odourant binding
odourant into nose → caught in mucus → binds to olfactory receptor on cilia of odourant receptor neuron (ORN)
odourant signal transduction
binding to olfactory receptor on odourant receptor neuron → triggers depolarisation → olfactory bulb → AP in olfactory nerve (cranial nerve I) neuron
odourant signal to brain
olfactory nerve (cranial nerve I) neuron → to primary olfactory cortex → then projects to OFC, thalamus, amygdala and hippocampus
odourant cortical processing/projections
- primary olfactory cortex = piriform cortex
- PC projects to lots of other brain regions
- amygdala ⇒ emotion
- hippocampus ⇒ memory
- thalamus ⇒ relay to/from other areas, attention
- orbitofrontal cortex (OFC) ⇒ emotion recognition, behavioural inhibition, hedonic assessment, decision making, taste
specificity in odourant detection
- there is 1 type of receptor per ORN but this can bind to multiple types of odourant
- ~350 different types of ORNs
- Each odourant has a unique ‘ORN recognition pattern’
- A population code emerges at every level!
Hyposmia
reduced ability to detect smells (e.g. when sick)
Anosmia
complete inability to detect smell. In rare cases a person might be born with a condition called congenital anosmia
Parosmia
a change in the normal perception of smell (something that once smelled good now smells unpleasant (due to an infection or trauma)
Phantosmia
- Sensation of an odour that isn’t there! (hallucinating a smell, not usually pleasant)
- could be problems in nose, passage, or brain (e.g. tumor)
retronasal olfaction
food particles from mouth enter nose → bind & produce smell sensations that are associated with the taste or flavour
2 pathways of odourants to brain
- nostrils to POC → smell
- top of throat to gustatory cortex → taste
interaction between smell and taste
- retronasal
- info both gustatory and olfactory cortices
- OFC receives info from gustatory and olfactory cortexes ⇒ integration of informatio
how many different olfactory stimuli can we discriminate?
can detect and discriminate > 1 trillion olfactory stimuli
factors that may influence an individual’s ability to detect or identify a certain odour
- genetic: number of receptors, sensitivity
- past experience/memory
- age, sex
- health
- nose anatomy
function of yummy baby smell
- activates reward circuitry in adult brain
- reminds us to look after our baby
armpit smell
- not volatile compounds (like in off food)
- 3 key components: thiols, steroids and diverse set of acids
- converted to smell compounds by microbes on our body
- people with lots of Corynebacterium, common in men = more smelly
- men also make more of the precursors = stinkier
chemically-mediated emotional reactivity and contagion
- reactivity: smelling someone’s anxiety can promote empathy
- contagion: smelling happiness can produce feel good chemicals in others
olfactory signals re immune system
- we can detect via sweat when someone’s immune system is ramped up, and our own leaps into action
- can learn to identify particular diseases by smell from specific smells given off in urine, sweat, breath, faeces
approach and avoidance behaviour
- approach: bonding
- incest avoidance: parents dislike the smell of their opposite sex kids once they hit puberty
lexicon for smell
- we lack the words to describe many specific scents, compared to Umpilla (aboriginal Australian hunter gatherer language)
- less words = less appreciation and awareness for smell
- but also language reflects lower reliance on scent for communication
pheromones
odourants that are detected unconsciously and can convey communication/affect behaviour
- distinction is functional, based on the response they promote in the person smelling them
kin recognition
- mothers recognise or at least prefer the smell of their kids
- HLA (set of genes involved in immunity) play a role, prefer smell of people with diverse HLA, may drive sexual attraction
odourants across the placenta
odour chemicals cross the placenta, baby attracted to smell of human milk, flavoured by mother’s diet
