
3) These Mitral cell pathways activate a specific pattern of olfactory cortical neurons
Each level of this pathway has its own computation
A population code emerges at each level and therefore there are different representations at every level
Step one in Olfaction
Is to take in the odourant with smell. In this process an odourant travels the nasal cavity to the olfactory epithelium
Odourant in the air –> Is inhaled through sniffing –> Places the odouran tin the nasal cavity
Olfactory Receptor Neurons in Smelling
The olfactory receptor neurons project cilia into the mucus membrane of the olfactory epithelium to detect inhaled odourants. This is the site of signal transduction
Synaptic Pathway in olfactory system
Olfactory Receptor Neurons –> Mitral cells (in olfactory bulb –> Olfactory Cortical Neurons in the olf cortex
Odour Memories
= Emotional, autobiographical memories evoked by an odour
eg. Proust effect - the vivid reliving of event from the past thorugh sensory stimuli (smelt a biscuit and thought of an Aunt)
