Smell and Trigemnial lecture
Sensory food perception
Sense of smell
Functions of smell
Flavour, eating beahviour
- Anticipation and consumption
Social communication
- Perfumes, hygiene
- Pheromones (chemosignals) --> we can smell health or emotional status on others
- Emotions and memory --> smell memory
Safety, warning
- Fire, gas leak, spoiled food
Anatomy and physiology of the olfactory system
From nose to brain
Smell are volatile molecules, which enter our nose and bind to receptors at the olfactory
epithelium --> the olfactory nerve to end in the olfactory bulb
,Every neuron in the olfactory epithelium express one type of receptor
The axons gather in the glomerulus which only get information from a specific type of
receptor but a lot of odor neurons.
Odorant receptor --> olfactory epithelium --> olfactory nerve --> olfactory bulb
Odor receptors
- Family of olfactory receptor genes totals more than 1000 genes
- Humans have 350-400 functional OR genes
- Able to detect and discriminate wide variety of chemicals
GPCR (G protein-coupled receptor) family
- Receptors recognize small structural features on each molecule
- A combination of activated receptors is responsible for a smell
This means that all the receptors look alike but differ on amino acids which is why different
receptors can have different odors
One molecule can bind to different receptor types in different places of the molecule.
Odor quality coding – pattern of activation
- One odorant can activate multiple odor receptors
- One odor receptor can be activated by multiple odorants
- Pattern of activated glomeruli codes for odor identity
Beyond the olfactory bulb
- Olfactory bulb
- Orbital prefrontal cortex
- Piriform cortex
All other senses have contralateral projections, so when the left hand moves the right side of
the brain is activated. But for smell this is not the case, smell is ipsilateral. When the signal
goes to the brain it does not go via the thalamus, while all other senses/information does do
that.
Olfactory bulb --> primary olfactory cortex (piriform cortex) which explains why smell is
closely related to emotions and memory
The insula (taste cortex) gets activated during smelling
The orbital prefrontal cortex is the secondary olfactory cortex.
,Olfactory neuroanatomy
- Ipsilateral
- No thalamus
Odor quality coding – central mechanisms
- When smells were perceived to be more similar they became more similar.
Odor quality coding – experience
- Enantiomer pairs of odors (indiscriminable)
- Humans are able to train and develop their sense of smell
Enantiomer = molecules that are non-superimposable mirror images of each other
Target odor --> paired with electric shock
Mirror odor --> not paired with shock
Interim summary
- Structures involved in odor coding from nose to brain: odor receptors on olfactory
epithelium, olfactory nerve, glomeruli in olfactory bulb, piriform cortex and OFC
- Molecular structure is important
, - A combination of activated receptors leads to an activation pattern of glomeruli
coding for a smell
- Overlapping but unique spatial pattern in piriform cortex for odor quality
- Dependent upon experience
Odor perception and measurement
Odor quality and categorization
- Plato: pleasant vs unpleasant --> to classify odors --> does not work
It is hard to characterize cause their are many, and hard to track down.
- Smell is abstract, associative, synthetic (not analytic)
Predicting odor quality perception?
- Many many volatile molecules exist
- Humans have 350-400 different odor receptors
- One odorant can activate multiple OR, and nor OR can be activated by multiple
odorants
- Odor coding and perception is flexible and depends on experience
Measuring olfactory function
- Sniffin sticks
- Identification
- Discrimination
- Detection threshold
You have normative values to see whether someone is still able to smell normally
Odor sensitivity
Thresholds for odor sensitivity
- Ppm range (10-6): terpenes (citrus, herbs)
- Ppb range (10-9): ethyl-mercaptan (gas/cabbage)
- Ppt range (10-12): alcohol compounds (ethanol)
Poor intensity rating
Odor discrimination – alcohols
- Structurally similar odors are difficult to distinguish
The more different they were at carbon atoms the better people are at discriminating them.
Odor discrimination – fat perception
People could small the difference between fat contents --> its not the fat molecule itself that
was smelled
Factors influencing your olfactory ability
- Age
Sensory food perception
Sense of smell
Functions of smell
Flavour, eating beahviour
- Anticipation and consumption
Social communication
- Perfumes, hygiene
- Pheromones (chemosignals) --> we can smell health or emotional status on others
- Emotions and memory --> smell memory
Safety, warning
- Fire, gas leak, spoiled food
Anatomy and physiology of the olfactory system
From nose to brain
Smell are volatile molecules, which enter our nose and bind to receptors at the olfactory
epithelium --> the olfactory nerve to end in the olfactory bulb
,Every neuron in the olfactory epithelium express one type of receptor
The axons gather in the glomerulus which only get information from a specific type of
receptor but a lot of odor neurons.
Odorant receptor --> olfactory epithelium --> olfactory nerve --> olfactory bulb
Odor receptors
- Family of olfactory receptor genes totals more than 1000 genes
- Humans have 350-400 functional OR genes
- Able to detect and discriminate wide variety of chemicals
GPCR (G protein-coupled receptor) family
- Receptors recognize small structural features on each molecule
- A combination of activated receptors is responsible for a smell
This means that all the receptors look alike but differ on amino acids which is why different
receptors can have different odors
One molecule can bind to different receptor types in different places of the molecule.
Odor quality coding – pattern of activation
- One odorant can activate multiple odor receptors
- One odor receptor can be activated by multiple odorants
- Pattern of activated glomeruli codes for odor identity
Beyond the olfactory bulb
- Olfactory bulb
- Orbital prefrontal cortex
- Piriform cortex
All other senses have contralateral projections, so when the left hand moves the right side of
the brain is activated. But for smell this is not the case, smell is ipsilateral. When the signal
goes to the brain it does not go via the thalamus, while all other senses/information does do
that.
Olfactory bulb --> primary olfactory cortex (piriform cortex) which explains why smell is
closely related to emotions and memory
The insula (taste cortex) gets activated during smelling
The orbital prefrontal cortex is the secondary olfactory cortex.
,Olfactory neuroanatomy
- Ipsilateral
- No thalamus
Odor quality coding – central mechanisms
- When smells were perceived to be more similar they became more similar.
Odor quality coding – experience
- Enantiomer pairs of odors (indiscriminable)
- Humans are able to train and develop their sense of smell
Enantiomer = molecules that are non-superimposable mirror images of each other
Target odor --> paired with electric shock
Mirror odor --> not paired with shock
Interim summary
- Structures involved in odor coding from nose to brain: odor receptors on olfactory
epithelium, olfactory nerve, glomeruli in olfactory bulb, piriform cortex and OFC
- Molecular structure is important
, - A combination of activated receptors leads to an activation pattern of glomeruli
coding for a smell
- Overlapping but unique spatial pattern in piriform cortex for odor quality
- Dependent upon experience
Odor perception and measurement
Odor quality and categorization
- Plato: pleasant vs unpleasant --> to classify odors --> does not work
It is hard to characterize cause their are many, and hard to track down.
- Smell is abstract, associative, synthetic (not analytic)
Predicting odor quality perception?
- Many many volatile molecules exist
- Humans have 350-400 different odor receptors
- One odorant can activate multiple OR, and nor OR can be activated by multiple
odorants
- Odor coding and perception is flexible and depends on experience
Measuring olfactory function
- Sniffin sticks
- Identification
- Discrimination
- Detection threshold
You have normative values to see whether someone is still able to smell normally
Odor sensitivity
Thresholds for odor sensitivity
- Ppm range (10-6): terpenes (citrus, herbs)
- Ppb range (10-9): ethyl-mercaptan (gas/cabbage)
- Ppt range (10-12): alcohol compounds (ethanol)
Poor intensity rating
Odor discrimination – alcohols
- Structurally similar odors are difficult to distinguish
The more different they were at carbon atoms the better people are at discriminating them.
Odor discrimination – fat perception
People could small the difference between fat contents --> its not the fat molecule itself that
was smelled
Factors influencing your olfactory ability
- Age
